| Message ID | 20201208052113.1641514-1-penny.zheng@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] design: design doc for 1:1 direct-map | expand |
Hi Penny, I am adding Paul and Zheng in the thread as there are similar interest for the x86 side. On 08/12/2020 05:21, Penny Zheng wrote: > This is one draft design about the infrastructure for now, not ready > for upstream yet (hence the RFC tag), thought it'd be useful to firstly > start a discussion with the community. > > Create one design doc for 1:1 direct-map. > It aims to describe why and how we allocate 1:1 direct-map(guest physical > == physical) domains. > > This document is partly based on Stefano Stabellini's patch serie v1: > [direct-map DomUs]( > https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg00707.html). May I ask why a different approach? > > Signed-off-by: Penny Zheng <penny.zheng@arm.com> > --- > For the part regarding allocating 1:1 direct-map domains with user-defined > memory regions, it will be included in next design of static memory > allocation. I don't think you can do without user-defined memory regions (see more below). > --- > docs/designs/1_1_direct-map.md | 87 ++++++++++++++++++++++++++++++++++ > 1 file changed, 87 insertions(+) > create mode 100644 docs/designs/1_1_direct-map.md > > diff --git a/docs/designs/1_1_direct-map.md b/docs/designs/1_1_direct-map.md > new file mode 100644 > index 0000000000..ce3e2c77fd > --- /dev/null > +++ b/docs/designs/1_1_direct-map.md > @@ -0,0 +1,87 @@ > +# Preface > + > +The document is an early draft for direct-map memory map > +(`guest physical == physical`) of domUs. And right now, it constrains to ARM s/constrains/limited/ Aside the interface to the user, you should be able to re-use the same code on x86. Note that because the memory layout on x86 is fixed (always starting at 0), you would only be able to have only one direct-mapped domain. > +architecture. > + > +It aims to describe why and how the guest would be created as direct-map domain. > + > +This document is partly based on Stefano Stabellini's patch serie v1: > +[direct-map DomUs]( > +https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg00707.html). > + > +This is a first draft and some questions are still unanswered. When this is the > +case, the text shall contain XXX. > + > +# Introduction > + > +## Background > + > +Cases where domU needs direct-map memory map: > + > + * IOMMU not present in the system. > + * IOMMU disabled, since it doesn't cover a specific device. If the device is not covered by the IOMMU, then why would you want to disable the IOMMUs for the rest of the system? > + * IOMMU disabled, since it doesn't have enough bandwidth. I am not sure to understand this one. > + * IOMMU disabled, since it adds too much latency. The list above sounds like direct-map memory would be necessary even without device-passthrough. Can you clarify it? > + > +*WARNING: > +Users should be careful that it is not always secure to assign a device without s/careful/aware/ I think. Also, it is never secure to assign a device without IOMMU/SMMU unless you have a replacement. I would suggest to reword it something like: "When the device is not protected by the IOMMU, the administrator should make sure that: - The device is assigned to a trusted guest - You have an additional security mechanism on the platform (e.g MPU) to protect the memory." > +IOMMU/SMMU protection. > +Users must be aware of this risk, that guests having access to hardware with > +DMA capacity must be trusted, or it could use the DMA engine to access any > +other memory area. > +Guests could use additional security hardware component like NOC, System MPU > +to protect the memory. What's the NOC? > + > +## Design > + > +The implementation may cover following aspects: > + > +### Native Address and IRQ numbers for GIC and UART(vPL011) > + > +Today, fixed addresses and IRQ numbers are used to map GIC and UART(vPL011) > +in DomUs. And it may cause potential clash on direct-map domains. > +So, Using native addresses and irq numbers for GIC, UART(vPL011), in > +direct-map domains is necessary. > +e.g. To me e.g. means example. But below this is not an example, this is a requirement in order to use the vpl011 on system without pl011 UART. > +For the virtual interrupt of vPL011: instead of always using `GUEST_VPL011_SPI`, > +try to reuse the physical SPI number if possible. How would you find the following region for guest using PV drivers; - Event channel interrupt - Grant table area > + > +### Device tree option: `direct_map` > + > +Introduce a new device tree option `direct_map` for direct-map domains. > +Then, when users try to allocate one direct-map domain(except DOM0), > +`direct-map` property needs