Re: [RFC PATCH v9 13/23] vhost: Add vhost_svq_inject

[Eugenio Perez Martin]

On Mon, Jul 11, 2022 at 11:14 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/7/7 02:39, Eugenio Pérez 写道:
> > This allows qemu to inject buffers to the device.
>
>
> Not a native speaker but we probably need a better terminology than
> inject here.
>
> Since the CVQ is totally under the control of the Qemu anyhow.
>

I'm totally fine to change terminology

>
> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >   hw/virtio/vhost-shadow-virtqueue.h |  2 ++
> >   hw/virtio/vhost-shadow-virtqueue.c | 34 ++++++++++++++++++++++++++++++
> >   2 files changed, 36 insertions(+)
> >
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.h 
> > b/hw/virtio/vhost-shadow-virtqueue.h
> > index a811f90e01..d01d2370db 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.h
> > +++ b/hw/virtio/vhost-shadow-virtqueue.h
> > @@ -98,6 +98,8 @@ bool vhost_svq_valid_features(uint64_t features, Error 
> > **errp);
> >
> >   void vhost_svq_push_elem(VhostShadowVirtqueue *svq,
> >                            const VirtQueueElement *elem, uint32_t len);
> > +int vhost_svq_inject(VhostShadowVirtqueue *svq, const struct iovec *iov,
> > +                     size_t out_num, size_t in_num, void *opaque);
> >   void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue *svq, int 
> > svq_kick_fd);
> >   void vhost_svq_set_svq_call_fd(VhostShadowVirtqueue *svq, int call_fd);
> >   void vhost_svq_get_vring_addr(const VhostShadowVirtqueue *svq,
> > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > b/hw/virtio/vhost-shadow-virtqueue.c
> > index 492bb12b5f..bd9e34b413 100644
> > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > @@ -283,6 +283,40 @@ static bool vhost_svq_add_element(VhostShadowVirtqueue 
> > *svq,
> >       return ok;
> >   }
> >
> > +/**
> > + * Inject a chain of buffers to the device
> > + *
> > + * @svq: Shadow VirtQueue
> > + * @iov: I/O vector
> > + * @out_num: Number of front out descriptors
> > + * @in_num: Number of last input descriptors
> > + * @opaque: Contextual data to store in descriptor
> > + *
> > + * Return 0 on success, -ENOMEM if cannot inject
> > + */
> > +int vhost_svq_inject(VhostShadowVirtqueue *svq, const struct iovec *iov,
> > +                     size_t out_num, size_t in_num, void *opaque)
>
>
> If we manage to embed opaque into VirtqueueElement, we can simply use
> vhost_svq_add() here.
>

That works fine as long as SVQ only forwards elements, but it needs to
do more than that: We need to inject new elements without guest
notice.

How could we track elements that do not have corresponding
VirtQueueElement, like the elements sent to restore the status at the
LM destination?

I'll try to make it clearer in the patch message.

Thanks!

> Thanks
>
>
> > +{
> > +    bool ok;
> > +
> > +    /*
> > +     * All vhost_svq_inject calls are controlled by qemu so we won't hit 
> > this
> > +     * assertions.
> > +     */
> > +    assert(out_num || in_num);
> > +    assert(svq->ops);
> > +
> > +    if (unlikely(svq->next_guest_avail_elem)) {
> > +        error_report("Injecting in a full queue");
> > +        return -ENOMEM;
> > +    }
> > +
> > +    ok = vhost_svq_add(svq, iov, out_num, iov + out_num, in_num, opaque);
> > +    assert(ok);
> > +    vhost_svq_kick(svq);
> > +    return 0;
> > +}
> > +
> >   /**
> >    * Forward available buffers.
> >    *
>