linux-kernel - Re: Kernel broken on processors without performance counters

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Date:	Wed, 22 Jul 2015 06:24:03 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Jason Baron <jasonbaron0@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Vince Weaver <vince@...ter.net>,
	"hillf.zj" <hillf.zj@...baba-inc.com>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Kernel broken on processors without performance counters

On Tue, Jul 21, 2015 at 02:50:25PM -0400, Jason Baron wrote:
> hmmm...so this is a case where need to the default the branch
> to the out-of-line branch at boot. That is, we can't just enable
> the out-of-line branch at boot time, b/c it might be too late at
> that point? IE native_sched_clock() gets called very early?

Well, even the layout is wrong here. The optimal thing would be to have:

	NOP
	rdtsc

unlikely:
	/* read jiffies */

at build time. And then at boot time, patch in the JMP over the NOP on
!use_tsc boxes. And RDTSC works always, no matter how early.

I'm fairly sure we can do that now with alternatives instead of jump
labels.

The problem currently is that the 5-byte NOP gets patched in with a JMP
so we have an unconditional forwards JMP to the RDTSC.

Now I'd put my money on most arches handling NOPs better then
unconditional JMPs and this is a hot path...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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