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

["Michael S. Tsirkin" <mst@redhat.com>]

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?