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

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

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.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.
> 
> 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.
> >