On Tue, Dec 25, 2018 at 06:09:19PM +0800, Jason Wang wrote: > > On 2018/12/25 上午2:12, Michael S. Tsirkin wrote: > > On Mon, Dec 24, 2018 at 04:32:39PM +0800, Jason Wang wrote: > > > On 2018/12/14 下午8:33, Michael S. Tsirkin wrote: > > > > On Fri, Dec 14, 2018 at 11:42:18AM +0800, Jason Wang wrote: > > > > > On 2018/12/13 下午11:27, Michael S. Tsirkin wrote: > > > > > > On Thu, Dec 13, 2018 at 06:10:19PM +0800, Jason Wang wrote: > > > > > > > Hi: > > > > > > > > > > > > > > This series tries to access virtqueue metadata through kernel virtual > > > > > > > address instead of copy_user() friends since they had too much > > > > > > > overheads like checks, spec barriers or even hardware feature > > > > > > > toggling. > > > > > > Userspace accesses through remapping tricks and next time there's a need > > > > > > for a new barrier we are left to figure it out by ourselves. > > > > > I don't get here, do you mean spec barriers? > > > > I mean the next barrier people decide to put into userspace > > > > memory accesses. > > > > > > > > > It's completely unnecessary for > > > > > vhost which is kernel thread. > > > > It's defence in depth. Take a look at the commit that added them. > > > > And yes quite possibly in most cases we actually have a spec > > > > barrier in the validation phase. If we do let's use the > > > > unsafe variants so they can be found. > > > > > > unsafe variants can only work if you can batch userspace access. This is not > > > necessarily the case for light load. > > > > Do we care a lot about the light load? How would you benchmark it? > > > > If we can reduce the latency that's will be more than what we expect. > > 1 byte TCP_RR shows 1.5%-2% improvement. It's nice but not great. E.g. adaptive polling would be a better approach to work on latency imho. > > > > > > And even if you're right, vhost is not the > > > > > only place, there's lots of vmap() based accessing in kernel. > > > > For sure. But if one can get by without get user pages, one > > > > really should. Witness recently uncovered mess with file > > > > backed storage. > > > > > > We only pin metadata pages, I don't believe they will be used by any DMA. > > It doesn't matter really, if you dirty pages behind the MM back > > the problem is there. > > > Ok, but the usual case is anonymous pages, do we use file backed pages for > user of vhost? Some people use file backed pages for vms. Nothing prevents them from using vhost as well. > And even if we use sometime, according to the pointer it's > not something that can fix, RFC has been posted to solve this issue. > > Thanks Except it's not broken if we don't to gup + write. So yea, wait for rfc to be merged.
