Re: [Xen-devel] [PATCH 1/3] mwait-idle: add support for using halt

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.03.19 at 17:12, <Brian.Woods@xxxxxxx> wrote:
> On 3/15/19 3:37 AM, Jan Beulich wrote:
>> Furthermore I'm then once again wondering what the gain is
>> over using the ACPI driver: The suggested _CST looks to exactly
>> match the data you enter into the table in the later patch. IOW
>> my fundamental concern didn't go away yet: As per the name
>> of the driver, it shouldn't really need to support HLT (or anything
>> other than MWAIT) as an entry method. Hence I think that at
>> the very least you need to extend the description of the change
>> quite a bit to explain why the ACPI driver is not suitable.
>> 
>> Depending on how this comes out, it may then still be a matter
>> of discussing whether, rather than fiddling with mwait-idle, it
>> wouldn't be better to have an AMD-specific driver instead. Are
>> there any thoughts in similar directions for Linux?
> 
> Because:
> #1 getting the ACPI tables from dom0 is either unreliable (PV dom0) or 
> not possible (PVH dom0).
> #2 the changes to the Intel code are minimal.
> #3 worse case, Xen thinks it's using CC6 when it's using CC1.  Not 
> perfect but far from fatal or breaking.

Having thought about this some more, I agree that an AMD-specific
driver would likely go too far. However, that's still no reason to fiddle
with the mwait-idle one - I think you could as well populate the data
as necessary for the ACPI driver to use, removing the dependency
on Dom0. After all that driver already knows of all the entry methods
you may want/need to use (see acpi_idle_do_entry()).

Jan



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