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

["Woods, Brian" <Brian.Woods@xxxxxxx>]

On 2/27/19 2:51 AM, Jan Beulich wrote:
>>>> On 26.02.19 at 17:54, <Brian.Woods@xxxxxxx> wrote:
>> On 2/26/19 10:37 AM, Jan Beulich wrote:
>>>>>> On 26.02.19 at 17:25, <Brian.Woods@xxxxxxx> wrote:
>>>> 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.
>>>
>>> For C2 and deeper (using entering methods other than HLT) - yes.
>>> The use of HLT is the default with the assumption that this will put
>>> the system in C1 (i.e. with a pretty low wakeup latency); see
>>> default_idle(), cpuidle_init_cpu(), and acpi_idle_do_entry().
>>
>> Well, assuming C2 is enabled (which I was assume is the default case),
>> HLT roughly puts the processor in C2 rather than C1.  On my test system,
>> the debug console output for the cx tables only output HLT for C1 (which
>> is wrong).
>>
>> Rather than depending on dom0, which is shaky, and not having an AML
>> interpreter, it seems the best solution is to hardcode the values in
>> like Intel does.  If Xen had an AML interpreter, I'd agree doing things
>> differently (reading in the ACPI tables) would be best.  But given the
>> resources Xen has at the moment, this seems like the safest solution and
>> is better than using HLT (which is C2 assuming it's enabled) as the
>> default idle method like Xen is using now.
>>
>> It comes down to sometimes (when C2 is diabled in BIOS) using C1
>> thinking it's C2, or without the patches in the common case using C2
>> thinking it's C1.
> 
> So in one of our idle routines, how would one go about entering
> C1 or C2 depending on wakeup latency requirements? I'm having a
> hard time seeing how HLT can be used for both (without a reboot
> cycle and a BIOS option change in between). Yet if there's only
> one state that can be entered, then it's merely cosmetic whether
> it gets called C1 or C2 in the debug output.
> 
> Anyway - I guess we need to continue this discussion (if necessary)
> once I got around to actually look at the patches.
> 
> Jan
> 
> Does this answer the questions?

C2/CC6 enabled (default our internal MBs [and in general I'd assume])
HLT -> C2/CC6*
mwait -> C1/CC1

C2/CC6 disabled in BIOS
HLT -> C1/CC1
mwait -> C1/CC1

* HLT doesn't directly call C2/CC6 but it has a small timer then flushes 
the caches and puts it in C2/CC6.  Effectively it's the same but not 
exactly.

Brian
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