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

From: Jason Wang
Date: Mon Aug 05 2019 - 00:41:53 EST

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

I start with synchronize_rcu() but both you and Michael raise some
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
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?


Now as long as all this code is disabled anyway, we can experiment a

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

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.


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