Re: [Xen-devel] RE: Saving/Restoring IA32_TSC_AUX MSR

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 12/13/09 10:06, Dan Magenheimer wrote:
> I agree there are some cases where the TSC_AUX value
> set by a guest OS may be useful.  But ensuring that its
> is always useful (NEVER incorrect) requires too many restrictions,
> such as pinning.
>    
At least with respect to Linux guests [*], this objection to rdtscp is 
moot, because if it isn't present then Linux will fall back to another 
mechanism which is always present.  Guest usermode will get the same 
info, good/bad/misleading/whatever, either way; rdtscp can't make it 
worse.  The only question is whether specifically adding rdtscp/TSC_AUX 
support adds any overall improvement.
(* I don't know if any other rdtscp-users attempt to put NUMA or other 
physical topology info into TSC_AUX.  If they just stick to 
setting/using the cpu number, then they will get a net win from rdtscp.)
    J

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