to be added under the appropriate `/chosen/domUx`. > + > + > + chosen { > + ... > + domU1 { > + compatible = "xen, domain"; > + #address-cells = <0x2>; > + #size-cells = <0x1>; > + direct-map; > + ... > + }; > + ... > + }; > + > +If users are using imagebuilder, they can add to boot.source something like the This documentations ounds like more something for imagebuilder rather than Xen itself. > +following: > + > + fdt set /chosen/domU1 direct-map > + > +Users could also use `xl` to create direct-map domains, just use the following > +config option: `direct-map=true` > + > +### direct-map guest memory allocation > + > +Func `allocate_memory_direct_map` is based on `allocate_memory_11`, and shall > +be refined to allocate memory for all direct-map domains, including DOM0. > +Roughly speaking, firstly, it tries to allocate arbitrary memory chunk of > +requested size from domain sub-allocator(`alloc_domheap_pages`). If fail, > +split the chunk into halves, and re-try, until it succeed or bail out with the > +smallest chunk size. If you have a mix of domain with direct-mapped and normal domain, you may end up to have the memory so small that your direct-mapped domain will have many small banks. This is going to be a major problem if you are creating the domain at runtime (you suggest xl can be used). In addition, some users may want to be able to control the location of the memory as this reduced the amount of work in the guest (e.g you don't have to dynamically discover the memory). I think it would be best to always require the admin to select the RAM bank used by a direct mapped domain. Alternatively, we could have a pool of memory that can only be used for direct mapped domain. This should limit the fragmentation of the memory. > +Then, `insert_11_bank` shall insert above allocated pages into a memory bank, > +which are ordered by address, and also set up guest P2M mapping( > +`guest_physmap_add_page`) to ensure `gfn == mfn`. Cheers,
On 08.12.2020 10:07, Julien Grall wrote: > On 08/12/2020 05:21, Penny Zheng wrote: >> --- /dev/null >> +++ b/docs/designs/1_1_direct-map.md >> @@ -0,0 +1,87 @@ >> +# Preface >> + >> +The document is an early draft for direct-map memory map >> +(`guest physical == physical`) of domUs. And right now, it constrains to ARM > > s/constrains/limited/ > > Aside the interface to the user, you should be able to re-use the same > code on x86. Note that because the memory layout on x86 is fixed (always > starting at 0), you would only be able to have only one direct-mapped > domain. Even one seems challenging, if it's truly meant to have all of the domain's memory direct-mapped: The use of space in the first Mb is different between host and guest. Jan
On 2020-12-08 10:12, Jan Beulich wrote: > On 08.12.2020 10:07, Julien Grall wrote: > > On 08/12/2020 05:21, Penny Zheng wrote: > >> --- /dev/null > >> +++ b/docs/designs/1_1_direct-map.md > >> @@ -0,0 +1,87 @@ > >> +# Preface > >> + > >> +The document is an early draft for direct-map memory map > >> +(`guest physical == physical`) of domUs. And right now, it constrains to ARM > > > > s/constrains/limited/ > > > > Aside the interface to the user, you should be able to re-use the same > > code on x86. Note that because the memory layout on x86 is fixed (always > > starting at 0), you would only be able to have only one direct-mapped > > domain. > > Even one seems challenging, if it's truly meant to have all of the > domain's memory direct-mapped: The use of space in the first Mb is > different between host and guest. Speaking about the case of x86, we can still direct-map the ram regions to the single direct-mapped DomU because neither Xen nor dom0 require those low mem. We don't worry about (i.e. don't direct-map) non-ram regions (or any range that is not reported as usable ram from DomU's PoV (dictated by e820 table), so those can be MMIO or arbitrary mapping with EPT. Fam
On 2020-12-08 13:21, Penny Zheng wrote: > +The document is an early draft for direct-map memory map > +(`guest physical == physical`) of domUs. And right now, it constrains to ARM > +architecture. I'm also working on direct-map DomU on x86, so let's coordinate and cover both arches. > + > +It aims to describe why and how the guest would be created as direct-map domain. > + > +This document is partly based on Stefano Stabellini's patch serie v1: > +[direct-map DomUs]( > +https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg00707.html). > + > +This is a first draft and some questions are still unanswered. When this is the > +case, the text shall contain XXX. > + > +# Introduction > + > +## Background > + > +Cases where domU needs direct-map memory map: > + > + * IOMMU not present in the system. > + * IOMMU disabled, since