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Re: [PATCH RFC 2/2] KVM: x86/vPMU: ignore access to LBR-related MSRs




On Wed, Dec 06, 2017 at 08:02:07PM +0300, Jan Dakinevich wrote:
> On Wed, 6 Dec 2017 07:57:28 -0800
> Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> 
> > 
> > If you do all this it's only a small step to fully enable LBRs for
> > guests. 
> 
> It is quite simple in a case where guest LBR-related MSRs matches host
> ones. They could be handled by MSR load/store areas, I suppose.

There is already a LBR control to enable/disable I believe.
You don't want to save/restore all MSRs on every entry/exit 
because that would be slow. The normal Linux context switch can do it.
 
> 
> In other cases, it could be expected the different amount of these MSRs
> and different theirs base values (e.g. Nehalem vs Core). Guest MSRs
> could be both subset and superset of host MSRs, so additional efforts
> to support this would be required.

In this case ignoring would be sufficient I suppose. But for the case
when everything matches it should work.

-Andi