Re: [PATCH for-9.0 v3] vdpa-dev: Fix initialisation order to restore VDUSE compatibility

[Eugenio Perez Martin]

On Tue, Mar 19, 2024 at 11:00 AM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 18.03.2024 um 20:27 hat Eugenio Perez Martin geschrieben:
> > On Mon, Mar 18, 2024 at 10:02 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Mon, Mar 18, 2024 at 12:31:26PM +0800, Jason Wang wrote:
> > > > On Fri, Mar 15, 2024 at 11:59 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > > > >
> > > > > VDUSE requires that virtqueues are first enabled before the DRIVER_OK
> > > > > status flag is set; with the current API of the kernel module, it is
> > > > > impossible to enable the opposite order in our block export code 
> > > > > because
> > > > > userspace is not notified when a virtqueue is enabled.
> > > > >
> > > > > This requirement also mathces the normal initialisation order as done 
> > > > > by
> > > > > the generic vhost code in QEMU. However, commit 6c482547 accidentally
> > > > > changed the order for vdpa-dev and broke access to VDUSE devices with
> > > > > this.
> > > > >
> > > > > This changes vdpa-dev to use the normal order again and use the 
> > > > > standard
> > > > > vhost callback .vhost_set_vring_enable for this. VDUSE devices can be
> > > > > used with vdpa-dev again after this fix.
> > > > >
> > > > > vhost_net intentionally avoided enabling the vrings for vdpa and does
> > > > > this manually later while it does enable them for other vhost 
> > > > > backends.
> > > > > Reflect this in the vhost_net code and return early for vdpa, so that
> > > > > the behaviour doesn't change for this device.
> > > > >
> > > > > Cc: qemu-stable@nongnu.org
> > > > > Fixes: 6c4825476a4351530bcac17abab72295b75ffe98
> > > > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > > > ---
> > > > > v2:
> > > > > - Actually make use of the @enable parameter
> > > > > - Change vhost_net to preserve the current behaviour
> > > > >
> > > > > v3:
> > > > > - Updated trace point [Stefano]
> > > > > - Fixed typo in comment [Stefano]
> > > > >
> > > > >  hw/net/vhost_net.c     | 10 ++++++++++
> > > > >  hw/virtio/vdpa-dev.c   |  5 +----
> > > > >  hw/virtio/vhost-vdpa.c | 29 ++++++++++++++++++++++++++---
> > > > >  hw/virtio/vhost.c      |  8 +++++++-
> > > > >  hw/virtio/trace-events |  2 +-
> > > > >  5 files changed, 45 insertions(+), 9 deletions(-)
> > > > >
> > > > > diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> > > > > index e8e1661646..fd1a93701a 100644
> > > > > --- a/hw/net/vhost_net.c
> > > > > +++ b/hw/net/vhost_net.c
> > > > > @@ -541,6 +541,16 @@ int vhost_set_vring_enable(NetClientState *nc, 
> > > > > int enable)
> > > > >      VHostNetState *net = get_vhost_net(nc);
> > > > >      const VhostOps *vhost_ops = net->dev.vhost_ops;
> > > > >
> > > > > +    /*
> > > > > +     * vhost-vdpa network devices need to enable dataplane 
> > > > > virtqueues after
> > > > > +     * DRIVER_OK, so they can recover device state before starting 
> > > > > dataplane.
> > > > > +     * Because of that, we don't enable virtqueues here and leave it 
> > > > > to
> > > > > +     * net/vhost-vdpa.c.
> > > > > +     */
> > > > > +    if (nc->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > > +        return 0;
> > > > > +    }
> > > >
> > > > I think we need some inputs from Eugenio, this is only needed for
> > > > shadow virtqueue during live migration but not other cases.
> > > >
> > > > Thanks
> > >
> > >
> > > Yes I think we had a backend flag for this, right? Eugenio can you
> > > comment please?
> > >
> >
> > We have the VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK backend flag,
> > right. If the backend does not offer it, it is better to enable all
> > the queues here and add a migration blocker in net/vhost-vdpa.c.
> >
> > So the check should be:
> > nc->info->type == VHOST_VDPA && (backend_features &
> > VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK).
> >
> > I can manage to add the migration blocker on top of this patch.
>
> Note that my patch preserves the current behaviour for vhost_net. The
> callback wasn't implemented for vdpa so far, so we never called anything
> even if the flag wasn't set. This patch adds an implementation for the
> callback, so we have to skip it here to have everything in vhost_net
> work as before - which is what the condition as written does.
>
> If we add a check for the flag now (I don't know if that's correct or
> not), that would be a second, unrelated change of behaviour in the same
> patch. So if it's necessary, that's a preexisting problem and I'd argue
> it doesn't belong in this patch, but should be done separately.
>

Right, that's a very good point. I'll add proper checking on top of
your patch when it is merged.

Reviewed-by: Eugenio Pérez <eperezma@redhat.com>

Thanks!