Re: [Qemu-devel] [PATCH] docs: clarify multiqueue vs multiple virtqueues

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jun 24, 2019 at 10:13:04AM +0100, Stefan Hajnoczi wrote:
> The vhost-user specification does not explain when
> VHOST_USER_PROTOCOL_F_MQ must be implemented.  This may lead
> implementors of vhost-user masters to believe that this protocol feature
> is required for any device that has multiple virtqueues.  That would be
> a mistake since existing vhost-user slaves offer multiple virtqueues but
> do not advertise VHOST_USER_PROTOCOL_F_MQ.
> 
> For example, a vhost-net device with one rx/tx queue pair is not
> multiqueue.  The slave does not need to advertise
> VHOST_USER_PROTOCOL_F_MQ.  Therefore the master must assume it has these
> virtqueues and cannot rely on askingt the slave how many virtqueues
> exist.
> 
> Extend the specification to explain the different between true
> multiqueue and regular devices with a fixed virtqueue layout.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
> Based-on: <20190621094005.4134-1-stefanha@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)

Ping?

> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 5750668aba..7827b710aa 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -324,6 +324,15 @@ must support changing some configuration aspects on the fly.
>  Multiple queue support
>  ----------------------
>  
> +Many devices have a fixed number of virtqueues.  In this case the master
> +already knows the number of available virtqueues without communicating with the
> +slave.
> +
> +Some devices do not have a fixed number of virtqueues.  Instead the maximum
> +number of virtqueues is chosen by the slave.  The number can depend on host
> +resource availability or slave implementation details.  Such devices are called
> +multiple queue devices.
> +
>  Multiple queue support allows the slave to advertise the maximum number of
>  queues.  This is treated as a protocol extension, hence the slave has to
>  implement protocol features first. The multiple queues feature is supported
> @@ -339,6 +348,14 @@ queue in the sent message to identify a specified queue.
>  The master enables queues by sending message ``VHOST_USER_SET_VRING_ENABLE``.
>  vhost-user-net has historically automatically enabled the first queue pair.
>  
> +Slaves should always implement the ``VHOST_USER_PROTOCOL_F_MQ`` protocol
> +feature, even for devices with a fixed number of virtqueues, since it is simple
> +to implement and offers a degree of introspection.
> +
> +Masters must not rely on the ``VHOST_USER_PROTOCOL_F_MQ`` protocol feature for
> +devices with a fixed number of virtqueues.  Only true multiqueue devices
> +require this protocol feature.
> +
>  Migration
>  ---------
>  
> -- 
> 2.21.0
> 

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