Date: Wed, 22 Jul 2015 16:43:30 -0400 (EDT)
From: Mikulas Patocka <mpatocka@redhat.com>
To: Borislav Petkov <bp@alien8.de>
cc: Jason Baron <jasonbaron0@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Vince Weaver <vince@deater.net>, "hillf.zj" <hillf.zj@alibaba-inc.com>,
        Valdis Kletnieks <Valdis.Kletnieks@vt.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: Kernel broken on processors without performance counters
In-Reply-To: <20150722042403.GA6345@nazgul.tnic>
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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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