From: Andy Lutomirski <luto@kernel.org>
To: David Laight <David.Laight@aculab.com>
Cc: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
Tom Lendacky <Thomas.Lendacky@amd.com>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
John Stultz <john.stultz@linaro.org>
Subject: Re: [RFC PATCH 4/4] x86/TSC: Use RDTSCP
Date: Sat, 15 Dec 2018 10:53:32 -0800 [thread overview]
Message-ID: <CALCETrW6QgD4SLo7S+b+MgX4X4w1wAv1oJoi8px_q4+XGydyOQ@mail.gmail.com> (raw)
In-Reply-To: <d7d04973d6d54a969c9324ab3240b401@AcuMS.aculab.com>
On Fri, Dec 14, 2018 at 5:39 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Borislav Petkov
> > Sent: 12 December 2018 18:45
> ...
> > > The property I want for RDTSC ordering is much weaker: I want it to be
> > > ordered like a load. Imagine that, instead of an on-chip TSC, the TSC
> > > is literally a location in main memory that gets incremented by an
> > > extra dedicated CPU every nanosecond or so. I want users of RDTSC to
> > > work as if they were reading such a location in memory using an
> > > ordinary load. I believe this gives the real desired property that it
> > > should be impossible to observe the TSC going backwards. This is a
> > > much weaker form of serialization.
> >
> > Well, in that case you need something new.
> >
> > Because, the moment you have a RDTSC in flight and a second RDTSC comes
> > in and that second RDTSC must *not* bypass the first one and execute
> > earlier due to OoO, you need to impose some ordering. And that's pretty
> > much uarch-dependent, I'd say.
> >
> > And I guess on AMD the way to do that is to stop dispatch until the
> > first RDTSC retires.
> >
> > Can it be done faster? Sure. And I'm pretty sure there's a lot of pesky
> > little hw details we're not even hearing of, which get in the way.
>
> ISTR one of the problems with RDTSC serialising is that it is used
> for micro-benchmarks.
If you're benchmarking with that level of detail, you're probably
doing RDTSC directly instead of using the vDSO. Or, even better,
RDPMC.
next prev parent reply other threads:[~2018-12-15 18:53 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-11 22:23 [RFC PATCH 0/4] x86/alternative: Add ALTERNATIVE_3 Borislav Petkov
2018-12-11 22:23 ` [RFC PATCH 1/4] x86/alternatives: Add macro comments Borislav Petkov
2019-01-16 11:57 ` [tip:x86/alternatives] " tip-bot for Borislav Petkov
2018-12-11 22:23 ` [RFC PATCH 2/4] x86/alternatives: Print containing function Borislav Petkov
2019-01-16 11:58 ` [tip:x86/alternatives] " tip-bot for Borislav Petkov
2018-12-11 22:23 ` [RFC PATCH 3/4] x86/alternatives: Add an ALTERNATIVE_3() macro Borislav Petkov
2019-01-16 11:59 ` [tip:x86/alternatives] " tip-bot for Borislav Petkov
2018-12-11 22:23 ` [RFC PATCH 4/4] x86/TSC: Use RDTSCP Borislav Petkov
2018-12-11 22:59 ` Andy Lutomirski
2018-12-11 23:12 ` Lendacky, Thomas
2018-12-11 23:39 ` Borislav Petkov
2018-12-12 2:24 ` Andy Lutomirski
2018-12-12 9:59 ` Peter Zijlstra
2018-12-12 12:02 ` Andrea Parri
2018-12-12 10:08 ` Borislav Petkov
2018-12-12 18:07 ` Andy Lutomirski
2018-12-12 18:44 ` Borislav Petkov
2018-12-12 18:50 ` Andy Lutomirski
2018-12-12 20:00 ` Borislav Petkov
2018-12-12 20:09 ` Andy Lutomirski
2018-12-12 20:29 ` Borislav Petkov
2018-12-14 13:39 ` David Laight
2018-12-15 18:53 ` Andy Lutomirski [this message]
2018-12-12 14:15 ` Lendacky, Thomas
2018-12-12 14:18 ` Lendacky, Thomas
2018-12-11 23:37 Alexey Dobriyan
2018-12-11 23:43 ` Borislav Petkov
2018-12-12 0:06 ` Borislav Petkov
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