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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Sep 25, 2023 at 10:58:05PM +0200, Ilya Maximets wrote:
> On 9/25/23 17:38, Stefan Hajnoczi wrote:
> > 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?
> 
> Hmm, I suppose we also need a read barrier after all even if we use
> a shadow index.  Assuming the index is correct, but the shadow variable
> was updated by a call outside of this function, then we may miss a
> barrier and read the descriptor out of order, in theory.  Read barrier
> is going to be a compiler barrier on x86, so the performance gain from
> this patch should still be mostly there.  I'll test that.

I can't say I understand generally. shadow is under qemu control,
I don't think it can be updated concurrently by multiple CPUs.


> >>
> >> 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.