linux-kernel - Re: [RFC PATCH V3 0/5] Hi:

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Message-ID: <CAPcyv4iQMQxZC8ehr2OP8_rqpuCeSWn5xyKN64JSyfR8_r7eOg@mail.gmail.com>
Date:   Mon, 7 Jan 2019 14:44:24 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, KVM list <kvm@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Netdev <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH V3 0/5] Hi:

On Mon, Jan 7, 2019 at 2:25 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Jan 07, 2019 at 01:39:15PM -0800, Dan Williams wrote:
> > On Mon, Jan 7, 2019 at 6:11 AM Michael S. Tsirkin <mst@...hat.com> wrote:
> > >
> > > On Sun, Jan 06, 2019 at 11:15:20PM -0800, Dan Williams wrote:
> > > > On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@...hat.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.
> > >
> > > Hmm it's way cheaper even though IIRC it's measureable.
> > > Jason, would you like to try?
> > > Although frankly __get_user being slower than get_user feels very wrong.
> > > Not yet sure what to do exactly but would you agree?
> >
> > Agree. __get_user() being faster than get_user() defeats the whole
> > point of converting code paths to the access_ok() + __get_user()
> > pattern.
>
> Did you mean the reverse?

Hmm, no... I'll rephrase: __get_user() should have lower overhead than
get_user().