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

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Message-ID: <49010D1E.8070400@zytor.com>
Date:	Thu, 23 Oct 2008 16:47:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.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.

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.

	-hpa
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