Re: [RFC PATCH V3 0/5] Hi:

Dan Williams
2019-01-07T07:15:20Z

On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote:
> >
> > On 2019/1/7 上午11:28, Michael S. Tsirkin wrote:
> > > On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
> > > > On 2019/1/3 上午4:47, Michael S. Tsirkin wrote:
> > > > > On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
> > > > > > 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.
> > > > > Will review, thanks!
> > > > > One questions that comes to mind is whether it's all about bypassing
> > > > > stac/clac. Could you please include a performance comparison with
> > > > > nosmap?
> > > > >
> > > > On machine without SMAP (Sandy Bridge):
> > > >
> > > > Before: 4.8Mpps
> > > >
> > > > After: 5.2Mpps
> > > OK so would you say it's really unsafe versus safe accesses?
> > > Or would you say it's just a better written code?
> >
> >
> > It's the effect of removing speculation barrier.
>
>
> You mean __uaccess_begin_nospec introduced by
> commit 304ec1b050310548db33063e567123fae8fd0301
> ?
>
> So fundamentally we do access_ok checks when supplying
> the memory table to the kernel thread, and we should
> do the spec barrier there.
>
> Then we can just create and use a variant of uaccess macros that does
> not include the barrier?
>
> Or, how about moving the barrier into access_ok?
> This way repeated accesses with a single access_ok get a bit faster.
> CC Dan Williams on this idea.

It would be interesting to see how expensive re-doing the address
limit check is compared to the speculation barrier. I.e. just switch
vhost_get_user() to use get_user() rather than __get_user(). That will
sanitize the pointer in the speculative path without a barrier.

I recall we had a convert access_ok() discussion with this result here:

https://lkml.org/lkml/2018/1/17/929

...but it sounds like you are proposing a smaller scope fixup for the
vhost use case? Something like barrier_nospec() in the success path
for all vhost access_ok() checks and then a get_user() variant that
disables the barrier.