Re: [PATCH v3] vhost: cache avail index in vhost_enable_notify()

[Stefan Hajnoczi]

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>

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