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.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.
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.