Re: [Qemu-devel] [PATCH 1/4] vhost-user: fix multiple queue specification

[Maxime Coquelin]

On 01/16/2018 04:00 AM, Michael S. Tsirkin wrote:
> On Fri, Jan 12, 2018 at 03:56:55PM +0100, Maxime Coquelin wrote:
> > The number of queues supported by the slave is queried with
> > message VHOST_USER_GET_QUEUE_NUM, not with message
> > VHOST_USER_GET_PROTOCOL_FEATURES.
> >
> > Also, looking at master and slave implemntations, the payload
> > returned by the slave is the number of queue pairs supported
> > by the slave, not the number of queues.
>
> virtio doesn't have a concept of queue pairs. virtio net does
> have a concept of a tx/rx pair for purposes of steering.

Ok, thanks for the clarification. I will have a look at how vhost-user
SCSI implements it.

> Would this be a slave bug then?

If I'm not mistaken, the bug is in QEMU:

VHOST_USER_GET_QUEUE_NUM is stored in (struct vhost_dev).max_queues.
vhost_net_get_max_queues() returns (struct vhost_dev).max_queues.
And vhost_user_start() from net/vhost-user.c calls
vhost_net_get_max_queues() to get the max number of tx/rx pairs.

If we want to fix QEMU, I think we will need a new flag for
compatibility with older/current backends that assume it represent a
queue pair.

> I've applied the 1st chunk for now.

Thanks.

> > Signed-off-by: Maxime Coquelin <address@hidden>
> > ---
> >   docs/interop/vhost-user.txt | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > index d49444e037..8a14191a1e 100644
> > --- a/docs/interop/vhost-user.txt
> > +++ b/docs/interop/vhost-user.txt
> > @@ -214,8 +214,8 @@ Multiple queue is treated as a protocol extension, hence 
> > the slave has to
> >   implement protocol features first. The multiple queues feature is supported
> >   only when the protocol feature VHOST_USER_PROTOCOL_F_MQ (bit 0) is set.
> >   
> > -The max number of queues the slave supports can be queried with message
> > -VHOST_USER_GET_PROTOCOL_FEATURES. Master should stop when the number of
> > +The max number of queue pairs the slave supports can be queried with message
> > +VHOST_USER_GET_QUEUE_NUM. Master should stop when the number of
> >   requested queues is bigger than that.
> >   
> >   As all queues share one connection, the master uses a unique index for each
> > @@ -537,7 +537,7 @@ Master message types
> >         Master payload: N/A
> >         Slave payload: u64
> >   
> > -      Query how many queues the backend supports. This request should be
> > +      Query how many queue pairs the backend supports. This request should be
> >         sent only when VHOST_USER_PROTOCOL_F_MQ is set in queried protocol
> >         features by VHOST_USER_GET_PROTOCOL_FEATURES.
> >   
> > --
> > 2.14.3