Re: [PATCH v4 0/9] vhost-user: Add SHMEM_MAP/UNMAP requests

[David Hildenbrand <david@redhat.com>]

On 24.02.25 09:54, Albert Esteve wrote:
> On Mon, Feb 17, 2025 at 9:01 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 17.02.25 17:40, Albert Esteve wrote:
>>> Hi all,
>>>
>>
>> Hi,
>>
>> looks like our debugging session was successfu :)
>>
>> One question below.
>>
>>> v3->v4
>>> - Change mmap strategy to use RAM blocks
>>>     and subregions.
>>> - Add new bitfield to qmp feature map
>>> - Followed most review comments from
>>>     last iteration.
>>> - Merged documentation patch again with
>>>     this one. Makes more sense to
>>>     review them together after all.
>>> - Add documentation for MEM_READ/WRITE
>>>     messages.
>>>
>>> The goal of this patch is to support
>>> dynamic fd-backed memory maps initiated
>>> from vhost-user backends.
>>> There are many devices that could already
>>> benefit of this feature, e.g.,
>>> virtiofs or virtio-gpu.
>>>
>>> After receiving the SHMEM_MAP/UNMAP request,
>>> the frontend creates the RAMBlock form the
>>> fd and maps it by adding it as a subregion
>>> of the shared memory region container.
>>>
>>> The VIRTIO Shared Memory Region list is
>>> declared in the `VirtIODevice` struct
>>> to make it generic.
>>>
>>> TODO: There was a conversation on the
>>> previous version around adding tests
>>> to the patch (which I have acknowledged).
>>> However, given the numerous changes
>>> that the patch already has, I have
>>> decided to send it early and collect
>>> some feedback while I work on the
>>> tests for the next iteration.
>>> Given that I have been able to
>>> test the implementation with
>>> my local setup, I am more or less
>>> confident that, at least, the code
>>> is in a relatively sane state
>>> so that no reviewing time is
>>> wasted on broken patches.
>>>
>>> This patch also includes:
>>> - SHMEM_CONFIG frontend request that is
>>> specifically meant to allow generic
>>> vhost-user-device frontend to be able to
>>> query VIRTIO Shared Memory settings from the
>>> backend (as this device is generic and agnostic
>>> of the actual backend configuration).
>>>
>>> - MEM_READ/WRITE backend requests are
>>> added to deal with a potential issue when having
>>> multiple backends sharing a file descriptor.
>>> When a backend calls SHMEM_MAP it makes
>>> accessing to the region fail for other
>>> backend as it is missing from their translation
>>> table. So these requests are a fallback
>>> for vhost-user memory translation fails.
>>
>> Can you elaborate what the issue here is?
>>
>> Why would SHMEM_MAP make accessing the region fail for other backends --
>> what makes this missing from their translation?
> 
> This issue was raised by Stefan Hajnoczi in one of the first
> iterations of this patchset, based upon previous David Gilbert's work
> on the virtiofs DAX Window.
> 
> Let me paste here some of his remarks:
> 
> """
> Other backends don't see these mappings. If the guest submits a vring
> descriptor referencing a mapping to another backend, then that backend
> won't be able to access this memory.
> """
> [...]
> """
> A bit more detail:
> 
> Device A has a VIRTIO Shared Memory Region. An application mmaps that
> memory (examples: guest userspace driver using Linux VFIO, a guest
> kernel driver that exposes the memory to userspace via mmap, or guest
> kernel DAX). The application passes that memory as an I/O buffer to
> device B (e.g. O_DIRECT disk I/O).
> 
> The result is that device B's vhost-user backend receives a vring
> descriptor that points to a guest memory address in device A's VIRTIO
> Shared Memory Region. Since device B does not have this memory in its
> table, it cannot translate the address and the device breaks.
> """
> 
> I have not triggered the issue myself. So the idea is that the next
> patch will *definitively* include some testing for the commits that I
> cannot verify with my local setup.

Hah! But isn't that exact problem which is now solved by our rework?

Whatever is mapped in the VIRTIO Shared Memory Region will be 
communicated to all other vhost-user devices. So they should have that 
memory in their map and should be able to access it.

The only thing vhost-user devices cannot access are IIRC ram_device_ptr 
memory regions (e.g., from vfio devices). But that is independent shared 
memory regions.

-- 
Cheers,

David / dhildenb