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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 27.03.19 at 18:28, <Brian.Woods@xxxxxxx> wrote:
> This also lacks some of the features of mwait-idle has and duplicates 
> the limited functionality.

Would you mind clarifying the lack-of-features aspect? The
only difference to your patches that I can spot is that you set
.target_residency in the static tables. If the value wanted
for CC6 is really 1000 instead of the 800 the default
calculation would produce, then this would be easy enough
to arrange for in my variant of the patch as well.

The mwait-idle driver would not have been needed at all if all
BIOSes surfaced complete data via ACPI. Therefore, by
suitably populating tables, it ought to be possible in theory to
use just that one driver. It's just that for Intel CPUs we've
decided to follow what Linux does, hence the separate
driver. There's no Linux precedent for AMD (afaict).

>  There's also a lack of comments which may or 
> may not be needed.  So that would add to the line change count if you 
> care about that.
> 
> I'm not sure why you're so adverse to the mwait-idle patches.  We're 
> hard coding values in and using mwait (just like Intel is), but the only 
> real change we need is using halt for one c-state.

But that's precisely what I dislike, as getting us further away
from the Linux driver. And btw, if we were to go that route,
then I think we'd better call acpi_idle_do_entry() than to
duplicate further parts of it. But that would also remove some
of that other part of the benefits of mwait_idle() over
acpi_processor_idle(): The former is much more streamlined,
due to not having to care about anything other than MWAIT.

As an aside, despite having followed the HLT approach in my
draft patch, I continue to be unconvinced that this is what we
actually want. There's a respective TBD remark there.

Jan



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel