| Message ID | 1471944454-13895-2-git-send-email-caoj.fnst@cn.fujitsu.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Cao jin <caoj.fnst@cn.fujitsu.com> writes: > The input parameters is used for creating the msix capable device, so > they must obey the PCI spec, or else, it should be programming error. True when the the parameters come from a device model attempting to define a PCI device violating the spec. But what if the parameters come from an actual PCI device violating the spec, via device assignment? For what it's worth, the new behavior seems consistent with msi_init(), which is good. > CC: Markus Armbruster <armbru@redhat.com> > CC: Marcel Apfelbaum <marcel@redhat.com> > CC: Michael S. Tsirkin <mst@redhat.com> > Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com> > --- > hw/pci/msix.c | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/hw/pci/msix.c b/hw/pci/msix.c > index 0ec1cb1..384a29d 100644 > --- a/hw/pci/msix.c > +++ b/hw/pci/msix.c > @@ -253,9 +253,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, > return -ENOTSUP; > } > > - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { > - return -EINVAL; > - } > + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); > > table_size = nentries * PCI_MSIX_ENTRY_SIZE; > pba_size = QEMU_ALIGN_UP(nentries, 64) / 8; > @@ -266,7 +264,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, /* Sanity test: table & pba don't overlap, fit within BARs, min aligned */ if ((table_bar_nr == pba_bar_nr && ranges_overlap(table_offset, table_size, pba_offset, pba_size)) || > table_offset + table_size > memory_region_size(table_bar) || > pba_offset + pba_size > memory_region_size(pba_bar) || > (table_offset | pba_offset) & PCI_MSIX_FLAGS_BIRMASK) { > - return -EINVAL; > + assert(0); > } Instead of if (... complicated condition ...) { assert(0); } let's write assert(... negation of the complicated condition ...); > > cap = pci_add_capability(dev, PCI_CAP_ID_MSIX, cap_pos, MSIX_CAP_LENGTH);
On 09/12/2016 09:29 PM, Markus Armbruster wrote: > Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> The input parameters is used for creating the msix capable device, so >> they must obey the PCI spec, or else, it should be programming error. > > True when the the parameters come from a device model attempting to > define a PCI device violating the spec. But what if the parameters come > from an actual PCI device violating the spec, via device assignment? Before the patch, on invalid param, the vfio behaviour is: error_report("vfio: msix_init failed"); then, device create fail. After the patch, its behaviour is: asserted. Do you mean we should still report some useful info to user on invalid params? Cao jin > > For what it's worth, the new behavior seems consistent with msi_init(), > which is good. > >> CC: Markus Armbruster <armbru@redhat.com> >> CC: Marcel Apfelbaum <marcel@redhat.com> >> CC: Michael S. Tsirkin <mst@redhat.com> >> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com> >> --- >> hw/pci/msix.c | 6 ++---- >> 1 file changed, 2 insertions(+), 4 deletions(-) >> >> diff --git a/hw/pci/msix.c b/hw/pci/msix.c >> index 0ec1cb1..384a29d 100644 >> --- a/hw/pci/msix.c >> +++ b/hw/pci/msix.c >> @@ -253,9 +253,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, >> return -ENOTSUP; >> } >> >> - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { >> - return -EINVAL; >> - } >> + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); >> >> table_size = nentries * PCI_MSIX_ENTRY_SIZE; >> pba_size = QEMU_ALIGN_UP(nentries, 64) / 8; >> @@ -266,7 +264,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, > /* Sanity test: table & pba don't overlap, fit within BARs, min aligned */ > if ((table_bar_nr == pba_bar_nr && > ranges_overlap(table_offset, table_size, pba_offset, pba_size)) || >> table_offset + table_size > memory_region_size(table_bar) || >> pba_offset + pba_size > memory_region_size(pba_bar) || >> (table_offset | pba_offset) & PCI_MSIX_FLAGS_BIRMASK) { >> - return -EINVAL; >> + assert(0); >> } > > Instead of > > if (... complicated condition ...) { > assert(0); > } > > let's write > > assert(... negation of the complicated condition ...); > >> >> cap = pci_add_capability(dev, PCI_CAP_ID_MSIX, cap_pos, MSIX_CAP_LENGTH); > > > . >
Cc: Alex for device assignment expertise. Cao jin <caoj.fnst@cn.fujitsu.com> writes: > On 09/12/2016 09:29 PM, Markus Armbruster wrote: >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >>> The input parameters is used for creating the msix capable device, so >>> they must obey the PCI spec, or else, it should be programming error. >> >> True when the the parameters come from a device model attempting to >> define a PCI device violating the spec. But what if the parameters come >> from an actual PCI device violating the spec, via device assignment? > > Before the patch, on invalid param, the vfio behaviour is: > error_report("vfio: msix_init failed"); > then, device create fail. > > After the patch, its behaviour is: > asserted. > > Do you mean we should still report some useful info to user on invalid > params? In the normal case, asking msix_init() to create MSI-X that are out of spec is a programming error: the code that does it is broken and needs fixing. Device assignment might be the exception: there, the parameters for msix_init() come from the assigned device, not the program. If they violate the spec, the device is broken. This wouldn't be a programming error. Alex, can this happen? If yes, we may want to handle it by failing device assignment. > Cao jin >> >> For what it's worth, the new behavior seems consistent with msi_init(), >> which is good. Whatever behavior on out-of-spec parameters we choose, msi_init() and msix_init() should behave the same.
On Tue, 13 Sep 2016 08:16:20 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Cc: Alex for device assignment expertise. > > Cao jin <caoj.fnst@cn.fujitsu.com> writes: > > > On 09/12/2016 09:29 PM, Markus Armbruster wrote: > >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> > >>> The input parameters is used for creating the msix capable device, so > >>> they must obey the PCI spec, or else, it should be programming error. > >> > >> True when the the parameters come from a device model attempting to > >> define a PCI device violating the spec. But what if the parameters come > >> from an actual PCI device violating the spec, via device assignment? > > > > Before the patch, on invalid param, the vfio behaviour is: > > error_report("vfio: msix_init failed"); > > then, device create fail. > > > > After the patch, its behaviour is: > > asserted. > > > > Do you mean we should still report some useful info to user on invalid > > params? > > In the normal case, asking msix_init() to create MSI-X that are out of > spec is a programming error: the code that does it is broken and needs > fixing. > > Device assignment might be the exception: there, the parameters for > msix_init() come from the assigned device, not the program. If they > violate the spec, the device is broken. This wouldn't be a programming > error. Alex, can this happen? > > If yes, we may want to handle it by failing device assignment. Generally, I think the entire premise of these sorts of patches is flawed. We take a working error path that allows a driver to robustly abort on unexpected date and turn it into a time bomb. Often the excuse for this is that "error handling is hard". Tough. Now a hot-add of a device that triggers this changes from a simple failure to a denial of service event. Furthermore, we base that time bomb on our interpretation of the spec, which we can only validate against in-tree devices. We have actually had assigned devices that fail the sanity test here, there's a quirk in vfio_msix_early_setup() for a Chelsio device with this bug. Do we really want user experiencing aborts when a simple device initialization failure is sufficient? Generally abort code paths like this cause me to do my own sanity testing, which is really poor practice since we should have that sanity testing in the common code. Thanks, Alex
Alex Williamson <alex.williamson@redhat.com> writes: > On Tue, 13 Sep 2016 08:16:20 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Cc: Alex for device assignment expertise. >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> >> >>> The input parameters is used for creating the msix capable device, so >> >>> they must obey the PCI spec, or else, it should be programming error. >> >> >> >> True when the the parameters come from a device model attempting to >> >> define a PCI device violating the spec. But what if the parameters come >> >> from an actual PCI device violating the spec, via device assignment? >> > >> > Before the patch, on invalid param, the vfio behaviour is: >> > error_report("vfio: msix_init failed"); >> > then, device create fail. >> > >> > After the patch, its behaviour is: >> > asserted. >> > >> > Do you mean we should still report some useful info to user on invalid >> > params? >> >> In the normal case, asking msix_init() to create MSI-X that are out of >> spec is a programming error: the code that does it is broken and needs >> fixing. >> >> Device assignment might be the exception: there, the parameters for >> msix_init() come from the assigned device, not the program. If they >> violate the spec, the device is broken. This wouldn't be a programming >> error. Alex, can this happen? >> >> If yes, we may want to handle it by failing device assignment. > > > Generally, I think the entire premise of these sorts of patches is > flawed. We take a working error path that allows a driver to robustly > abort on unexpected date and turn it into a time bomb. Often the > excuse for this is that "error handling is hard". Tough. Now a > hot-add of a device that triggers this changes from a simple failure to > a denial of service event. Furthermore, we base that time bomb on our > interpretation of the spec, which we can only validate against in-tree > devices. > > We have actually had assigned devices that fail the sanity test here, > there's a quirk in vfio_msix_early_setup() for a Chelsio device with > this bug. Do we really want user experiencing aborts when a simple > device initialization failure is sufficient? > > Generally abort code paths like this cause me to do my own sanity > testing, which is really poor practice since we should have that sanity > testing in the common code. Thanks, I prefer to assert on programming error, because 1. it does double duty as documentation, 2. error handling of impossible conditions is commonly wrong, and 3. assertion failures have a much better chance to get the program fixed. Even when presence of a working error path kills 2., the other two make me stick to assertions. However, input out-of-spec is not a programming error. For most users of msix_init(), the arguments are hard-coded, thus invalid arguments are a programming error. For device assignment, they come from a physical device, thus invalid arguments can either be a programming error (our idea of "invalid" is invalid) or bad input (the physical device is out-of-spec). Since we can't know, we better handle it rather than assert. Bottom line: you convinced me msix_init() should stay as it is. But now msi_init() looks like it needs a change: it asserts on invalid nr_vectors parameter. Does that need fixing, Alex?
On Thu, 29 Sep 2016 15:11:27 +0200 Markus Armbruster <armbru@redhat.com> wrote: > Alex Williamson <alex.williamson@redhat.com> writes: > > > On Tue, 13 Sep 2016 08:16:20 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Cc: Alex for device assignment expertise. > >> > >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> > >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: > >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> >> > >> >>> The input parameters is used for creating the msix capable device, so > >> >>> they must obey the PCI spec, or else, it should be programming error. > >> >> > >> >> True when the the parameters come from a device model attempting to > >> >> define a PCI device violating the spec. But what if the parameters come > >> >> from an actual PCI device violating the spec, via device assignment? > >> > > >> > Before the patch, on invalid param, the vfio behaviour is: > >> > error_report("vfio: msix_init failed"); > >> > then, device create fail. > >> > > >> > After the patch, its behaviour is: > >> > asserted. > >> > > >> > Do you mean we should still report some useful info to user on invalid > >> > params? > >> > >> In the normal case, asking msix_init() to create MSI-X that are out of > >> spec is a programming error: the code that does it is broken and needs > >> fixing. > >> > >> Device assignment might be the exception: there, the parameters for > >> msix_init() come from the assigned device, not the program. If they > >> violate the spec, the device is broken. This wouldn't be a programming > >> error. Alex, can this happen? > >> > >> If yes, we may want to handle it by failing device assignment. > > > > > > Generally, I think the entire premise of these sorts of patches is > > flawed. We take a working error path that allows a driver to robustly > > abort on unexpected date and turn it into a time bomb. Often the > > excuse for this is that "error handling is hard". Tough. Now a > > hot-add of a device that triggers this changes from a simple failure to > > a denial of service event. Furthermore, we base that time bomb on our > > interpretation of the spec, which we can only validate against in-tree > > devices. > > > > We have actually had assigned devices that fail the sanity test here, > > there's a quirk in vfio_msix_early_setup() for a Chelsio device with > > this bug. Do we really want user experiencing aborts when a simple > > device initialization failure is sufficient? > > > > Generally abort code paths like this cause me to do my own sanity > > testing, which is really poor practice since we should have that sanity > > testing in the common code. Thanks, > > I prefer to assert on programming error, because 1. it does double duty > as documentation, 2. error handling of impossible conditions is commonly > wrong, and 3. assertion failures have a much better chance to get the > program fixed. Even when presence of a working error path kills 2., the > other two make me stick to assertions. So we're looking at: > - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { > - return -EINVAL; > - } vs > + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); How do you argue that one of these provides better self documentation than the other? The assert may have a better chance of getting fixed, but it's because the existence of the assert itself exposes a vulnerability in the code. Which would you rather have in production, a VMM that crashes on the slightest deviance from the input it expects or one that simply errors the faulting code path and continues? Error handling is hard, which is why we need to look at it as a collection of smaller problems. We return an error at a leaf function and let callers of that function decide how to handle it. If some of those callers don't want to deal with error handling, abort there, we can come back to them later, but let the code paths that do want proper error handling to continue. If we add aborts into the leaf function, then any calling path that wants to be robust against an error needs to fully sanitize the input itself, at which point we have different drivers sanitizing in different ways, all building up walls to protect themselves from the time bombs in these leaf functions. It's crazy. > However, input out-of-spec is not a programming error. For most users > of msix_init(), the arguments are hard-coded, thus invalid arguments are > a programming error. For device assignment, they come from a physical > device, thus invalid arguments can either be a programming error (our > idea of "invalid" is invalid) or bad input (the physical device is > out-of-spec). Since we can't know, we better handle it rather than > assert. So are we going to flag every call path that device assignment might use as one that needs "proper" error handling any anything that's only used by emulated devices can assert? How will anyone ever know? vfio tries really hard to be just another device in the QEMU ecosystem. > Bottom line: you convinced me msix_init() should stay as it is. But now > msi_init() looks like it needs a change: it asserts on invalid > nr_vectors parameter. Does that need fixing, Alex? IMHO, they all need to be fixed. Besides, look at the callers of msi_init(), almost every one will assert on its own if msi_init() fails, all we're doing is hindering drivers like vfio-pci that can gracefully handle a failure. I think that's exactly how each of these should be handled, find a leaf function with asserts, convert it to proper error handling, change the callers that don't already handle the error or assert to assert, then work down through each code path to figure out how they can more robustly handle an error. I don't buy the argument that error handling is too hard or that we're more likely to get it wrong. It needs to be handled as percolating small errors, each of which is trivial to handle on its own. Thanks, Alex
Alex Williamson <alex.williamson@redhat.com> writes: > On Thu, 29 Sep 2016 15:11:27 +0200 > Markus Armbruster <armbru@redhat.com> wrote: > >> Alex Williamson <alex.williamson@redhat.com> writes: >> >> > On Tue, 13 Sep 2016 08:16:20 +0200 >> > Markus Armbruster <armbru@redhat.com> wrote: >> > >> >> Cc: Alex for device assignment expertise. >> >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> >> >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> >> >> >> >>> The input parameters is used for creating the msix capable device, so >> >> >>> they must obey the PCI spec, or else, it should be programming error. >> >> >> >> >> >> True when the the parameters come from a device model attempting to >> >> >> define a PCI device violating the spec. But what if the parameters come >> >> >> from an actual PCI device violating the spec, via device assignment? >> >> > >> >> > Before the patch, on invalid param, the vfio behaviour is: >> >> > error_report("vfio: msix_init failed"); >> >> > then, device create fail. >> >> > >> >> > After the patch, its behaviour is: >> >> > asserted. >> >> > >> >> > Do you mean we should still report some useful info to user on invalid >> >> > params? >> >> >> >> In the normal case, asking msix_init() to create MSI-X that are out of >> >> spec is a programming error: the code that does it is broken and needs >> >> fixing. >> >> >> >> Device assignment might be the exception: there, the parameters for >> >> msix_init() come from the assigned device, not the program. If they >> >> violate the spec, the device is broken. This wouldn't be a programming >> >> error. Alex, can this happen? >> >> >> >> If yes, we may want to handle it by failing device assignment. >> > >> > >> > Generally, I think the entire premise of these sorts of patches is >> > flawed. We take a working error path that allows a driver to robustly >> > abort on unexpected date and turn it into a time bomb. Often the >> > excuse for this is that "error handling is hard". Tough. Now a >> > hot-add of a device that triggers this changes from a simple failure to >> > a denial of service event. Furthermore, we base that time bomb on our >> > interpretation of the spec, which we can only validate against in-tree >> > devices. >> > >> > We have actually had assigned devices that fail the sanity test here, >> > there's a quirk in vfio_msix_early_setup() for a Chelsio device with >> > this bug. Do we really want user experiencing aborts when a simple >> > device initialization failure is sufficient? >> > >> > Generally abort code paths like this cause me to do my own sanity >> > testing, which is really poor practice since we should have that sanity >> > testing in the common code. Thanks, >> >> I prefer to assert on programming error, because 1. it does double duty >> as documentation, 2. error handling of impossible conditions is commonly >> wrong, and 3. assertion failures have a much better chance to get the >> program fixed. Even when presence of a working error path kills 2., the >> other two make me stick to assertions. > > So we're looking at: > >> - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { >> - return -EINVAL; >> - } > > vs > >> + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); > > How do you argue that one of these provides better self documentation > than the other? The first one says "this can happen, and when it does, the function fails cleanly." For a genuine programming error, this is in part misleading. The second one says "I assert this can't happen. We'd be toast if I was wrong." > The assert may have a better chance of getting fixed, but it's because > the existence of the assert itself exposes a vulnerability in the code. > Which would you rather have in production, a VMM that crashes on the > slightest deviance from the input it expects or one that simply errors > the faulting code path and continues? Invalid input to a program should never be treated as programming error. > Error handling is hard, which is why we need to look at it as a > collection of smaller problems. We return an error at a leaf function > and let callers of that function decide how to handle it. If some of > those callers don't want to deal with error handling, abort there, we > can come back to them later, but let the code paths that do want proper > error handling to continue. If we add aborts into the leaf function, > then any calling path that wants to be robust against an error needs to > fully sanitize the input itself, at which point we have different > drivers sanitizing in different ways, all building up walls to protect > themselves from the time bombs in these leaf functions. It's crazy. It depends on the kind of error in the leaf function. I suspect we're talking past each other because we got different kinds of errors in mind. Programming is impossible without things like preconditions, postconditions, invariants. If a section of code is entered when its precondition doesn't hold, we're toast. This is the archetypical programming error. If it can actually happen, the program is incorrect, and needs fixing. Checking preconditions is often (but not always) practical. In my opinion, checking is good practice, and the proper way to check is assert(). Makes the incorrect program fail before it can do further damage, and helps with finding the programming error. A preconditions is part of the contract between a function and its users. An strong precondition can make the function's job easier, but that's no use if the resulting function is inconvenient to use. On the other hand, complicating the function to get a weaker precondition nobody actually needs is just as dumb. Returning an error is *not* checking preconditions. Remember, if the precondition doesn't hold, we're toast. If we're toast when we return an error, we're clearly doing it wrong. You are arguing for weaker preconditions. I'm not actually disagreeing with you! I'm merely expressing my opinion that checking preconditions with assert() is a good idea. >> However, input out-of-spec is not a programming error. For most users >> of msix_init(), the arguments are hard-coded, thus invalid arguments are >> a programming error. For device assignment, they come from a physical >> device, thus invalid arguments can either be a programming error (our >> idea of "invalid" is invalid) or bad input (the physical device is >> out-of-spec). Since we can't know, we better handle it rather than >> assert. > > So are we going to flag every call path that device assignment might > use as one that needs "proper" error handling any anything that's only > used by emulated devices can assert? How will anyone ever know? vfio > tries really hard to be just another device in the QEMU ecosystem. It tries, but it can't help to add a few things. Consider the number of MSI vectors. It can only be 1, 2, 4, 8, 16 or 32. When the callers of msi_init() pass literal numbers, making "the number is valid" a precondition is quite sensible. If the numbers come from the user via configuration, they need to be checked. Two sane ways to do that: check close to where the configuration is processed, and check where it is used. The former will likely produce better error messages. But the latter has its advantages, too. Checking next to its use in msi_init() involves making it handle invalid numbers, i.e. weakening its precondition. Making vectors configurable turned moves them from the realm of preconditions to the realm of program input. Code needs to be updated for that. What device assignment adds is moving many more bits to the program input realm. More code needs to be updated for that. >> Bottom line: you convinced me msix_init() should stay as it is. But now >> msi_init() looks like it needs a change: it asserts on invalid >> nr_vectors parameter. Does that need fixing, Alex? > > IMHO, they all need to be fixed. Besides, look at the callers of > msi_init(), almost every one will assert on its own if msi_init() > fails, all we're doing is hindering drivers like vfio-pci that can > gracefully handle a failure. I think that's exactly how each of these > should be handled, find a leaf function with asserts, convert it to > proper error handling, change the callers that don't already handle the > error or assert to assert, then work down through each code path to > figure out how they can more robustly handle an error. I don't buy the > argument that error handling is too hard or that we're more likely to > get it wrong. It needs to be handled as percolating small errors, each > of which is trivial to handle on its own. Thanks, Once there's a need to handle a certain condition as an error, we should do that, no argument. This also provides a way to test the error path. However, I wouldn't buy an argument that preconditions should be made as weak as possible in leaf functions (let alone always) regardless of the cost in complexity, and non-testability of error paths. I'm strictly a pay as you go person. Back to the problem at hand. Cao jin, would you be willing to fix msi_init()?
* Markus Armbruster (armbru@redhat.com) wrote: > Alex Williamson <alex.williamson@redhat.com> writes: > > > On Thu, 29 Sep 2016 15:11:27 +0200 > > Markus Armbruster <armbru@redhat.com> wrote: > > > >> Alex Williamson <alex.williamson@redhat.com> writes: > >> > >> > On Tue, 13 Sep 2016 08:16:20 +0200 > >> > Markus Armbruster <armbru@redhat.com> wrote: > >> > > >> >> Cc: Alex for device assignment expertise. > >> >> > >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> >> > >> >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: > >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> >> >> > >> >> >>> The input parameters is used for creating the msix capable device, so > >> >> >>> they must obey the PCI spec, or else, it should be programming error. > >> >> >> > >> >> >> True when the the parameters come from a device model attempting to > >> >> >> define a PCI device violating the spec. But what if the parameters come > >> >> >> from an actual PCI device violating the spec, via device assignment? > >> >> > > >> >> > Before the patch, on invalid param, the vfio behaviour is: > >> >> > error_report("vfio: msix_init failed"); > >> >> > then, device create fail. > >> >> > > >> >> > After the patch, its behaviour is: > >> >> > asserted. > >> >> > > >> >> > Do you mean we should still report some useful info to user on invalid > >> >> > params? > >> >> > >> >> In the normal case, asking msix_init() to create MSI-X that are out of > >> >> spec is a programming error: the code that does it is broken and needs > >> >> fixing. > >> >> > >> >> Device assignment might be the exception: there, the parameters for > >> >> msix_init() come from the assigned device, not the program. If they > >> >> violate the spec, the device is broken. This wouldn't be a programming > >> >> error. Alex, can this happen? > >> >> > >> >> If yes, we may want to handle it by failing device assignment. > >> > > >> > > >> > Generally, I think the entire premise of these sorts of patches is > >> > flawed. We take a working error path that allows a driver to robustly > >> > abort on unexpected date and turn it into a time bomb. Often the > >> > excuse for this is that "error handling is hard". Tough. Now a > >> > hot-add of a device that triggers this changes from a simple failure to > >> > a denial of service event. Furthermore, we base that time bomb on our > >> > interpretation of the spec, which we can only validate against in-tree > >> > devices. > >> > > >> > We have actually had assigned devices that fail the sanity test here, > >> > there's a quirk in vfio_msix_early_setup() for a Chelsio device with > >> > this bug. Do we really want user experiencing aborts when a simple > >> > device initialization failure is sufficient? > >> > > >> > Generally abort code paths like this cause me to do my own sanity > >> > testing, which is really poor practice since we should have that sanity > >> > testing in the common code. Thanks, > >> > >> I prefer to assert on programming error, because 1. it does double duty > >> as documentation, 2. error handling of impossible conditions is commonly > >> wrong, and 3. assertion failures have a much better chance to get the > >> program fixed. Even when presence of a working error path kills 2., the > >> other two make me stick to assertions. > > > > So we're looking at: > > > >> - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { > >> - return -EINVAL; > >> - } > > > > vs > > > >> + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); > > > > How do you argue that one of these provides better self documentation > > than the other? > > The first one says "this can happen, and when it does, the function > fails cleanly." For a genuine programming error, this is in part > misleading. > > The second one says "I assert this can't happen. We'd be toast if I was > wrong." > > > The assert may have a better chance of getting fixed, but it's because > > the existence of the assert itself exposes a vulnerability in the code. > > Which would you rather have in production, a VMM that crashes on the > > slightest deviance from the input it expects or one that simply errors > > the faulting code path and continues? > > Invalid input to a program should never be treated as programming error. > > > Error handling is hard, which is why we need to look at it as a > > collection of smaller problems. We return an error at a leaf function > > and let callers of that function decide how to handle it. If some of > > those callers don't want to deal with error handling, abort there, we > > can come back to them later, but let the code paths that do want proper > > error handling to continue. If we add aborts into the leaf function, > > then any calling path that wants to be robust against an error needs to > > fully sanitize the input itself, at which point we have different > > drivers sanitizing in different ways, all building up walls to protect > > themselves from the time bombs in these leaf functions. It's crazy. > > It depends on the kind of error in the leaf function. > > I suspect we're talking past each other because we got different kinds > of errors in mind. > > Programming is impossible without things like preconditions, > postconditions, invariants. > > If a section of code is entered when its precondition doesn't hold, > we're toast. This is the archetypical programming error. > > If it can actually happen, the program is incorrect, and needs fixing. > > Checking preconditions is often (but not always) practical. In my > opinion, checking is good practice, and the proper way to check is > assert(). Makes the incorrect program fail before it can do further > damage, and helps with finding the programming error. > > A preconditions is part of the contract between a function and its > users. An strong precondition can make the function's job easier, but > that's no use if the resulting function is inconvenient to use. On the > other hand, complicating the function to get a weaker precondition > nobody actually needs is just as dumb. > > Returning an error is *not* checking preconditions. Remember, if the > precondition doesn't hold, we're toast. If we're toast when we return > an error, we're clearly doing it wrong. > > You are arguing for weaker preconditions. I'm not actually disagreeing > with you! I'm merely expressing my opinion that checking preconditions > with assert() is a good idea. I have a fairly strong dislike for asserts in qemu, and although I'm not always consistent, my reasoning is mainly to do with asserts once a guest is running. Lets imagine you have a happily running guest and then you try and do something new and complex (e.g. hotplug a vfio-device); now lets say that new thing has something very broken about it, do you really want the previously running guest to die? My view is it can very much depend on how broken you think the world is; you've got to remember that crashing at this point is going to lose the user a VM, and that could mean losing data - so at that point you have to make a decision about whether your lack of confidence in the state of the VM due to the failed precondition is worse than your knowledge that the VM is going to fail. Perhaps giving the user an error and disabling the device lets the admin gravefully shutdown the VM and walk away with all their data intact. So I wouldn't argue for weaker preconditions, just what the result is if the precondition fails. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes: > * Markus Armbruster (armbru@redhat.com) wrote: >> Alex Williamson <alex.williamson@redhat.com> writes: >> >> > On Thu, 29 Sep 2016 15:11:27 +0200 >> > Markus Armbruster <armbru@redhat.com> wrote: >> > >> >> Alex Williamson <alex.williamson@redhat.com> writes: >> >> >> >> > On Tue, 13 Sep 2016 08:16:20 +0200 >> >> > Markus Armbruster <armbru@redhat.com> wrote: >> >> > >> >> >> Cc: Alex for device assignment expertise. >> >> >> >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> >> >> >> >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: >> >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: >> >> >> >> >> >> >> >>> The input parameters is used for creating the msix capable device, so >> >> >> >>> they must obey the PCI spec, or else, it should be programming error. >> >> >> >> >> >> >> >> True when the the parameters come from a device model attempting to >> >> >> >> define a PCI device violating the spec. But what if the parameters come >> >> >> >> from an actual PCI device violating the spec, via device assignment? >> >> >> > >> >> >> > Before the patch, on invalid param, the vfio behaviour is: >> >> >> > error_report("vfio: msix_init failed"); >> >> >> > then, device create fail. >> >> >> > >> >> >> > After the patch, its behaviour is: >> >> >> > asserted. >> >> >> > >> >> >> > Do you mean we should still report some useful info to user on invalid >> >> >> > params? >> >> >> >> >> >> In the normal case, asking msix_init() to create MSI-X that are out of >> >> >> spec is a programming error: the code that does it is broken and needs >> >> >> fixing. >> >> >> >> >> >> Device assignment might be the exception: there, the parameters for >> >> >> msix_init() come from the assigned device, not the program. If they >> >> >> violate the spec, the device is broken. This wouldn't be a programming >> >> >> error. Alex, can this happen? >> >> >> >> >> >> If yes, we may want to handle it by failing device assignment. >> >> > >> >> > >> >> > Generally, I think the entire premise of these sorts of patches is >> >> > flawed. We take a working error path that allows a driver to robustly >> >> > abort on unexpected date and turn it into a time bomb. Often the >> >> > excuse for this is that "error handling is hard". Tough. Now a >> >> > hot-add of a device that triggers this changes from a simple failure to >> >> > a denial of service event. Furthermore, we base that time bomb on our >> >> > interpretation of the spec, which we can only validate against in-tree >> >> > devices. >> >> > >> >> > We have actually had assigned devices that fail the sanity test here, >> >> > there's a quirk in vfio_msix_early_setup() for a Chelsio device with >> >> > this bug. Do we really want user experiencing aborts when a simple >> >> > device initialization failure is sufficient? >> >> > >> >> > Generally abort code paths like this cause me to do my own sanity >> >> > testing, which is really poor practice since we should have that sanity >> >> > testing in the common code. Thanks, >> >> >> >> I prefer to assert on programming error, because 1. it does double duty >> >> as documentation, 2. error handling of impossible conditions is commonly >> >> wrong, and 3. assertion failures have a much better chance to get the >> >> program fixed. Even when presence of a working error path kills 2., the >> >> other two make me stick to assertions. >> > >> > So we're looking at: >> > >> >> - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { >> >> - return -EINVAL; >> >> - } >> > >> > vs >> > >> >> + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); >> > >> > How do you argue that one of these provides better self documentation >> > than the other? >> >> The first one says "this can happen, and when it does, the function >> fails cleanly." For a genuine programming error, this is in part >> misleading. >> >> The second one says "I assert this can't happen. We'd be toast if I was >> wrong." >> >> > The assert may have a better chance of getting fixed, but it's because >> > the existence of the assert itself exposes a vulnerability in the code. >> > Which would you rather have in production, a VMM that crashes on the >> > slightest deviance from the input it expects or one that simply errors >> > the faulting code path and continues? >> >> Invalid input to a program should never be treated as programming error. >> >> > Error handling is hard, which is why we need to look at it as a >> > collection of smaller problems. We return an error at a leaf function >> > and let callers of that function decide how to handle it. If some of >> > those callers don't want to deal with error handling, abort there, we >> > can come back to them later, but let the code paths that do want proper >> > error handling to continue. If we add aborts into the leaf function, >> > then any calling path that wants to be robust against an error needs to >> > fully sanitize the input itself, at which point we have different >> > drivers sanitizing in different ways, all building up walls to protect >> > themselves from the time bombs in these leaf functions. It's crazy. >> >> It depends on the kind of error in the leaf function. >> >> I suspect we're talking past each other because we got different kinds >> of errors in mind. >> >> Programming is impossible without things like preconditions, >> postconditions, invariants. >> >> If a section of code is entered when its precondition doesn't hold, >> we're toast. This is the archetypical programming error. >> >> If it can actually happen, the program is incorrect, and needs fixing. >> >> Checking preconditions is often (but not always) practical. In my >> opinion, checking is good practice, and the proper way to check is >> assert(). Makes the incorrect program fail before it can do further >> damage, and helps with finding the programming error. >> >> A preconditions is part of the contract between a function and its >> users. An strong precondition can make the function's job easier, but >> that's no use if the resulting function is inconvenient to use. On the >> other hand, complicating the function to get a weaker precondition >> nobody actually needs is just as dumb. >> >> Returning an error is *not* checking preconditions. Remember, if the >> precondition doesn't hold, we're toast. If we're toast when we return >> an error, we're clearly doing it wrong. >> >> You are arguing for weaker preconditions. I'm not actually disagreeing >> with you! I'm merely expressing my opinion that checking preconditions >> with assert() is a good idea. > > I have a fairly strong dislike for asserts in qemu, and although I'm not > always consistent, my reasoning is mainly to do with asserts once a guest > is running. > > Lets imagine you have a happily running guest and then you try and do > something new and complex (e.g. hotplug a vfio-device); now lets say that > new thing has something very broken about it, do you really want the previously > running guest to die? If a precondition doesn't hold, we're toast. The best we can do is crash before we mess up things further. A problematic condition we can safely recover from can be made an error condition. I think the crux of our misunderstandings (I hesitate to call it an argument) is confusing recoverable error conditions with violated preconditions. We all agree (violently, perhaps) that assert() is not an acceptable error handling mechanism. > My view is it can very much depend on how broken you think the > world is; you've got to remember that crashing at this point > is going to lose the user a VM, and that could mean losing > data - so at that point you have to make a decision about whether > your lack of confidence in the state of the VM due to the failed > precondition is worse than your knowledge that the VM is going to fail. > > Perhaps giving the user an error and disabling the device lets > the admin gravefully shutdown the VM and walk away with all > their data intact. This is risky business unless you can prove the problematic condition is safely isolated. To elaborate on your device example: say some logic error in device emulation code put the device instance in some broken state. If you detect that before the device could mess up anything else, fencing the device is safe. But if device state is borked because some other code overran an array, continuing risks making things worse. Crashing the guest is bad. Letting it first overwrite good data with bad data is worse. Sadly, such proof is hardly ever possible in unrestricted C. So we're down to probabilities and tradeoffs. I'd reject a claim that once the guest is running the tradeoffs *always* favour trying to hobble on. If you want a less bleak isolation and recovery story, check out Erlang. Note that its "let it crash" philosophy is very much in accordance with my views on what can safely be done after detecting a programming error / violated precondition. > So I wouldn't argue for weaker preconditions, just what the > result is if the precondition fails. I respectfully disagree with your use of the concept "precondition".