it doesn't cover a specific device. > + * IOMMU disabled, since it doesn't have enough bandwidth. > + * IOMMU disabled, since it adds too much latency. > + > +*WARNING: > +Users should be careful that it is not always secure to assign a device without > +IOMMU/SMMU protection. > +Users must be aware of this risk, that guests having access to hardware with > +DMA capacity must be trusted, or it could use the DMA engine to access any > +other memory area. > +Guests could use additional security hardware component like NOC, System MPU > +to protect the memory. > + > +## Design > + > +The implementation may cover following aspects: > + > +### Native Address and IRQ numbers for GIC and UART(vPL011) > + > +Today, fixed addresses and IRQ numbers are used to map GIC and UART(vPL011) > +in DomUs. And it may cause potential clash on direct-map domains. > +So, Using native addresses and irq numbers for GIC, UART(vPL011), in > +direct-map domains is necessary. > +e.g. > +For the virtual interrupt of vPL011: instead of always using `GUEST_VPL011_SPI`, > +try to reuse the physical SPI number if possible. > + > +### Device tree option: `direct_map` > + > +Introduce a new device tree option `direct_map` for direct-map domains. > +Then, when users try to allocate one direct-map domain(except DOM0), > +`direct-map` property needs to be added under the appropriate `/chosen/domUx`. > + > + > + chosen { > + ... > + domU1 { > + compatible = "xen, domain"; > + #address-cells = <0x2>; > + #size-cells = <0x1>; > + direct-map; > + ... > + }; > + ... > + }; > + > +If users are using imagebuilder, they can add to boot.source something like the > +following: > + > + fdt set /chosen/domU1 direct-map > + > +Users could also use `xl` to create direct-map domains, just use the following > +config option: `direct-map=true` > + > +### direct-map guest memory allocation > + > +Func `allocate_memory_direct_map` is based on `allocate_memory_11`, and shall > +be refined to allocate memory for all direct-map domains, including DOM0. > +Roughly speaking, firstly, it tries to allocate arbitrary memory chunk of > +requested size from domain sub-allocator(`alloc_domheap_pages`). If fail, > +split the chunk into halves, and re-try, until it succeed or bail out with the > +smallest chunk size. > +Then, `insert_11_bank` shall insert above allocated pages into a memory bank, > +which are ordered by address, and also set up guest P2M mapping( > +`guest_physmap_add_page`) to ensure `gfn == mfn`. A high level comment from x86 PoV: in the mfn addr space, we want to explicitly reserve range for direct-map. This ensures Xen or Dom0 will leave the pages for DomU at boot time, since as Julien mentioned, x86 machines have fixed mem layout starting from 0, so the corresponding pages mustn't go into xenheap/domheap in the first place. IOW x86 depends on some mechanism very similar to what badpage= does. But I wouldn't overload/abuse the parameter for direct-map. Maybe introduce a new option, like "identpage=". Fam
On 08.12.2020 11:22, Fam Zheng wrote: > On 2020-12-08 10:12, Jan Beulich wrote: >> On 08.12.2020 10:07, Julien Grall wrote: >>> On 08/12/2020 05:21, Penny Zheng wrote: >>>> --- /dev/null >>>> +++ b/docs/designs/1_1_direct-map.md >>>> @@ -0,0 +1,87 @@ >>>> +# Preface >>>> + >>>> +The document is an early draft for direct-map memory map >>>> +(`guest physical == physical`) of domUs. And right now, it constrains to ARM >>> >>> s/constrains/limited/ >>> >>> Aside the interface to the user, you should be able to re-use the same >>> code on x86. Note that because the memory layout on x86 is fixed (always >>> starting at 0), you would only be able to have only one direct-mapped >>> domain. >> >> Even one seems challenging, if it's truly meant to have all of the >> domain's memory direct-mapped: The use of space in the first Mb is >> different between host and guest. > > Speaking about the case of x86, we can still direct-map the ram regions > to the single direct-mapped DomU because neither Xen nor dom0 require > those low mem. > > We don't worry about (i.e. don't direct-map) non-ram regions (or any > range that is not reported as usable ram from DomU's PoV (dictated by > e820 table), so those can be MMIO or arbitrary mapping with EPT. For one, the very first page is considered special in x86 Xen. No guest should gain access to MFN 0, unless you first audit all code and address all the issues you find. And then there's also Xen's low-memory trampoline living there. Plus besides the BDA (at real-mode address 0040:0000) I suppose the EBDA also shouldn't be exposed to a guest, nor anything else that the host finds reserved in E820. IOW it would be the host E820 to dictate some of the guest E820 in such a case. Jan
