Re: [PATCH] virtio: use shadow_avail_idx while checking number of heads

[Stefan Hajnoczi]

On Mon, 25 Sept 2023 at 11:02, Ilya Maximets <i.maximets@ovn.org> wrote:
>
> On 9/25/23 16:23, Stefan Hajnoczi wrote:
> > On Fri, 25 Aug 2023 at 13:04, Ilya Maximets <i.maximets@ovn.org> wrote:
> >>
> >> We do not need the most up to date number of heads, we only want to
> >> know if there is at least one.
> >>
> >> Use shadow variable as long as it is not equal to the last available
> >> index checked.  This avoids expensive qatomic dereference of the
> >> RCU-protected memory region cache as well as the memory access itself
> >> and the subsequent memory barrier.
> >>
> >> The change improves performance of the af-xdp network backend by 2-3%.
> >>
> >> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
> >> ---
> >>  hw/virtio/virtio.c | 10 +++++++++-
> >>  1 file changed, 9 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> >> index 309038fd46..04bf7cc977 100644
> >> --- a/hw/virtio/virtio.c
> >> +++ b/hw/virtio/virtio.c
> >> @@ -999,7 +999,15 @@ void virtqueue_push(VirtQueue *vq, const 
> >> VirtQueueElement *elem,
> >>  /* Called within rcu_read_lock().  */
> >>  static int virtqueue_num_heads(VirtQueue *vq, unsigned int idx)
> >>  {
> >> -    uint16_t num_heads = vring_avail_idx(vq) - idx;
> >> +    uint16_t num_heads;
> >> +
> >> +    if (vq->shadow_avail_idx != idx) {
> >> +        num_heads = vq->shadow_avail_idx - idx;
> >> +
> >> +        return num_heads;
> >
> > This still needs to check num_heads > vq->vring.num and return -EINVAL
> > as is done below.
>
> Hmm, yeas, you're right.  If the value was incorrect initially, the shadow
> will be incorrect.  However, I think we should just not return here in this
> case and let vring_avail_idx() to grab an actual new value below.  Otherwise
> we may never break out of this error.
>
> Does that make sense?

No, because virtio_error() marks the device as broken. The device
requires a reset in order to function again. Fetching
vring_avail_idx() again won't help.

>
> >
> >> +    }
> >> +
> >> +    num_heads = vring_avail_idx(vq) - idx;
> >>
> >>      /* Check it isn't doing very strange things with descriptor numbers. 
> >> */
> >>      if (num_heads > vq->vring.num) {
> >> --
> >> 2.40.1
> >>
> >>
>