On 9/25/23 17:38, Stefan Hajnoczi wrote: > On Mon, 25 Sept 2023 at 11:36, Ilya Maximets <i.maxim...@ovn.org> wrote: >> >> On 9/25/23 17:12, Stefan Hajnoczi wrote: >>> On Mon, 25 Sept 2023 at 11:02, Ilya Maximets <i.maxim...@ovn.org> wrote: >>>> >>>> On 9/25/23 16:23, Stefan Hajnoczi wrote: >>>>> On Fri, 25 Aug 2023 at 13:04, Ilya Maximets <i.maxim...@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.maxim...@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. >> >> 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.