* Markus Armbruster (armbru@redhat.com) wrote: > "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes: > > > * Markus Armbruster (armbru@redhat.com) wrote: > >> Alex Williamson <alex.williamson@redhat.com> writes: > >> > >> > On Thu, 29 Sep 2016 15:11:27 +0200 > >> > Markus Armbruster <armbru@redhat.com> wrote: > >> > > >> >> Alex Williamson <alex.williamson@redhat.com> writes: > >> >> > >> >> > On Tue, 13 Sep 2016 08:16:20 +0200 > >> >> > Markus Armbruster <armbru@redhat.com> wrote: > >> >> > > >> >> >> Cc: Alex for device assignment expertise. > >> >> >> > >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> >> >> > >> >> >> > On 09/12/2016 09:29 PM, Markus Armbruster wrote: > >> >> >> >> Cao jin <caoj.fnst@cn.fujitsu.com> writes: > >> >> >> >> > >> >> >> >>> The input parameters is used for creating the msix capable device, so > >> >> >> >>> they must obey the PCI spec, or else, it should be programming error. > >> >> >> >> > >> >> >> >> True when the the parameters come from a device model attempting to > >> >> >> >> define a PCI device violating the spec. But what if the parameters come > >> >> >> >> from an actual PCI device violating the spec, via device assignment? > >> >> >> > > >> >> >> > Before the patch, on invalid param, the vfio behaviour is: > >> >> >> > error_report("vfio: msix_init failed"); > >> >> >> > then, device create fail. > >> >> >> > > >> >> >> > After the patch, its behaviour is: > >> >> >> > asserted. > >> >> >> > > >> >> >> > Do you mean we should still report some useful info to user on invalid > >> >> >> > params? > >> >> >> > >> >> >> In the normal case, asking msix_init() to create MSI-X that are out of > >> >> >> spec is a programming error: the code that does it is broken and needs > >> >> >> fixing. > >> >> >> > >> >> >> Device assignment might be the exception: there, the parameters for > >> >> >> msix_init() come from the assigned device, not the program. If they > >> >> >> violate the spec, the device is broken. This wouldn't be a programming > >> >> >> error. Alex, can this happen? > >> >> >> > >> >> >> If yes, we may want to handle it by failing device assignment. > >> >> > > >> >> > > >> >> > Generally, I think the entire premise of these sorts of patches is > >> >> > flawed. We take a working error path that allows a driver to robustly > >> >> > abort on unexpected date and turn it into a time bomb. Often the > >> >> > excuse for this is that "error handling is hard". Tough. Now a > >> >> > hot-add of a device that triggers this changes from a simple failure to > >> >> > a denial of service event. Furthermore, we base that time bomb on our > >> >> > interpretation of the spec, which we can only validate against in-tree > >> >> > devices. > >> >> > > >> >> > We have actually had assigned devices that fail the sanity test here, > >> >> > there's a quirk in vfio_msix_early_setup() for a Chelsio device with > >> >> > this bug. Do we really want user experiencing aborts when a simple > >> >> > device initialization failure is sufficient? > >> >> > > >> >> > Generally abort code paths like this cause me to do my own sanity > >> >> > testing, which is really poor practice since we should have that sanity > >> >> > testing in the common code. Thanks, > >> >> > >> >> I prefer to assert on programming error, because 1. it does double duty > >> >> as documentation, 2. error handling of impossible conditions is commonly > >> >> wrong, and 3. assertion failures have a much better chance to get the > >> >> program fixed. Even when presence of a working error path kills 2., the > >> >> other two make me stick to assertions. > >> > > >> > So we're looking at: > >> > > >> >> - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { > >> >> - return -EINVAL; > >> >> - } > >> > > >> > vs > >> > > >> >> + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); > >> > > >> > How do you argue that one of these provides better self documentation > >> > than the other? > >> > >> The first one says "this can happen, and when it does, the function > >> fails cleanly." For a genuine programming error, this is in part > >> misleading. > >> > >> The second one says "I assert this can't happen. We'd be toast if I was > >> wrong." > >> > >> > The assert may have a better chance of getting fixed, but it's because > >> > the existence of the assert itself exposes a vulnerability in the code. > >> > Which would you rather have in production, a VMM that crashes on the > >> > slightest deviance from the input it expects or one that simply errors > >> > the faulting code path and continues? > >> > >> Invalid input to a program should never be treated as programming error. > >> > >> > Error handling is hard, which is why we need to look at it as a > >> > collection of smaller problems. We return an error at a leaf function > >> > and let callers of that function decide how to handle it. If some of > >> > those callers don't want to deal with error handling, abort there, we > >> > can come back to them later, but let the code paths that do want proper > >> > error handling to continue. If we add aborts into the leaf function, > >> > then any calling path that wants to be robust against an error needs to > >> > fully sanitize the input itself, at which point we have different > >> > drivers sanitizing in different ways, all building up walls to protect > >> > themselves from the time bombs in these leaf functions. It's crazy. > >> > >> It depends on the kind of error in the leaf function. > >> > >> I suspect we're talking past each other because we got different kinds > >> of errors in mind. > >> > >> Programming is impossible without things like preconditions, > >> postconditions, invariants. > >> > >> If a section of code is entered when its precondition doesn't hold, > >> we're toast. This is the archetypical programming error. > >> > >> If it can actually happen, the program is incorrect, and needs fixing. > >> > >> Checking preconditions is often (but not always) practical. In my > >> opinion, checking is good practice, and the proper way to check is > >> assert(). Makes the incorrect program fail before it can do further > >> damage, and helps with finding the programming error. > >> > >> A preconditions is part of the contract between a function and its > >> users. An strong precondition can make the function's job easier, but > >> that's no use if the resulting function is inconvenient to use. On the > >> other hand, complicating the function to get a weaker precondition > >> nobody actually needs is just as dumb. > >> > >> Returning an error is *not* checking preconditions. Remember, if the > >> precondition doesn't hold, we're toast. If we're toast when we return > >> an error, we're clearly doing it wrong. > >> > >> You are arguing