Re: [PATCH] Skip tsc synchronization checks if CONSTANT_TSC bit isset.

From: H. Peter Anvin
Date: Thu Oct 23 2008 - 19:48:23 EST

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.

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