Re: [PATCH 0/7] Enable shared device assignment

David Hildenbrand
2025-01-10T08:26:09Z

On 10.01.25 08:06, Chenyi Qiang wrote:
>
>
> On 1/10/2025 9:42 AM, Alexey Kardashevskiy wrote:
>>
>>
>> On 9/1/25 19:49, Chenyi Qiang wrote:
>>>
>>>
>>> On 1/9/2025 4:18 PM, Alexey Kardashevskiy wrote:
>>>>
>>>>
>>>> On 9/1/25 18:52, Chenyi Qiang wrote:
>>>>>
>>>>>
>>>>> On 1/8/2025 7:38 PM, Alexey Kardashevskiy wrote:
>>>>>>
>>>>>>
>>>>>> On 8/1/25 17:28, Chenyi Qiang wrote:
>>>>>>> Thanks Alexey for your review!
>>>>>>>
>>>>>>> On 1/8/2025 12:47 PM, Alexey Kardashevskiy wrote:
>>>>>>>> On 13/12/24 18:08, Chenyi Qiang wrote:
>>>>>>>>> Commit 852f0048f3 ("RAMBlock: make guest_memfd require
>>>>>>>>> uncoordinated
>>>>>>>>> discard") effectively disables device assignment when using
>>>>>>>>> guest_memfd.
>>>>>>>>> This poses a significant challenge as guest_memfd is essential for
>>>>>>>>> confidential guests, thereby blocking device assignment to these
>>>>>>>>> VMs.
>>>>>>>>> The initial rationale for disabling device assignment was due to
>>>>>>>>> stale
>>>>>>>>> IOMMU mappings (see Problem section) and the assumption that TEE
>>>>>>>>> I/O
>>>>>>>>> (SEV-TIO, TDX Connect, COVE-IO, etc.) would solve the device-
>>>>>>>>> assignment
>>>>>>>>> problem for confidential guests [1]. However, this assumption has
>>>>>>>>> proven
>>>>>>>>> to be incorrect. TEE I/O relies on the ability to operate devices
>>>>>>>>> against
>>>>>>>>> "shared" or untrusted memory, which is crucial for device
>>>>>>>>> initialization
>>>>>>>>> and error recovery scenarios. As a result, the current
>>>>>>>>> implementation
>>>>>>>>> does
>>>>>>>>> not adequately support device assignment for confidential guests,
>>>>>>>>> necessitating
>>>>>>>>> a reevaluation of the approach to ensure compatibility and
>>>>>>>>> functionality.
>>>>>>>>>
>>>>>>>>> This series enables shared device assignment by notifying VFIO of
>>>>>>>>> page
>>>>>>>>> conversions using an existing framework named RamDiscardListener.
>>>>>>>>> Additionally, there is an ongoing patch set [2] that aims to add 1G
>>>>>>>>> page
>>>>>>>>> support for guest_memfd. This patch set introduces in-place page
>>>>>>>>> conversion,
>>>>>>>>> where private and shared memory share the same physical pages as
>>>>>>>>> the
>>>>>>>>> backend.
>>>>>>>>> This development may impact our solution.
>>>>>>>>>
>>>>>>>>> We presented our solution in the guest_memfd meeting to discuss its
>>>>>>>>> compatibility with the new changes and potential future directions
>>>>>>>>> (see [3]
>>>>>>>>> for more details). The conclusion was that, although our
>>>>>>>>> solution may
>>>>>>>>> not be
>>>>>>>>> the most elegant (see the Limitation section), it is sufficient for
>>>>>>>>> now and
>>>>>>>>> can be easily adapted to future changes.
>>>>>>>>>
>>>>>>>>> We are re-posting the patch series with some cleanup and have
>>>>>>>>> removed
>>>>>>>>> the RFC
>>>>>>>>> label for the main enabling patches (1-6). The newly-added patch
>>>>>>>>> 7 is
>>>>>>>>> still
>>>>>>>>> marked as RFC as it tries to resolve some extension concerns
>>>>>>>>> related to
>>>>>>>>> RamDiscardManager for future usage.
>>>>>>>>>
>>>>>>>>> The overview of the patches:
>>>>>>>>> - Patch 1: Export a helper to get intersection of a
>>>>>>>>> MemoryRegionSection
>>>>>>>>>       with a given range.
>>>>>>>>> - Patch 2-6: Introduce a new object to manage the guest-memfd with
>>>>>>>>>       RamDiscardManager, and notify the shared/private state change
>>>>>>>>> during
>>>>>>>>>       conversion.
>>>>>>>>> - Patch 7: Try to resolve a semantics concern related to
>>>>>>>>> RamDiscardManager
>>>>>>>>>       i.e. RamDiscardManager is used to manage memory plug/unplug
>>>>>>>>> state
>>>>>>>>>       instead of shared/private state. It would affect future
>>>>>>>>> users of
>>>>>>>>>       RamDiscardManger in confidential VMs. Attach it behind as
>>>>>>>>> a RFC
>>>>>>>>> patch[4].
>>>>>>>>>
>>>>>>>>> Changes since last version:
>>>>>>>>> - Add a patch to export some generic helper functions from
>>>>>>>>> virtio-mem
>>>>>>>>> code.
>>>>>>>>> - Change the bitmap in guest_memfd_manager from default shared to
>>>>>>>>> default
>>>>>>>>>       private. This keeps alignment with virtio-mem that 1-
>>>>>>>>> setting in
>>>>>>>>> bitmap
>>>>>>>>>       represents the populated state and may help to export more
>>>>>>>>> generic
>>>>>>>>> code
>>>>>>>>>       if necessary.
>>>>>>>>> - Add the helpers to initialize/uninitialize the
>>>>>>>>> guest_memfd_manager
>>>>>>>>> instance
>>>>>>>>>       to make it more clear.
>>>>>>>>> - Add a patch to distinguish between the shared/private state
>>>>>>>>> change
>>>>>>>>> and
