Re: [XEN PATCH] x86/msr: Allow hardware domain to read package C-state residency counters

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

Andrew Cooper:
> On 18/07/2023 2:17 pm, Simon Gaiser wrote:
>> Since it's limited to the hardware domain it should be safe and it's
>> very useful to have access to this directly in dom0 when debugging power
>> related things for example S0ix.
> 
> You need a SoB.

Yeah, sorry.

> But, this is an area there things are subtly broken.  For package-scope
> MSRs on single socket systems (which does include client systems), then
> this happens to function.
> 
> It does not function for core-scoped MSRs, or at all in a multi-socket
> system.  In such scenarios, dom0 can be rescheduled to a CPU in a
> different scope while it thinks it is sampling a single scope.
> 
> This is one of the areas where dom0 and Xen end up fighting over the system.
> 
> I agree that we want some way for dom0 to get this information, but I'm
> afraid it's not as simple as just permitting access to the MSRs like this.

I see. So a generic solution is not so easy. Also even if there would be
an interface for dom0, my main motivation was to be able to just use
existing code like /sys/kernel/debug/pmc_core/package_cstate_show and
turbostat. You can already read those PC-states via Xen's debug
interface, but that's less convenient.

For those package-scoped MSRs, how about limiting them to single-socket
systems?

Simon

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