linux-kernel - Re: [PATCH V2 4/9] vhost: reset invalidate_count in vhost_set_vring_num

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Date:   Wed, 31 Jul 2019 09:41:24 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V2 4/9] vhost: reset invalidate_count in
 vhost_set_vring_num_addr()

On Wed, Jul 31, 2019 at 04:46:50AM -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>

Did Michael report this as well?

> 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 2a3154976277..2a7217c33668 100644
> +++ b/drivers/vhost/vhost.c
> @@ -2073,6 +2073,10 @@ 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;
>  	vhost_uninit_vq_maps(vq);
>  #endif
>