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

["H. Peter Anvin" <hpa@zytor.com>]

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