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

[David Hildenbrand <david@redhat.com>]

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?

-- 
Cheers,

David / dhildenb