Re: [PATCH 7/7] vhost-user: call VHOST_USER_SET_VRING_ENABLE synchronously

[Stefan Hajnoczi]

On Tue, 3 Oct 2023 at 10:40, Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Tue, Oct 03, 2023 at 09:08:15AM -0400, Stefan Hajnoczi wrote:
> > On Tue, 3 Oct 2023 at 08:27, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Mon, Oct 02, 2023 at 05:13:26PM -0400, Stefan Hajnoczi wrote:
> > > > One more question:
> > > >
> > > > Why is the disabled state not needed by regular (non-vhost) virtio-net 
> > > > devices?
> > >
> > > Tap does the same - it purges queued packets:
> > >
> > > int tap_disable(NetClientState *nc)
> > > {
> > >     TAPState *s = DO_UPCAST(TAPState, nc, nc);
> > >     int ret;
> > >
> > >     if (s->enabled == 0) {
> > >         return 0;
> > >     } else {
> > >         ret = tap_fd_disable(s->fd);
> > >         if (ret == 0) {
> > >             qemu_purge_queued_packets(nc);
> > >             s->enabled = false;
> > >             tap_update_fd_handler(s);
> > >         }
> > >         return ret;
> > >     }
> > > }
> >
> > tap_disable() is not equivalent to the vhost-user "started but
> > disabled" ring state. tap_disable() is a synchronous one-time action,
> > while "started but disabled" is a continuous state.
>
> well, yes. but practically guests do not queue too many buffers
> after disabling a queue. I don't know if they reliably don't
> or it's racy and we didn't notice it yet - I think it
> was mostly dpdk that had this and that's usually
> used with vhost-user.
>
> > The "started but disabled" ring state isn't needed to achieve this.
> > The back-end can just drop tx buffers upon receiving
> > VHOST_USER_SET_VRING_ENABLE .num=0.
>
> yes, maybe that would have been a better way to do this.
>
>
> > The history of the spec is curious. VHOST_USER_SET_VRING_ENABLE was
> > introduced before the the "started but disabled" state was defined,
> > and it explicitly mentions tap attach/detach:
> >
> > commit 7263a0ad7899994b719ebed736a1119cc2e08110
> > Author: Changchun Ouyang <changchun.ouyang@intel.com>
> > Date:   Wed Sep 23 12:20:01 2015 +0800
> >
> >     vhost-user: add a new message to disable/enable a specific virt queue.
> >
> >     Add a new message, VHOST_USER_SET_VRING_ENABLE, to enable or disable
> >     a specific virt queue, which is similar to attach/detach queue for
> >     tap device.
> >
> > and then later:
> >
> > commit c61f09ed855b5009f816242ce281fd01586d4646
> > Author: Michael S. Tsirkin <mst@redhat.com>
> > Date:   Mon Nov 23 12:48:52 2015 +0200
> >
> >     vhost-user: clarify start and enable
> >
> > >
> > > what about non tap backends? I suspect they just aren't
> > > used widely with multiqueue so no one noticed.
> >
> > I still don't understand why "started but disabled" is needed instead
> > of just two ring states: enabled and disabled.
>
> With dropping packets when ring is disabled? Maybe that would
> have been enough. I also failed to realize it's specific to
> net, seemed generic to me :(
>
> > It seems like the cleanest path going forward is to keep the "ignore
> > rx, discard tx" semantics for virtio-net devices but to clarify in the
> > spec that other device types do not process the ring:
> >
> > "
> > * started but disabled: the back-end must not process the ring. For legacy
> >   reasons there is an exception for the networking device, where the
> >   back-end must process and discard any TX packets and not process
> >   other rings.
> > "
> >
> > What do you think?
> >
> > Stefan
>
> Okay... I hope we are not missing any devices which need virtio net
> semantics. Care checking them all?

Sure, I will check them. I'm very curious myself whether virtio-vsock
is affected (it has rx and tx queues).

I will report back.

Stefan

>
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> MST
>