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.
-hpa
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