On Fri, Jan 14, 2022 at 10:05:08AM +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 are not caching the avail index, 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. > > Anyway, 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. > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx> ... and can in theory even hurt due to an extra memory write. So ... performance test restults pls? > --- > v1: > - improved the commit description [MST, Jason] > --- > 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); > > -- > 2.31.1