On Tue, 2020-12-08 at 11:53 +0100, Jan Beulich wrote: > On 08.12.2020 11:22, Fam Zheng wrote: > > On 2020-12-08 10:12, Jan Beulich wrote: > > > On 08.12.2020 10:07, Julien Grall wrote: > > > > On 08/12/2020 05:21, Penny Zheng wrote: > > > > > --- /dev/null > > > > > +++ b/docs/designs/1_1_direct-map.md > > > > > @@ -0,0 +1,87 @@ > > > > > +# Preface > > > > > + > > > > > +The document is an early draft for direct-map memory map > > > > > +(`guest physical == physical`) of domUs. And right now, it > > > > > constrains to ARM > > > > > > > > s/constrains/limited/ > > > > > > > > Aside the interface to the user, you should be able to re-use > > > > the same > > > > code on x86. Note that because the memory layout on x86 is > > > > fixed (always > > > > starting at 0), you would only be able to have only one direct- > > > > mapped > > > > domain. > > > > > > Even one seems challenging, if it's truly meant to have all of > > > the > > > domain's memory direct-mapped: The use of space in the first Mb > > > is > > > different between host and guest. > > > > Speaking about the case of x86, we can still direct-map the ram > > regions > > to the single direct-mapped DomU because neither Xen nor dom0 > > require > > those low mem. > > > > We don't worry about (i.e. don't direct-map) non-ram regions (or > > any > > range that is not reported as usable ram from DomU's PoV (dictated > > by > > e820 table), so those can be MMIO or arbitrary mapping with EPT. > > For one, the very first page is considered special in x86 Xen. No > guest should gain access to MFN 0, unless you first audit all > code and address all the issues you find. And then there's also > Xen's low-memory trampoline living there. Plus besides the BDA > (at real-mode address 0040:0000) I suppose the EBDA also shouldn't > be exposed to a guest, nor anything else that the host finds > reserved in E820. IOW it would be the host E820 to dictate some > of the guest E820 in such a case. > You're right about the trampoline area, it has to be specially taken care of. Not a problem if we could disable cpu hotplug. I don't think the guest will ever try to DMA from/to MFN 0, BDA or EBDA, so even not direct mapping those should not make any functional difference. In general, I agree the guest E820 as well as all direct mapping areas mustn't break out of host E820 limitation, otherwise it will not work. Fam
Hi Julien Thanks for the nice and detailed comments. (*^▽^*) Here are the replies: > -----Original Message----- > From: Julien Grall <julien@xen.org> > Sent: Tuesday, December 8, 2020 5:07 PM > To: Penny Zheng <Penny.Zheng@arm.com>; xen-devel@lists.xenproject.org; > sstabellini@kernel.org > Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>; Kaly Xin > <Kaly.Xin@arm.com>; Wei Chen <Wei.Chen@arm.com>; nd <nd@arm.com>; > Paul Durrant <paul@xen.org>; famzheng@amazon.com > Subject: Re: [RFC] design: design doc for 1:1 direct-map > > Hi Penny, > > I am adding Paul and Zheng in the thread as there are similar interest for the > x86 side. > > On 08/12/2020 05:21, Penny Zheng wrote: > > This is one draft design about the infrastructure for now, not ready > > for upstream yet (hence the RFC tag), thought it'd be useful to > > firstly start a discussion with the community. > > > > Create one design doc for 1:1 direct-map. > > It aims to describe why and how we allocate 1:1 direct-map(guest > > physical == physical) domains. > > > > This document is partly based on Stefano Stabellini's patch serie v1: > > [direct-map DomUs]( > > https://lists.xenproject.org/archives/html/xen-devel/2020- > 04/msg00707.html). > > May I ask why a different approach? In Stefano original design, he'd like to allocate 1:1 direct-map with user-defined memory regions and he prefers allocating it from sub-domain allocator. And it brings quite a discussion there and in the last, everyone kinds of all agrees that it is not workable. Since if requested memory ever goes into any allocators, no matter boot, or sub-domain allocator, we could not ensure that before actually allocating it for one 1:1 direct-map domain, it will not be into any other use. So I'd prefer to split original design into two parts: one is here, that user only wants to allocate one 1:1 direct-map domain, not caring about where the ram will be located into. Think about dom0. Then, we could stick to allocate memory still from sub-domain allocator. Another part which I said in below commits, "For the part regarding allocating 1:1 direct- map domains with user-defined memory regions, it will be included in next design of static memory allocation". But of course, If a combination can make community to better understand our ideas, We're willing to combine them in next version.
