Virtualization - Nov 2021 - [RFC PATCH v5 21/26] vhost: Add vhost_svq_valid_guest_features to shadow vq

On Tue, Nov 2, 2021 at 4:10 PM Eugenio Perez Martin <eperezma at
redhat.com> wrote:
>
> On Tue, Nov 2, 2021 at 6:26 AM Jason Wang <jasowang at redhat.com>
wrote:
> >
> > On Sat, Oct 30, 2021 at 2:44 AM Eugenio P?rez <eperezma at
redhat.com> wrote:
> > >
> > > This allows it to test if the guest has aknowledge an invalid
transport
> > > feature for SVQ. This will include packed vq layout or event_idx,
> > > where VirtIO device needs help from SVQ.
> > >
> > > There is not needed at this moment, but since SVQ will not
re-negotiate
> > > features again with the guest, a failure in acknowledge them is
fatal
> > > for SVQ.
> > >
> >
> > It's not clear to me why we need this. Maybe you can give me an
> > example. E.g isn't it sufficient to filter out the device with
> > event_idx?
> >
>
> If the guest did negotiate _F_EVENT_IDX, it expects to be notified
> only when device marks as used a specific number of descriptors.
>
> If we use VirtQueue notification, the VirtQueue code handles it
> transparently. But if we want to be able to change the guest VQ's
> call_fd, we cannot use VirtQueue's, so this needs to be handled by SVQ
> code. And that is still not implemented.
>
> Of course in the event_idx case we could just ignore it and notify in
> all used descriptors, but it seems not polite to me :). I will develop
> event_idx on top of this, either exposing the needed pieces in
> VirtQueue (I prefer this) or rolling our own in SVQ.
Yes, but what I meant is, we can fail the SVQ enabling if the device
supports event_idx. Then we're sure guests won't negotiate event_idx.

Thanks
>
> Same reasoning can be applied to unknown transport features.
>
> Thanks!
>
> > Thanks
> >
> > > Signed-off-by: Eugenio P?rez <eperezma at redhat.com>
> > > ---
> > >  hw/virtio/vhost-shadow-virtqueue.h | 1 +
> > >  hw/virtio/vhost-shadow-virtqueue.c | 6 ++++++
> > >  2 files changed, 7 insertions(+)
> > >
> > > diff --git a/hw/virtio/vhost-shadow-virtqueue.h
b/hw/virtio/vhost-shadow-virtqueue.h
> > > index 946b2c6295..ac55588009 100644
> > > --- a/hw/virtio/vhost-shadow-virtqueue.h
> > > +++ b/hw/virtio/vhost-shadow-virtqueue.h
> > > @@ -16,6 +16,7 @@
> > >  typedef struct VhostShadowVirtqueue VhostShadowVirtqueue;
> > >
> > >  bool vhost_svq_valid_device_features(uint64_t *features);
> > > +bool vhost_svq_valid_guest_features(uint64_t *features);
> > >
> > >  void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue *svq, int
svq_kick_fd);
> > >  void vhost_svq_set_guest_call_notifier(VhostShadowVirtqueue
*svq, int call_fd);
> > > diff --git a/hw/virtio/vhost-shadow-virtqueue.c
b/hw/virtio/vhost-shadow-virtqueue.c
> > > index 6e0508a231..cb9ffcb015 100644
> > > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > > @@ -62,6 +62,12 @@ bool vhost_svq_valid_device_features(uint64_t
*dev_features)
> > >      return true;
> > >  }
> > >
> > > +/* If the guest is using some of these, SVQ cannot communicate
*/
> > > +bool vhost_svq_valid_guest_features(uint64_t *guest_features)
> > > +{
> > > +    return true;
> > > +}
> > > +
> > >  /* Forward guest notifications */
> > >  static void vhost_handle_guest_kick(EventNotifier *n)
> > >  {
> > > --
> > > 2.27.0
> > >
> >
>