On Sat, Aug 03, 2019 at 05:36:13PM -0400, Michael S. Tsirkin wrote: > On Fri, Aug 02, 2019 at 02:24:18PM -0300, Jason Gunthorpe wrote: > > On Fri, Aug 02, 2019 at 10:27:21AM -0400, 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. > > > > Yikes, I'm not sure you can ever win against copy_from_user using > > mmu_notifiers? > > Ever since copy_from_user started playing with flags (for SMAP) and > added speculation barriers there's a chance we can win by accessing > memory through the kernel address. You think copy_to_user will be more expensive than the minimum two atomics required to synchronize with another thread? > > Also, why can't this just permanently GUP the pages? In fact, where > > does it put_page them anyhow? Worrying that 7f466 adds a get_user page > > but does not add a put_page?? You didn't answer this.. Why not just use GUP? Jason _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization