Subject: Re: [PATCH net-next 0/3] vhost: accelerate metadata access through vmap()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 25 Dec 2018 07:52:42 -0500

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.

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