On Wed, Feb 02, 2022 at 11:14:30AM +0000, Stefan Hajnoczi wrote: > On Fri, Jan 28, 2022 at 10:41:29AM +0100, Stefano Garzarella wrote: > > In vhost_enable_notify() we enable the notifications and we read > > the avail index to check if new buffers have become available in > > the meantime. > > > > We do not update the cached avail index value, so when the device > > will call vhost_get_vq_desc(), it will find the old value in the > > cache and it will read the avail index again. > > > > It would be better to refresh the cache every time we read avail > > index, so let's change vhost_enable_notify() caching the value in > > `avail_idx` and compare it with `last_avail_idx` to check if there > > are new buffers available. > > > > We don't expect a significant performance boost because > > the above path is not very common, indeed vhost_enable_notify() > > is often called with unlikely(), expecting that avail index has > > not been updated. > > > > We ran virtio-test/vhost-test and noticed minimal improvement as > > expected. To stress the patch more, we modified vhost_test.ko to > > call vhost_enable_notify()/vhost_disable_notify() on every cycle > > when calling vhost_get_vq_desc(); in this case we observed a more > > evident improvement, with a reduction of the test execution time > > of about 3.7%. > > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx> > > --- > > v3 > > - reworded commit description [Stefan] > > --- > > drivers/vhost/vhost.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > > index 59edb5a1ffe2..07363dff559e 100644 > > --- a/drivers/vhost/vhost.c > > +++ b/drivers/vhost/vhost.c > > @@ -2543,8 +2543,9 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq) > > &vq->avail->idx, r); > > return false; > > } > > + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); > > > > - return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx; > > + return vq->avail_idx != vq->last_avail_idx; > > } > > EXPORT_SYMBOL_GPL(vhost_enable_notify); > > This changes behavior (fixes a bug?): previously the function returned > false when called with avail buffers still pending (vq->last_avail_idx < > vq->avail_idx). Now it returns true because we compare against > vq->last_avail_idx and I think that's reasonable. > > Reviewed-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx> I don't see the behaviour change... could you explain the scanario in more detail pls? Thanks!