for weaker preconditions. I'm not actually disagreeing > >> with you! I'm merely expressing my opinion that checking preconditions > >> with assert() is a good idea. > > > > I have a fairly strong dislike for asserts in qemu, and although I'm not > > always consistent, my reasoning is mainly to do with asserts once a guest > > is running. > > > > Lets imagine you have a happily running guest and then you try and do > > something new and complex (e.g. hotplug a vfio-device); now lets say that > > new thing has something very broken about it, do you really want the previously > > running guest to die? > > If a precondition doesn't hold, we're toast. The best we can do is > crash before we mess up things further. > > A problematic condition we can safely recover from can be made an error > condition. > > I think the crux of our misunderstandings (I hesitate to call it an > argument) is confusing recoverable error conditions with violated > preconditions. We all agree (violently, perhaps) that assert() is not > an acceptable error handling mechanism. I think perhaps part of the problem maybe trying to place all types of screwups into only two categories; 'errors' and 'violations of preconditions'. Consider some cases: a) The user tries to specify an out of range value to a setting; an error, probably not fatal (except if it was commandline) b) An inconsistency is found in the MMU state violation of precondition, fatal. c) A host device used for passthrough does something which according to the USB/PCI/SCSI specs is illegal violation of precondition - but you probably don't want that to be fatal. d) An inconsistency is found in a specific device emulation violation of precondition - but I might not want that to be fatal. I think we agree on (a),(b), disagree on (d) and I think this case might be (c). > > My view is it can very much depend on how broken you think the > > world is; you've got to remember that crashing at this point > > is going to lose the user a VM, and that could mean losing > > data - so at that point you have to make a decision about whether > > your lack of confidence in the state of the VM due to the failed > > precondition is worse than your knowledge that the VM is going to fail. > > > > Perhaps giving the user an error and disabling the device lets > > the admin gravefully shutdown the VM and walk away with all > > their data intact. > > This is risky business unless you can prove the problematic condition is > safely isolated. To elaborate on your device example: say some logic > error in device emulation code put the device instance in some broken > state. If you detect that before the device could mess up anything > else, fencing the device is safe. But if device state is borked because > some other code overran an array, continuing risks making things worse. > Crashing the guest is bad. Letting it first overwrite good data with > bad data is worse. > > Sadly, such proof is hardly ever possible in unrestricted C. So we're > down to probabilities and tradeoffs. Agreed. > I'd reject a claim that once the guest is running the tradeoffs *always* > favour trying to hobble on. Agreed; this is the difference between my case (b) and (d). My preference is to fail the device in question if it's not a core device; that way if it's a disk you can't write any more to it to mess it's contents up further, and you won't read bad data from it - that's about as much isolation as you're going to get. However, some of it is also down to our expections of the stability of the code in question - if the inconsistency is in some code that you know is complex probably with untested cases and which isn't core to the VM continuing (e.g. outgoing migration or hotplugging a host device) then I believe it's OK to issue a scary warning, disable/error the device in question and hobble on. I'd say it's OK to argue that a piece of core code should be heavily isolated from the bits you think are still a bit touchy - so it's reasonable to me to have an assert in some core code (b) as long as it's possible to stop any of the (c) and (d) cases triggering it if they're coded defensively enough to error out before that assert could be hit. But then again someone might worry they just can't deal with all the types of screwup (c) might present. > If you want a less bleak isolation and recovery story, check out Erlang. > Note that its "let it crash" philosophy is very much in accordance with > my views on what can safely be done after detecting a programming error > / violated precondition. > > > So I wouldn't argue for weaker preconditions, just what the > > result is if the precondition fails. > > I respectfully disagree with your use of the concept "precondition". I generally avoid using the word precondition; it's too formal for my liking given the level we're programming at and the lack of any formal defs. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
On 09/30/2016 10:06 PM, Markus Armbruster wrote: > Alex Williamson <alex.williamson@redhat.com> writes: > > > Once there's a need to handle a certain condition as an error, we should > do that, no argument. This also provides a way to test the error path. > > However, I wouldn't buy an argument that preconditions should be made as > weak as possible in leaf functions (let alone always) regardless of the > cost in complexity, and non-testability of error paths. I'm strictly a > pay as you go person. > > Back to the problem at hand. Cao jin, would you be willing to fix > msi_init()? > > Sorry for the holiday delay. Sure, will fix it, and add it as a new patch in this series.
diff --git a/hw/pci/msix.c b/hw/pci/msix.c index 0ec1cb1..384a29d 100644 --- a/hw/pci/msix.c +++ b/hw/pci/msix.c @@ -253,9 +253,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, return -ENOTSUP; } - if (nentries < 1 || nentries > PCI_MSIX_FLAGS_QSIZE + 1) { - return -EINVAL; - } + assert(nentries >= 1 && nentries <= PCI_MSIX_FLAGS_QSIZE + 1); table_size = nentries * PCI_MSIX_ENTRY_SIZE; pba_size = QEMU_ALIGN_UP(nentries, 64) / 8; @@ -266,7 +264,7 @@ int msix_init(struct PCIDevice *dev, unsigned short nentries, table_offset + table_size > memory_region_size(table_bar) || pba_offset + pba_size > memory_region_size(pba_bar) || (table_offset | pba_offset) & PCI_MSIX_FLAGS_BIRMASK) { - return -EINVAL; + assert(0); } cap = pci_add_capability(dev, PCI_CAP_ID_MSIX, cap_pos, MSIX_CAP_LENGTH);
The input parameters is used for creating the msix capable device, so they must obey the PCI spec, or else, it should be programming error. CC: Markus Armbruster <armbru@redhat.com> CC: Marcel Apfelbaum <marcel@redhat.com> CC: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com> --- hw/pci/msix.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)