On Tue, Sep 26, 2023 at 12:13:11AM +0200, Ilya Maximets wrote: > On 9/25/23 23:24, Michael S. Tsirkin wrote: > > 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. > > It can't, I agree. Scenario I'm thinking about is the following: > > 1. vring_avail_idx() is called from one of the places other than > virtqueue_num_heads(). Shadow is updated with the current value. > Some users of vring_avail_idx() do not use barriers after the call. > > 2. virtqueue_split_get_avail_bytes() is called. > > 3. virtqueue_split_get_avail_bytes() calls virtqueue_num_heads(). > > 4. virtqueue_num_heads() checks the shadow and returns early. > > 5. virtqueue_split_get_avail_bytes() calls vring_split_desc_read() and > reads the descriptor. > > If between steps 1 and 5 we do not have a read barrier, we potentially > risk reading descriptor data that is not yet fully written, because > there is no guarantee that reading the last_avail_idx on step 1 wasn't > reordered with the descriptor read. > > In current code we always have smp_rmb() in virtqueue_num_heads(). > But if we return from this function without a barrier, we may have an > issue, IIUC. > > I agree that it's kind of a very unlikely scenario and we will probably > have a control dependency between steps 1 and 5 that will prevent the > issue, but it might be safer to just have an explicit barrier in > virtqueue_num_heads(). > > Does that make sense? Or am I missing something else here?
Aha, got it. Good point, yes. Pls document in a code comment. > > > > > >>>> > >>>> 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. > >