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@xxxxxxxxxx>
> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> 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
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