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

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

On 3/26/19 10:54 AM, Jan Beulich wrote:
>>>> 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
> 
> 
I did a rough example of how that might work and lines of code changed 
for adding it to cpu_idle was roughly 125.  Seeing as this doesn't 
compile and doesn't even have comments, I'd say at least 140 lines of 
code/change (most of those are additive too), a lot of is functionally 
copied from mwait-idle and how it reads data out of the structures, 
checks, and populates the cx structures.  The first set of mwait patches 
is 87 lines changed total.

I _could_ try and refactor some of the code and get it down from 
125-140, but that would most likely make porting changes even harder for 
mwait-idle.

What are your thoughts?

Brian
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