On 10/12/2020 07:02, Penny Zheng wrote: > Hi Julien Hi Penny, Apologies for the late answer. > > Thanks for the nice and detailed comments. (*^▽^*) > Here are the replies: > >> -----Original Message----- >> From: Julien Grall <julien@xen.org> >> Sent: Tuesday, December 8, 2020 5:07 PM >> To: Penny Zheng <Penny.Zheng@arm.com>; xen-devel@lists.xenproject.org; >> sstabellini@kernel.org >> Cc: Bertrand Marquis <Bertrand.Marquis@arm.com>; Kaly Xin >> <Kaly.Xin@arm.com>; Wei Chen <Wei.Chen@arm.com>; nd <nd@arm.com>; >> Paul Durrant <paul@xen.org>; famzheng@amazon.com >> Subject: Re: [RFC] design: design doc for 1:1 direct-map >> >> Hi Penny, >> >> I am adding Paul and Zheng in the thread as there are similar interest for the >> x86 side. >> >> On 08/12/2020 05:21, Penny Zheng wrote: >>> This is one draft design about the infrastructure for now, not ready >>> for upstream yet (hence the RFC tag), thought it'd be useful to >>> firstly start a discussion with the community. >>> >>> Create one design doc for 1:1 direct-map. >>> It aims to describe why and how we allocate 1:1 direct-map(guest >>> physical == physical) domains. >>> >>> This document is partly based on Stefano Stabellini's patch serie v1: >>> [direct-map DomUs]( >>> https://lists.xenproject.org/archives/html/xen-devel/2020- >> 04/msg00707.html). >> >> May I ask why a different approach? > > In Stefano original design, he'd like to allocate 1:1 direct-map with user-defined > memory regions and he prefers allocating it from sub-domain allocator. I am not entirely sure what you are referring to with "sub-domain allocator". > > And it brings quite a discussion there and in the last, everyone kinds of all > agrees that it is not workable. Since if requested memory ever goes into any > allocators, no matter boot, or sub-domain allocator, we could not ensure that > before actually allocating it for one 1:1 direct-map domain, it will not be into > any other use. Yes, you cannot give the memory to the heap allocator and expect the region to always be free. However, you can mark them as reserve so the allocator doesn't touch it. We (AWS) also needs to reserve memory for later use in the case of LiveUpdate. In our case, the memory already contain guest data, so it is not possible to give them to any allocator. We solved it by excluding the page from any allocator and then marking then page as allocated/used when giving to the domain. There are some corner cases unsolved when using NUMA. Aside that this work because the heap allocator don't keep a list of in-use pages. > > So I'd prefer to split original design into two parts: one is here, that user only > wants to allocate one 1:1 direct-map domain, not caring about where the ram > will be located into. While I understand that a user may not care where the direct-map memory is allocated. However, I question the usefulness because: 1) This doesn't work with MPU 2) You may end up with provide the guest with many small regions if the guest is not created right after boot or rebooting. Can you outline what would be your use case here? > >>> +architecture. >>> + >>> +It aims to describe why and how the guest would be created as direct-map >> domain. >>> + >>> +This document is partly based on Stefano Stabellini's patch serie v1: >>> +[direct-map DomUs]( >>> +https://lists.xenproject.org/archives/html/xen-devel/2020- >> 04/msg00707.html). >>> + >>> +This is a first draft and some questions are still unanswered. When >>> +this is the case, the text shall contain XXX. >>> + >>> +# Introduction >>> + >>> +## Background >>> + >>> +Cases where domU needs direct-map memory map: >>> + >>> + * IOMMU not present in the system. >>> + * IOMMU disabled, since it doesn't cover a specific device. >> >> If the device is not covered by the IOMMU, then why would you want to >> disable the IOMMUs for the rest of the system? >> > > This is a mixed scenario. We pass some devices to VM with SMMU, and we > pass other devices to VM without SMMU. We could not guarantee guest > DMA security. Not