Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

[Jason Wang]

On 2019/8/5 äå12:36, Jason Wang wrote:
> On 2019/8/2 äå10:27, Michael S. Tsirkin wrote:
> > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote:
> > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
> > > > > This must be a proper barrier, like a spinlock, mutex, or
> > > > > synchronize_rcu.
> > > >
> > > > I start with synchronize_rcu() but both you and Michael raise some
> > > > concern.
> > > I've also idly wondered if calling synchronize_rcu() under the various
> > > mm locks is a deadlock situation.
> > >
> > > > Then I try spinlock and mutex:
> > > >
> > > > 1) spinlock: add lots of overhead on datapath, this leads 0 
> > > > performance
> > > > improvement.
> > > I think the topic here is correctness not performance improvement
> > The topic is whether we should revert
> > commit 7f466032dc9 ("vhost: access vq metadata through kernel virtual 
> > address")
> >
> > or keep it in. The only reason to keep it is performance.
>
>
> Maybe it's time to introduce the config option?


Or does it make sense if I post a V3 with:

- introduce config option and disable the optimization by default

- switch from synchronize_rcu() to vhost_flush_work(), but the rest are 
the same

This can give us some breath to decide which way should go for next release?

Thanks


> > Now as long as all this code is disabled anyway, we can experiment a
> > bit.
> >
> > I personally feel we would be best served by having two code paths:
> >
> > - Access to VM memory directly mapped into kernel
> > - Access to userspace
> >
> >
> > Having it all cleanly split will allow a bunch of optimizations, for
> > example for years now we planned to be able to process an incoming short
> > packet directly on softirq path, or an outgoing on directly within
> > eventfd.
>
>
> It's not hard consider we've already had our own accssors. But the 
> question is (as asked in another thread), do you want permanent GUP or 
> still use MMU notifiers.
>
> Thanks
>
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