Re: [RFC PATCH v5 21/26] vhost: Add vhost_svq_valid_guest_features to shadow vq

[Jason Wang]

On Wed, Nov 3, 2021 at 3:44 PM Eugenio Perez Martin <eperezma@redhat.com> wrote:
>
> On Wed, Nov 3, 2021 at 4:18 AM Jason Wang <jasowang@redhat.com> wrote:
> >
> > On Tue, Nov 2, 2021 at 4:10 PM Eugenio Perez Martin <eperezma@redhat.com> 
> > wrote:
> > >
> > > On Tue, Nov 2, 2021 at 6:26 AM Jason Wang <jasowang@redhat.com> wrote:
> > > >
> > > > On Sat, Oct 30, 2021 at 2:44 AM Eugenio Pérez <eperezma@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.
> >
>
> We can go that way for sure, but then we leave out the scenario where
> the device supports event_idx but the guest has not acked it. This is
> a valid scenario for SVQ to work in.

If SVQ supports event idx in the future, we can remove it from the
blacklist. But I think it should be simpler to let SVQ use the same
features as guests. So in this case SVQ won't use the event index.

Thanks

>
> Thanks!
>
> > Thanks
> >
> > >
> > > Same reasoning can be applied to unknown transport features.
> > >
> > > Thanks!
> > >
> > > > Thanks
> > > >
> > > > > Signed-off-by: Eugenio Pérez <eperezma@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
> > > > >
> > > >
> > >
> >
>