Subject: Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit is
	set.
From: Alok Kataria <akataria@vmware.com>
Reply-To: akataria@vmware.com
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <andi@firstfloor.org>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>,
       the arch/x86 maintainers <x86@kernel.org>,
       Daniel Hecht <dhecht@vmware.com>
In-Reply-To: <49010D1E.8070400@zytor.com>
References: <20081021181536.GI12825@one.firstfloor.org>
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	 <1224712846.13953.37.camel@alok-dev1>
	 <20081022221316.GW12825@one.firstfloor.org>
	 <1224713518.13953.46.camel@alok-dev1>
	 <20081022225409.GB27492@one.firstfloor.org>
	 <1224728478.13953.79.camel@alok-dev1>
	 <20081023081052.GI27492@one.firstfloor.org>
	 <1224805162.21776.45.camel@alok-dev1>  <49010D1E.8070400@zytor.com>
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Organization: VMware INC.
Date: Thu, 23 Oct 2008 18:12:43 -0700
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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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