linux-kernel - Re: [PATCH] Skip tsc synchronization checks if CONSTANT

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Message-Id: <1224810764.21776.74.camel@alok-dev1>
Date:	Thu, 23 Oct 2008 18:12:43 -0700
From:	Alok Kataria <akataria@...are.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Daniel Hecht <dhecht@...are.com>
Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is
	set.

On Thu, 2008-10-23 at 16:47 -0700, H. Peter Anvin wrote:
> Alok Kataria wrote:
> >
> > I am ok with the CONSTANT_TSC bit check, but if people think that its
> > not important to skip this for native, i think adding a new flag to skip
> > this should be safe enough.
> >
> > Ingo, HPA your views on this whole detection and skipping thing ?
> >
> 
> Okay, first of all, I'm somewhat leery (to put it mildly) of trusting a
> CPUID bit to tell me a *system* property, which is that all cores in the
> system are synchronized.  The CPU designer will know that all the cores
> in the *package* are synchronized, but if that extends system-wide is a
> property beyond the CPU.  Now, if I'm not completely mistaken, in the
> case of AMD this bit is actually set by the BIOS via a magic MSR, but
> that doesn't mean it can't be wrong.
> 
> As far as skipping the check, it makes sense for me in the case of known
> virtualization platforms; a CPU feature bit, real or synthetic, is a
> very clean way to do that.  In general we should centralize CPU
> knowledge to arch/x86/kernel/cpu and have the code outside look for
> specific feature flags, and that applies to virtualization platforms, too.

I agree with the synthetic cpu feature thing.
Do you think i should use one of the existing word like the word 3 which
is for synthesized feature bits ? Or is it better to define a new
virtualization specific word ?

Thanks,
Alok

> 
>         -hpa

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