linux-kernel - Re: [PATCH 4/6] vhost: reset invalidate_count in vhost_set_vring_num

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Message-ID: <20190723042143-mutt-send-email-mst@kernel.org>
Date:   Tue, 23 Jul 2019 05:17:28 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] vhost: reset invalidate_count in
 vhost_set_vring_num_addr()

On Tue, Jul 23, 2019 at 03:57:16AM -0400, Jason Wang wrote:
> The vhost_set_vring_num_addr() could be called in the middle of
> invalidate_range_start() and invalidate_range_end(). If we don't reset
> invalidate_count after the un-registering of MMU notifier, the
> invalidate_cont will run out of sync (e.g never reach zero). This will
> in fact disable the fast accessor path. Fixing by reset the count to
> zero.
> 
> Reported-by: Michael S. Tsirkin <mst@...hat.com>
> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
> Signed-off-by: Jason Wang <jasowang@...hat.com>
> ---
>  drivers/vhost/vhost.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 03666b702498..89c9f08b5146 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2074,6 +2074,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>  		d->has_notifier = false;
>  	}
>  
> +	/* reset invalidate_count in case we are in the middle of
> +	 * invalidate_start() and invalidate_end().
> +	 */
> +	vq->invalidate_count = 0;

I think that the code is ok but the comments are not very clear:
- we are never in the middle since we just removed the notifier
- the result is not just disabling optimization:
  if notifier becomes negative, then later we
  can think it's ok to map when it isn't since
  notifier is active.

>  	vhost_uninit_vq_maps(vq);
>  #endif
>  
> -- 
> 2.18.1