Re: [RFC PATCH 06/27] virtio: Add virtio_queue_get_used_notify_split

[Stefan Hajnoczi]

On Tue, Jan 12, 2021 at 07:21:27PM +0100, Eugenio Perez Martin wrote:
> On Mon, Dec 7, 2020 at 5:58 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > On Fri, Nov 20, 2020 at 07:50:44PM +0100, Eugenio Pérez wrote:
> > > This function is just used for a few commits, so SW LM is developed
> > > incrementally, and it is deleted after it is useful.
> > >
> > > For a few commits, only the events (irqfd, eventfd) are forwarded.
> >
> > s/eventfd/ioeventfd/ (irqfd is also an eventfd)
> >
> 
> Oops, will fix, thanks!
> 
> > > +bool virtio_queue_get_used_notify_split(VirtQueue *vq)
> > > +{
> > > +    VRingMemoryRegionCaches *caches;
> > > +    hwaddr pa = offsetof(VRingUsed, flags);
> > > +    uint16_t flags;
> > > +
> > > +    RCU_READ_LOCK_GUARD();
> > > +
> > > +    caches = vring_get_region_caches(vq);
> > > +    assert(caches);
> > > +    flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
> > > +    return !(VRING_USED_F_NO_NOTIFY & flags);
> > > +}
> >
> > QEMU stores the notification status:
> >
> > void virtio_queue_set_notification(VirtQueue *vq, int enable)
> > {
> >     vq->notification = enable; <---- here
> >
> >     if (!vq->vring.desc) {
> >         return;
> >     }
> >
> >     if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> >         virtio_queue_packed_set_notification(vq, enable);
> >     } else {
> >         virtio_queue_split_set_notification(vq, enable);
> >
> > I'm wondering why it's necessary to fetch from guest RAM instead of
> > using vq->notification? It also works for both split and packed
> > queues so the code would be simpler.
> 
> To use vq->notification makes sense at the end of the series.
> 
> However, at this stage (just routing notifications, not descriptors),
> vhost device is the one updating that flag, not qemu. Since we cannot
> just migrate used ring memory to qemu without migrating descriptors
> ring too, qemu needs to check guest's memory looking for vhost device
> updates on that flag.
> 
> I can see how that deserves better documentation or even a better
> name. Also, this function should be in the shadow vq file, not
> virtio.c.

I can't think of a reason why QEMU needs to know the flag value that the
vhost device has set. This flag is a hint to the guest driver indicating
whether the device wants to receive notifications.

Can you explain why QEMU needs to look at the value of the flag?

Stefan

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