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

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Message-ID: <alpine.LRH.2.02.1507221641310.31598@file01.intranet.prod.int.rdu2.redhat.com>
Date:	Wed, 22 Jul 2015 16:43:30 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
cc:	Jason Baron <jasonbaron0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	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 Wed, 22 Jul 2015, Borislav Petkov wrote:

> 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.

RDTSC doesn't work on 486...

Mikulas
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