>>>>>>>>>       the memory plug/unplug state change in RamDiscardManager.
>>>>>>>>> - RFC: https://lore.kernel.org/qemu-devel/20240725072118.358923-1-
>>>>>>>>> chenyi.qiang@intel.com/
>>>>>>>>>
>>>>>>>>> ---
>>>>>>>>>
>>>>>>>>> Background
>>>>>>>>> ==========
>>>>>>>>> Confidential VMs have two classes of memory: shared and private
>>>>>>>>> memory.
>>>>>>>>> Shared memory is accessible from the host/VMM while private
>>>>>>>>> memory is
>>>>>>>>> not. Confidential VMs can decide which memory is shared/private and
>>>>>>>>> convert memory between shared/private at runtime.
>>>>>>>>>
>>>>>>>>> "guest_memfd" is a new kind of fd whose primary goal is to serve
>>>>>>>>> guest
>>>>>>>>> private memory. The key differences between guest_memfd and normal
>>>>>>>>> memfd
>>>>>>>>> are that guest_memfd is spawned by a KVM ioctl, bound to its owner
>>>>>>>>> VM and
>>>>>>>>> cannot be mapped, read or written by userspace.
>>>>>>>>
>>>>>>>> The "cannot be mapped" seems to be not true soon anymore (if not
>>>>>>>> already).
>>>>>>>>
>>>>>>>> https://lore.kernel.org/all/20240801090117.3841080-1-
>>>>>>>> tabba@google.com/T/
>>>>>>>
>>>>>>> Exactly, allowing guest_memfd to do mmap is the direction. I
>>>>>>> mentioned
>>>>>>> it below with in-place page conversion. Maybe I would move it here to
>>>>>>> make it more clear.
>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>> In QEMU's implementation, shared memory is allocated with normal
>>>>>>>>> methods
>>>>>>>>> (e.g. mmap or fallocate) while private memory is allocated from
>>>>>>>>> guest_memfd. When a VM performs memory conversions, QEMU frees
>>>>>>>>> pages
>>>>>>>>> via
>>>>>>>>> madvise() or via PUNCH_HOLE on memfd or guest_memfd from one
>>>>>>>>> side and
>>>>>>>>> allocates new pages from the other side.
>>>>>>>>>
>>>>>>>
>>>>>>> [...]
>>>>>>>
>>>>>>>>>
>>>>>>>>> One limitation (also discussed in the guest_memfd meeting) is that
>>>>>>>>> VFIO
>>>>>>>>> expects the DMA mapping for a specific IOVA to be mapped and
>>>>>>>>> unmapped
>>>>>>>>> with
>>>>>>>>> the same granularity. The guest may perform partial conversions,
>>>>>>>>> such as
>>>>>>>>> converting a small region within a larger region. To prevent such
>>>>>>>>> invalid
>>>>>>>>> cases, all operations are performed with 4K granularity. The
>>>>>>>>> possible
>>>>>>>>> solutions we can think of are either to enable VFIO to support
>>>>>>>>> partial
>>>>>>>>> unmap
>>>>>>
>>>>>> btw the old VFIO does not split mappings but iommufd seems to be
>>>>>> capable
>>>>>> of it - there is iopt_area_split(). What happens if you try
>>>>>> unmapping a
>>>>>> smaller chunk that does not exactly match any mapped chunk? thanks,
>>>>>
>>>>> iopt_cut_iova() happens in iommufd vfio_compat.c, which is to make
>>>>> iommufd be compatible with old VFIO_TYPE1. IIUC, it happens with
>>>>> disable_large_page=true. That means the large IOPTE is also disabled in
>>>>> IOMMU. So it can do the split easily. See the comment in
>>>>> iommufd_vfio_set_iommu().
>>>>>
>>>>> iommufd VFIO compatible mode is a transition from legacy VFIO to
>>>>> iommufd. For the normal iommufd, it requires the iova/length must be a
>>>>> superset of a previously mapped range. If not match, will return error.
>>>>
>>>>
>>>> This is all true but this also means that "The former requires complex
>>>> changes in VFIO" is not entirely true - some code is already there.
>>>> Thanks,
>>>
>>> Hmm, my statement is a little confusing.  The bottleneck is that the
>>> IOMMU driver doesn't support the large page split. So if we want to
>>> enable large page and want to do partial unmap, it requires complex
>>> change.
>>
>> We won't need to split large pages (if we stick to 4K for now), we need
>> to split large mappings (not large pages) to allow partial unmapping and
>> iopt_area_split() seems to be doing this. Thanks,
>
> You mean we can disable large page in iommufd and then VFIO will be able
> to do partial unmap. Yes, I think it is doable and we can avoid many
> ioctl context switches overhead.

So I understand this correctly: the disable_large_pages=true will imply
that we never have PMD mappings such that we can atomically poke a hole
in a mapping, without temporarily having to remove a PMD mapping in the
iommu table to insert a PTE table?

batch_iommu_map_small() seems to document that behavior.

It's interesting that that comment points out that this is purely "VFIO
compatibility", and that it otherwise violates the iommufd invariant:
"pairing map/unmap". So, it is against the real iommufd design ...

Back when working on virtio-mem support (RAMDiscardManager), thought
there was not way to reliably do atomic partial unmappings.

--
Cheers,

David / dhildenb