linux-kernel - Re: [PATCH v1] vhost: cache avail index in vhost_enable

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Date:   Mon, 24 Jan 2022 11:31:49 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
        netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH v1] vhost: cache avail index in vhost_enable_notify()

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/

> 
> 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@...hat.com>
> ---
> 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;

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