On Tue, Jan 25, 2022 at 12:14:22PM +0100, Stefano Garzarella wrote: > On Mon, Jan 24, 2022 at 11:31:49AM +0000, Stefan Hajnoczi wrote: > > 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. > > > > I think this wording is clearer because we do keep a cached the avail > > index value, but the issue is we don't update it: > > s/We are not caching the avail index/We do not update the cached avail > > index value/ > > I'll fix in v3. > It seems I forgot to CC you on v2: https://lore.kernel.org/virtualization/20220121153108.187291-1-sgarzare@xxxxxxxxxx/ > > > > > > > > > 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> > > > --- > > > 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; > > > > vhost_vq_avail_empty() has a fast path that's missing in > > vhost_enable_notify(): > > > > if (vq->avail_idx != vq->last_avail_idx) > > return false; > > Yep, I thought about that, but devices usually call vhost_enable_notify() > right when vq->avail_idx == vq->last_avail_idx, so I don't know if it's an > extra check for a branch that will never be taken. > > Do you think it is better to add that check? (maybe with unlikely()) You're right. It's probably fine to omit it. Stefan
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