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Re: [Xen-devel] [PATCH 0/3] mwait support for AMD processors

On 2/26/19 4:49 AM, Jan Beulich wrote:
>>>> On 25.02.19 at 21:23, <Brian.Woods@xxxxxxx> wrote:
>> This patch series add support and enablement for mwait on AMD Naples
>> and Rome processors.  Newer AMD processors support mwait, but only for
>> c1, and for c2 halt is used.  The mwait-idle driver is modified to be
>> able to use both mwait and halt for idling.
> I recall you saying so elsewhere, but I continue to be confused. Afaik
> HLT is specified to mean C1. Without having looked at the patches,
> I'm also not happy to see you say you make the driver capable of using
> HLT. That's not its purpose, and I think the ACPI driver should instead
> be used for that.
> It is my understanding that the driver is there solely to overcome
> recurring issues with BIOSes not providing optimal (or even correct)
> ACPI tables. Since for C1 we don't even need any ACPI tables (we
> enter C1 [through HLT] whenever no other C states are defined),
> I'm having trouble seeing what problem would be addressed here.
> Are there really no deeper C states than C2 supported by your CPUs?
> Jan

On Naples/Rome HLT can mean different things depending on how the 
HW/BIOS is set up.  If C2/CC6 is enabled, HLT (after going through some 
checks etc), will put the system in a C2/CC6 state.  If C2/CC6 is 
disabled in BIOS, then HLT will act as C1/CC1. It may not be completely 
ideal but the system will function just fine.  It's the best that can be 
done without reading the tables (that I can think of).

Correct me if I'm wrong, but the Xen's acpi-idle implementation is 
dependent on dom0 using a AML interpreter and then giving that data back 
to Xen.  I've heard that this doesn't always work correctly on PV dom0s 
and doesn't work at all on PVH dom0s.

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