really, you can guarantee DMA security if devices not protected by an IOMMU are assigned to *trusted* domains. > > So users may want to disable the SMMU, at least, they can gain some > performance improvement from SMMU disabled. That's an understandable argument. Yet, I think this only works if you trust *all* your domains. So a user may still want to keep IOMMU on when assigning devices (as long as they are protected by an IOMMU) to a non-trusted domain. So I would suggest to rephrase your second bullet point with: "IOMMU disabled if all the guests are trusted" >>> + * IOMMU disabled, since it doesn't have enough bandwidth. >> >> I am not sure to understand this one. >> > > In some SoC, there would be multiple devices connected to one SMMU. > > In some extreme situation, multiple devices do DMA concurrency, The > translation requests can exceed SMMU's translation capacity. This will > cause DMA latency. Ok. So either the SoC doesn't fit your use-case or the SoC was not correctly designed. Therefore, I would call that a workaround :). I would suggest to update the design doc with more information. OOI, is it really necessary to turn off the IOMMU? Would it be possible to instead have a few devices by-passing the IOMMU when they are assigned to a trusted domain? > >>> + * IOMMU disabled, since it adds too much latency. >> >> The list above sounds like direct-map memory would be necessary even >> without device-passthrough. Can you clarify it? >> > > Okay. > > SMMU on different SoCs can be implemented differently. For example, some > SoC vendor may remove the TLB inside SMMU. > > In this case, the SMMU will add latency in DMA progress. Users may want to > disable the SMMU for some Realtime scenarios. Thanks for the explanation, however this wasn't my question. I was pointed out that your example gave the impression that domaion with not devices assigned would also need to be direct-mapped. Could you confirm whether this is the intended purpose? > >>> + >>> +*WARNING: >>> +Users should be careful that it is not always secure to assign a >>> +device without >> >> s/careful/aware/ I think. Also, it is never secure to assign a device without >> IOMMU/SMMU unless you have a replacement. >> >> I would suggest to reword it something like: >> >> "When the device is not protected by the IOMMU, the administrator should >> make sure that: >> - The device is assigned to a trusted guest >> - You have an additional security mechanism on the platform (e.g >> MPU) to protect the memory." >> > > Thanks for the rephrase. (*^▽^*) > >>> +IOMMU/SMMU protection. >>> +Users must be aware of this risk, that guests having access to >>> +hardware with DMA capacity must be trusted, or it could use the DMA >>> +engine to access any other memory area. >>> +Guests could use additional security hardware component like NOC, >>> +System MPU to protect the memory. >> >> What's the NOC? >> > > Network on Chip. > > Some kind of SoC level firewall that limits the devices' DMA access range > or CPU memory access range. I would suggest to use the longer term or introduce an accronym section. > >>> + >>> +## Design >>> + >>> +The implementation may cover following aspects: >>> + >>> +### Native Address and IRQ numbers for GIC and UART(vPL011) >>> + >>> +Today, fixed addresses and IRQ numbers are used to map GIC and >>> +UART(vPL011) in DomUs. And it may cause potential clash on direct-map >> domains. >>> +So, Using native addresses and irq numbers for GIC, UART(vPL011), in >>> +direct-map domains is necessary. >>> +e.g. >> >> To me e.g. means example. But below this is not an example, this is a >> requirement in order to use the vpl011 on system without pl011 UART. >> > > Yes, right. > I'll delete e.g. here > >>> +For the virtual interrupt of vPL011: instead of always using >>> +`GUEST_VPL011_SPI`, try to reuse the physical SPI number if possible. >> >> How would you find the following region for guest using PV drivers; >> - Event channel interrupt >> - Grant table area >> > Good catch! thousand thx.
