Re: [RFC PATCH v4 11/20] vhost: Route host->guest notification through shadow virtqueue

[Jason Wang]

在 2021/10/15 上午12:39, Eugenio Perez Martin 写道:
> On Wed, Oct 13, 2021 at 5:47 AM Jason Wang <jasowang@redhat.com> wrote:
> > 在 2021/10/1 下午3:05, Eugenio Pérez 写道:
> > > This will make qemu aware of the device used buffers, allowing it to
> > > write the guest memory with its contents if needed.
> > >
> > > Since the use of vhost_virtqueue_start can unmasks and discard call
> > > events, vhost_virtqueue_start should be modified in one of these ways:
> > > * Split in two: One of them uses all logic to start a queue with no
> > >     side effects for the guest, and another one tha actually assumes that
> > >     the guest has just started the device. Vdpa should use just the
> > >     former.
> > > * Actually store and check if the guest notifier is masked, and do it
> > >     conditionally.
> > > * Left as it is, and duplicate all the logic in vhost-vdpa.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >    hw/virtio/vhost-shadow-virtqueue.c | 19 +++++++++++++++
> > >    hw/virtio/vhost-vdpa.c             | 38 +++++++++++++++++++++++++++++-
> > >    2 files changed, 56 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> > > b/hw/virtio/vhost-shadow-virtqueue.c
> > > index 21dc99ab5d..3fe129cf63 100644
> > > --- a/hw/virtio/vhost-shadow-virtqueue.c
> > > +++ b/hw/virtio/vhost-shadow-virtqueue.c
> > > @@ -53,6 +53,22 @@ static void vhost_handle_guest_kick(EventNotifier *n)
> > >        event_notifier_set(&svq->kick_notifier);
> > >    }
> > >
> > > +/* Forward vhost notifications */
> > > +static void vhost_svq_handle_call_no_test(EventNotifier *n)
> > > +{
> > > +    VhostShadowVirtqueue *svq = container_of(n, VhostShadowVirtqueue,
> > > +                                             call_notifier);
> > > +
> > > +    event_notifier_set(&svq->guest_call_notifier);
> > > +}
> > > +
> > > +static void vhost_svq_handle_call(EventNotifier *n)
> > > +{
> > > +    if (likely(event_notifier_test_and_clear(n))) {
> > > +        vhost_svq_handle_call_no_test(n);
> > > +    }
> > > +}
> > > +
> > >    /*
> > >     * Obtain the SVQ call notifier, where vhost device notifies SVQ that there
> > >     * exists pending used buffers.
> > > @@ -180,6 +196,8 @@ VhostShadowVirtqueue *vhost_svq_new(struct vhost_dev *dev, 
> > > int idx)
> > >        }
> > >
> > >        svq->vq = virtio_get_queue(dev->vdev, vq_idx);
> > > +    event_notifier_set_handler(&svq->call_notifier,
> > > +                               vhost_svq_handle_call);
> > >        return g_steal_pointer(&svq);
> > >
> > >    err_init_call_notifier:
> > > @@ -195,6 +213,7 @@ err_init_kick_notifier:
> > >    void vhost_svq_free(VhostShadowVirtqueue *vq)
> > >    {
> > >        event_notifier_cleanup(&vq->kick_notifier);
> > > +    event_notifier_set_handler(&vq->call_notifier, NULL);
> > >        event_notifier_cleanup(&vq->call_notifier);
> > >        g_free(vq);
> > >    }
> > > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > > index bc34de2439..6c5f4c98b8 100644
> > > --- a/hw/virtio/vhost-vdpa.c
> > > +++ b/hw/virtio/vhost-vdpa.c
> > > @@ -712,13 +712,40 @@ static bool vhost_vdpa_svq_start_vq(struct vhost_dev 
> > > *dev, unsigned idx)
> > >    {
> > >        struct vhost_vdpa *v = dev->opaque;
> > >        VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, idx);
> > > -    return vhost_svq_start(dev, idx, svq);
> > > +    EventNotifier *vhost_call_notifier = vhost_svq_get_svq_call_notifier(svq);
> > > +    struct vhost_vring_file vhost_call_file = {
> > > +        .index = idx + dev->vq_index,
> > > +        .fd = event_notifier_get_fd(vhost_call_notifier),
> > > +    };
> > > +    int r;
> > > +    bool b;
> > > +
> > > +    /* Set shadow vq -> guest notifier */
> > > +    assert(v->call_fd[idx]);
> >
> > We need aovid the asser() here. On which case we can hit this?
> I would say that there is no way we can actually hit it, so let's remove it.
>
> > > +    vhost_svq_set_guest_call_notifier(svq, v->call_fd[idx]);
> > > +
> > > +    b = vhost_svq_start(dev, idx, svq);
> > > +    if (unlikely(!b)) {
> > > +        return false;
> > > +    }
> > > +
> > > +    /* Set device -> SVQ notifier */
> > > +    r = vhost_vdpa_set_vring_dev_call(dev, &vhost_call_file);
> > > +    if (unlikely(r)) {
> > > +        error_report("vhost_vdpa_set_vring_call for shadow vq failed");
> > > +        return false;
> > > +    }
> >
> > Similar to kick, do we need to set_vring_call() before vhost_svq_start()?
> It should not matter at this moment because the device should not be
> started at this point and device calls should not run
> vhost_svq_handle_call until BQL is released.


Yes, we stop virtqueue before.


> The "logic" of doing it after is to make clear that svq must be fully
> initialized before processing device calls, even in the case that we
> extract SVQ in its own iothread or similar. But this could be done
> before vhost_svq_start for sure.
>
> > > +
> > > +    /* Check for pending calls */
> > > +    event_notifier_set(vhost_call_notifier);
> >
> > Interesting, can this result spurious interrupt?
> This actually "queues" a vhost_svq_handle_call after the BQL release,
> where the device should be fully reset. In that regard, if there are
> no used descriptors there will not be an irq raised to the guest. Does
> that answer the question? Or have I missed something?


Yes, please explain this in the comment.


> > > +    return true;
> > >    }
> > >
> > >    static unsigned vhost_vdpa_enable_svq(struct vhost_vdpa *v, bool enable)
> > >    {
> > >        struct vhost_dev *hdev = v->dev;
> > >        unsigned n;
> > > +    int r;
> > >
> > >        if (enable == v->shadow_vqs_enabled) {
> > >            return hdev->nvqs;
> > > @@ -752,9 +779,18 @@ static unsigned vhost_vdpa_enable_svq(struct vhost_vdpa 
> > > *v, bool enable)
> > >        if (!enable) {
> > >            /* Disable all queues or clean up failed start */
> > >            for (n = 0; n < v->shadow_vqs->len; ++n) {
> > > +            struct vhost_vring_file file = {
> > > +                .index = vhost_vdpa_get_vq_index(hdev, n),
> > > +                .fd = v->call_fd[n],
> > > +            };
> > > +
> > > +            r = vhost_vdpa_set_vring_call(hdev, &file);
> > > +            assert(r == 0);
> > > +
> > >                unsigned vq_idx = vhost_vdpa_get_vq_index(hdev, n);
> > >                VhostShadowVirtqueue *svq = g_ptr_array_index(v->shadow_vqs, n);
> > >                vhost_svq_stop(hdev, n, svq);
> > > +            /* TODO: This can unmask or override call fd! */
> >
> > I don't get this comment. Does this mean the current code can't work
> > with mask_notifiers? If yes, this is something we need to fix.
> Yes, but it will be addressed in the next series. I should have
> explained it bette here, sorry :).


Ok.

Thanks


> Thanks!
>
> > Thanks
> >
> >
> > >                vhost_virtqueue_start(hdev, hdev->vdev, &hdev->vqs[n], vq_idx);
> > >            }