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

[Stefan Hajnoczi]

On Mon, 25 Sept 2023 at 11:36, Ilya Maximets <i.maximets@ovn.org> wrote:
>
> On 9/25/23 17:12, Stefan Hajnoczi wrote:
> > 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.
>
> OK, I see.  In this case we're talking about situation where
> vring_avail_idx() was called in some other place and stored a bad value
> in the shadow variable, then virtqueue_num_heads() got called.  Right?
>
> AFAIU, we can still just fall through here and let vring_avail_idx()
> to read the index again and fail the existing check.  That would happen
> today without this patch applied.

Yes, that is fine.

>
> I'm jut trying to avoid duplication of the virtio_error call, i.e.:
>
>     if (vq->shadow_avail_idx != idx) {
>         num_heads = vq->shadow_avail_idx - idx;
>
>         /* Check it isn't doing very strange things with descriptor numbers. 
> */
>         if (num_heads > vq->vring.num) {
>             virtio_error(vq->vdev, "Guest moved used index from %u to %u",
>                          idx, vq->shadow_avail_idx);
>             return -EINVAL;
>         }
>         return num_heads;
>     }
>
> vs
>
>     if (vq->shadow_avail_idx != idx) {
>         num_heads = vq->shadow_avail_idx - idx;
>
>         /* Only use the shadow value if it was good initially. */
>         if (num_heads <= vq->vring.num) {
>             return num_heads;
>         }
>     }
>
>
> What do you think?

Sounds good.

>
> Best regards, Ilya Maximets.