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

Message

Albert Esteve Feb. 17, 2025, 4:40 p.m. UTC

Hi all,

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.

Albert Esteve (9):
  vhost-user: Add VirtIO Shared Memory map request
  vhost_user.rst: Align VhostUserMsg excerpt members
  vhost_user.rst: Add SHMEM_MAP/_UNMAP to spec
  vhost_user: Add frontend get_shmem_config command
  vhost_user.rst: Add GET_SHMEM_CONFIG message
  qmp: add shmem feature map
  vhost-user-devive: Add shmem BAR
  vhost_user: Add mem_read/write backend requests
  vhost_user.rst: Add MEM_READ/WRITE messages

 docs/interop/vhost-user.rst               | 110 +++++++++
 hw/virtio/vhost-user-base.c               |  47 +++-
 hw/virtio/vhost-user-device-pci.c         |  36 ++-
 hw/virtio/vhost-user.c                    | 272 ++++++++++++++++++++--
 hw/virtio/virtio-qmp.c                    |   3 +
 hw/virtio/virtio.c                        |  81 +++++++
 include/hw/virtio/vhost-backend.h         |   9 +
 include/hw/virtio/vhost-user.h            |   1 +
 include/hw/virtio/virtio.h                |  29 +++
 subprojects/libvhost-user/libvhost-user.c | 160 +++++++++++++
 subprojects/libvhost-user/libvhost-user.h |  92 ++++++++
 11 files changed, 813 insertions(+), 27 deletions(-)

Comments

David Hildenbrand Feb. 17, 2025, 8:01 p.m. UTC | #1

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?