diff --git a/docs/designs/1_1_direct-map.md b/docs/designs/1_1_direct-map.md new file mode 100644 index 0000000000..ce3e2c77fd --- /dev/null +++ b/docs/designs/1_1_direct-map.md @@ -0,0 +1,87 @@ +# Preface + +The document is an early draft for direct-map memory map +(`guest physical == physical`) of domUs. And right now, it constrains to ARM +architecture. + +It aims to describe why and how the guest would be created as direct-map domain. + +This document is partly based on Stefano Stabellini's patch serie v1: +[direct-map DomUs]( +https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg00707.html). + +This is a first draft and some questions are still unanswered. When this is the +case, the text shall contain XXX. + +# Introduction + +## Background + +Cases where domU needs direct-map memory map: + + * IOMMU not present in the system. + * IOMMU disabled, since it doesn't cover a specific device. + * IOMMU disabled, since it doesn't have enough bandwidth. + * IOMMU disabled, since it adds too much latency. + +*WARNING: +Users should be careful that it is not always secure to assign a device without +IOMMU/SMMU protection. +Users must be aware of this risk, that guests having access to hardware with +DMA capacity must be trusted, or it could use the DMA engine to access any +other memory area. +Guests could use additional security hardware component like NOC, System MPU +to protect the memory. + +## Design + +The implementation may cover following aspects: + +### Native Address and IRQ numbers for GIC and UART(vPL011) + +Today, fixed addresses and IRQ numbers are used to map GIC and UART(vPL011) +in DomUs. And it may cause potential clash on direct-map domains. +So, Using native addresses and irq numbers for GIC, UART(vPL011), in +direct-map domains is necessary. +e.g. +For the virtual interrupt of vPL011: instead of always using `GUEST_VPL011_SPI`, +try to reuse the physical SPI number if possible. + +### Device tree option: `direct_map` + +Introduce a new device tree option `direct_map` for direct-map domains. +Then, when users try to allocate one direct-map domain(except DOM0), +`direct-map` property needs to be added under the appropriate `/chosen/domUx`. + + + chosen { + ... + domU1 { + compatible = "xen, domain"; + #address-cells = <0x2>; + #size-cells = <0x1>; + direct-map; + ... + }; + ... + }; + +If users are using imagebuilder, they can add to boot.source something like the +following: + + fdt set /chosen/domU1 direct-map + +Users could also use `xl` to create direct-map domains, just use the following +config option: `direct-map=true` + +### direct-map guest memory allocation + +Func `allocate_memory_direct_map` is based on `allocate_memory_11`, and shall +be refined to allocate memory for all direct-map domains, including DOM0. +Roughly speaking, firstly, it tries to allocate arbitrary memory chunk of +requested size from domain sub-allocator(`alloc_domheap_pages`). If fail, +split the chunk into halves, and re-try, until it succeed or bail out with the +smallest chunk size. +Then, `insert_11_bank` shall insert above allocated pages into a memory bank, +which are ordered by address, and also set up guest P2M mapping( +`guest_physmap_add_page`) to ensure `gfn == mfn`.
This is one draft design about the infrastructure for now, not ready for upstream yet (hence the RFC tag), thought it'd be useful to firstly start a discussion with the community. Create one design doc for 1:1 direct-map. It aims to describe why and how we allocate 1:1 direct-map(guest physical == physical) domains. This document is partly based on Stefano Stabellini's patch serie v1: [direct-map DomUs]( https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg00707.html). Signed-off-by: Penny Zheng <penny.zheng@arm.com> --- For the part regarding allocating 1:1 direct-map domains with user-defined memory regions, it will be included in next design of static memory allocation. --- docs/designs/1_1_direct-map.md | 87 ++++++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+) create mode 100644 docs/designs/1_1_direct-map.md