linux-kernel - Re: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks

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Date:   Thu, 11 Oct 2018 10:37:07 -0700
From:   Andres Freund <andres@...razel.de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Davidlohr Bueso <dbueso@...e.de>,
        virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v2 0/3] x86: faster mb()+other barrier.h tweaks

Hi,

On 2016-01-26 10:20:14 +0200, Michael S. Tsirkin wrote:
> On Tue, Jan 12, 2016 at 02:25:24PM -0800, H. Peter Anvin wrote:
> > On 01/12/16 14:10, Michael S. Tsirkin wrote:
> > > mb() typically uses mfence on modern x86, but a micro-benchmark shows that it's
> > > 2 to 3 times slower than lock; addl $0,(%%e/rsp) that we use on older CPUs.
> > > 
> > > So let's use the locked variant everywhere - helps keep the code simple as
> > > well.
> > > 
> > > While I was at it, I found some inconsistencies in comments in
> > > arch/x86/include/asm/barrier.h
> > > 
> > > I hope I'm not splitting this up too much - the reason is I wanted to isolate
> > > the code changes (that people might want to test for performance) from comment
> > > changes approved by Linus, from (so far unreviewed) comment change I came up
> > > with myself.
> > > 
> > > Lightly tested on my system.
> > > 
> > > Michael S. Tsirkin (3):
> > >   x86: drop mfence in favor of lock+addl
> > >   x86: drop a comment left over from X86_OOSTORE
> > >   x86: tweak the comment about use of wmb for IO
> > > 
> > 
> > I would like to get feedback from the hardware team about the
> > implications of this change, first.

> Any luck getting some feedback on this one?

Ping?  I just saw a bunch of kernel fences in a benchmark, making me
wonder why linux uses mfence rather than lock addl. Leading me to this
thread.

Greetings,

Andres Freund