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

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Date:   Fri, 14 Jan 2022 14:18:01 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        netdev <netdev@...r.kernel.org>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH] vhost: cache avail index in vhost_enable_notify()

On Thu, Jan 13, 2022 at 10:57 PM Stefano Garzarella <sgarzare@...hat.com> 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. In this case, the device would go to re-read avail
> index to access the descriptor.
>
> As we already do in other place, we can cache the value in `avail_idx`
> and compare it with `last_avail_idx` to check if there are new
> buffers available.
>
> Signed-off-by: Stefano Garzarella <sgarzare@...hat.com>

Patch looks fine but I guess we won't get performance improvement
since it doesn't save any userspace/VM memory access?

Thanks

> ---
>  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
>