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Re: [XEN PATCH] x86/msr: Allow hardware domain to read package C-state residency counters
- To: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 18 Jul 2023 14:25:11 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
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.
~Andrew
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