Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

Michael S. Tsirkin
2022-01-14T12:45:47Z

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@redhat.com>

... 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