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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 02, 2021 at 07:34:20PM +0100, Eugenio Perez Martin wrote:
> On Tue, Mar 2, 2021 at 12:22 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > 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
> 
> My bad, "need" is not the right word: SVQ could just forward the
> notification at this point without checking the flag. Taking into
> account that it's not used in later series, and it's even removed in
> patch 14/27 of this series, I can see that it just adds noise to the
> entire patchset
> 
> This function just allows svq to re-check the flag after the guest
> sends the notification. This way svq is able to drop the kick as a
> (premature?) optimization in case the device sets it just after the
> guest sends the kick.
> 
> Until patch 13/27, only notifications are forwarded, not buffers. VM
> guest's drivers and vhost device still read and write at usual
> addresses, but ioeventfd and kvmfd are intercepted by qemu. This
> allows us to test if the notification forwarding part is doing ok.
> From patch 14 of this series, svq offers a new vring to the device in
> qemu's VAS, so the former does not need to check the guest's memory
> anymore, and this function can be dropped.
> 
> Is it clearer now? Please let me know if I should add something else.

Thanks for explaining. You could drop it to simplify the code. If you
leave it in, please include a comment explaining the purpose.

Thanks,
Stefan

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