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

Alexey Kardashevskiy
2025-01-09T08:18:46Z

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,

--
Alexey