| Message ID | 20220110105044.94423-2-nsaenzju@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v4,1/2] Documentation: Fill the gaps about entry/noinstr constraints | expand |
On Mon, Jan 10, 2022 at 11:50:44AM +0100, Nicolas Saenz Julienne wrote: > The topic of nesting and reentrancy in the context of early entry code > hasn't been addressed so far. So do it. > > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> > --- > > Changes since v3: > - Introduce Paul's rewording suggestions > > Documentation/core-api/entry.rst | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst > index c6f8e22c88fe..e12f22ab33c7 100644 > --- a/Documentation/core-api/entry.rst > +++ b/Documentation/core-api/entry.rst > @@ -105,6 +105,8 @@ has to do extra work between the various steps. In such cases it has to > ensure that enter_from_user_mode() is called first on entry and > exit_to_user_mode() is called last on exit. > > +Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking > +to print a warning. > > KVM > --- > @@ -121,6 +123,8 @@ Task work handling is done separately for guest at the boundary of the > vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of > the work handled on return to user space. > > +Do not nest KVM entry/exit transitions because doing so is nonsensical. > + > Interrupts and regular exceptions > --------------------------------- > > @@ -180,6 +184,16 @@ before it handles soft interrupts, whose handlers must run in BH context rather > than irq-disabled context. In addition, irqentry_exit() might schedule, which > also requires that HARDIRQ_OFFSET has been removed from the preemption count. > > +Even though interrupt handlers are expected to run with local interrupts > +disabled, interrupt nesting is common from an entry/exit perspective. For > +example, softirq handling happens within an irqentry_{enter,exit}() block with > +local interrupts enabled. Also, although uncommon, nothing prevents an > +interrupt handler from re-enabling interrupts. > + > +Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it > +runs with local interrupts disabled. But NMIs can happen anytime, and a lot of > +the entry code is shared between the two. > + > NMI and NMI-like exceptions > --------------------------- > > @@ -259,3 +273,7 @@ and for e.g. a debug exception it can look like this: > > There is no combined irqentry_nmi_if_kernel() function available as the > above cannot be handled in an exception-agnostic way. > + > +NMIs can happen in any context. For example, an NMI-like exception triggered > +while handling an NMI. So NMI entry code has to be reentrant and state updates > +need to handle nesting. > -- > 2.34.1 >
On Mon, Jan 10, 2022 at 11:50:44AM +0100, Nicolas Saenz Julienne wrote: > The topic of nesting and reentrancy in the context of early entry code > hasn't been addressed so far. So do it. > > Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
diff --git a/Documentation/core-api/entry.rst b/Documentation/core-api/entry.rst index c6f8e22c88fe..e12f22ab33c7 100644 --- a/Documentation/core-api/entry.rst +++ b/Documentation/core-api/entry.rst @@ -105,6 +105,8 @@ has to do extra work between the various steps. In such cases it has to ensure that enter_from_user_mode() is called first on entry and exit_to_user_mode() is called last on exit. +Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking +to print a warning. KVM --- @@ -121,6 +123,8 @@ Task work handling is done separately for guest at the boundary of the vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of the work handled on return to user space. +Do not nest KVM entry/exit transitions because doing so is nonsensical. + Interrupts and regular exceptions --------------------------------- @@ -180,6 +184,16 @@ before it handles soft interrupts, whose handlers must run in BH context rather than irq-disabled context. In addition, irqentry_exit() might schedule, which also requires that HARDIRQ_OFFSET has been removed from the preemption count. +Even though interrupt handlers are expected to run with local interrupts +disabled, interrupt nesting is common from an entry/exit perspective. For +example, softirq handling happens within an irqentry_{enter,exit}() block with +local interrupts enabled. Also, although uncommon, nothing prevents an +interrupt handler from re-enabling interrupts. + +Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it +runs with local interrupts disabled. But NMIs can happen anytime, and a lot of +the entry code is shared between the two. + NMI and NMI-like exceptions --------------------------- @@ -259,3 +273,7 @@ and for e.g. a debug exception it can look like this: There is no combined irqentry_nmi_if_kernel() function available as the above cannot be handled in an exception-agnostic way. + +NMIs can happen in any context. For example, an NMI-like exception triggered +while handling an NMI. So NMI entry code has to be reentrant and state updates +need to handle nesting.
The topic of nesting and reentrancy in the context of early entry code hasn't been addressed so far. So do it. Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> --- Changes since v3: - Introduce Paul's rewording suggestions Documentation/core-api/entry.rst | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)