linux-kernel - Re: [PATCH] x86, TSC: Add a software TSC offset

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Date:	Tue, 22 Jul 2014 00:08:25 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] x86, TSC: Add a software TSC offset

On Mon, Jul 21, 2014 at 02:56:49PM -0700, Andy Lutomirski wrote:
> I expect that users of __vdso_clock_gettime (e.g. glibc) will get the
> correct time :)  They use vread_tsc, and they can't use
> preempt_disable, because they're in userspace.  They also can't
> directly access per-cpu variables.
> 
> Turning off vdso tsc support on these machines would be an option.

Right, this is what I was going to propose to tglx on IRC. Or we can try
to come up with something working for the vdso too, say RDTSCP :-)

But that still won't work as it needs the per-cpu variables. So I guess
vdso loses...

> I actually own one of these systems.  It's a Sandy Bridge Core-i7
> Extreme or something like that.

Ha, cool, so I've got my tester! :-)

> I wonder if that's a bug in get_cycles.
> 
> The basic issue is that rdtsc is not ordered with respect to nearby
> loads, so it's fairly easy to see it behaving non-monotonically across
> CPUs.  rdtscp is ordered, but it's a little slower.

Yah, that I know. But I don't see get_cycles() having the barriers. So
it might be a bug. We can certainly try to "fix" it and see what happens
:-)

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 94605c0e9cee..ad7d5e449c0b 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -27,7 +27,9 @@ static inline cycles_t get_cycles(void)
 	if (!cpu_has_tsc)
 		return 0;
 #endif
+	rdtsc_barrier();
 	rdtscll(ret);
+	rdtsc_barrier();
 
 	return ret;
 }

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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