Re: [Xen-devel] [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware

[Boris Ostrovsky <boris.ostrovsky@xxxxxxx>]

On 04/20/12 04:45, Keir Fraser wrote:
> On 20/04/2012 09:15, "Jan Beulich"<JBeulich@xxxxxxxx>  wrote:
>
> > > svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware
> > >
> > > When running in TSC_MODE_ALWAYS_EMULATE mode on processors that support
> > > TSC scaling we don't need to intercept RDTSC/RDTSCP instructions.
> > While the patch itself looks fine, I'm having difficulty to connect the
> > mentioning of TSC_MODE_ALWAYS_EMULATE to it - afaics all modes
> > are affected as long as they result in d->arch.vtsc to be set.
> I think the real point of this patch is they *never* want to trap and
> emulate RDTSC on these newer processors.

Right. Mentioning TSC_MODE_ALWAYS_EMULATE explicitly wasn't probably a 
particularly good idea.
The reason I did that was because with TSC_MODE_DEFAULT guests (at least 
Linux guests) typically don't use TSC since TSC ends up being declared 
non-invariant.
-boris


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