Re: [PATCH v12 5/7] vhost_user.rst: Add GET_SHMEM_CONFIG message

[Alyssa Ross <hi@alyssa.is>]

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Albert Esteve <aesteve@redhat.com> writes:

> On Sat, Feb 7, 2026 at 8:05 PM Alyssa Ross <hi@alyssa.is> wrote:
>>
>> Albert Esteve <aesteve@redhat.com> writes:
>>
>> > Add GET_SHMEM_CONFIG vhost-user frontend
>> > message to the spec documentation.
>> >
>> > Reviewed-by: Alyssa Ross <hi@alyssa.is>
>> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
>> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
>> > ---
>> >  docs/interop/vhost-user.rst | 41 +++++++++++++++++++++++++++++++++++++
>> >  1 file changed, 41 insertions(+)
>> >
>> > diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
>> > index 6c1d66d7d3..e314313806 100644
>> > --- a/docs/interop/vhost-user.rst
>> > +++ b/docs/interop/vhost-user.rst
>> > @@ -371,6 +371,22 @@ MMAP request
>> >    - 0: Pages are mapped read-only
>> >    - 1: Pages are mapped read-write
>> >
>> > +VIRTIO Shared Memory Region configuration
>> > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> > +
>> > ++-------------+---------+------------+----+--------------+
>> > +| num regions | padding | mem size 0 | .. | mem size 255 |
>> > ++-------------+---------+------------+----+--------------+
>> > +
>> > +:num regions: a 32-bit number of regions
>> > +
>> > +:padding: 32-bit
>> > +
>> > +:mem size: an array of 256 64-bit fields representing the size of each
>> > +           VIRTIO Shared Memory Region. ``num regions`` specifies the
>> > +           number of valid regions (non-zero size). The array index
>> > +           corresponds to the shared memory ID (shmid).
>> > +
>> >  C structure
>> >  -----------
>> >
>> > @@ -397,6 +413,7 @@ In QEMU the vhost-user message is implemented with the following struct:
>> >            VhostUserShared object;
>> >            VhostUserTransferDeviceState transfer_state;
>> >            VhostUserMMap mmap;
>> > +          VhostUserShMemConfig shmem;
>> >        };
>> >    } QEMU_PACKED VhostUserMsg;
>> >
>> > @@ -1761,6 +1778,30 @@ Front-end message types
>> >    Using this function requires prior negotiation of the
>> >    ``VHOST_USER_PROTOCOL_F_DEVICE_STATE`` feature.
>> >
>> > +``VHOST_USER_GET_SHMEM_CONFIG``
>> > +  :id: 44
>> > +  :equivalent ioctl: N/A
>> > +  :request payload: N/A
>> > +  :reply payload: ``struct VhostUserShMemConfig``
>> > +
>> > +  When the ``VHOST_USER_PROTOCOL_F_SHMEM`` protocol feature has been
>> > +  successfully negotiated, this message can be submitted by the front-end
>> > +  to gather the VIRTIO Shared Memory Region configuration. The back-end will
>> > +  respond with the number of VIRTIO Shared Memory Regions it requires, and
>> > +  each shared memory region size in an array. The shared memory IDs are
>> > +  represented by the array index. The information returned shall comply
>> > +  with the following rules:
>> > +
>> > +  * The shared information will remain valid and unchanged for the entire
>> > +    lifetime of the connection.
>> > +
>> > +  * The Shared Memory Region size must be a multiple of the page size
>> > +    supported by mmap(2).
>> > +
>> > +  * The size may be 0 if the region is unused. This can happen when the
>> > +    device does not support an optional feature but does support a feature
>> > +    that uses a higher shmid.
>>
>> This is still phrased as if unused regions after the last used one can
>> be ommitted.  If 256 sizes are always sent, then size must be 0 for all
>> unused regions.
>
> Had to read it a couple of times to see what you meant. But yeah, it
> implies that a zero is always followed by some valid region and is not
> always the case. I guess I can keep only the first sentence and delete
> the rest.

Sounds good to me.

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