Re: [PATCH 4/6] vhost: reset invalidate_count in vhost_set_vring_num_addr()

[Jason Wang]

On 2019/7/23 äå5:17, Michael S. Tsirkin wrote:
> 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


If I read the code correctly, mmu_notifier_unregister() can only 
guarantee to wait for the pending method to complete. So we can have:

invalidate_start()

mmu_notifier_unregister()

invalidate_end()


> - 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.


I don't get how it could be negative, the only possible thing is to have 
a positive value.

Thanks


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