Re: [RFC PATCH 2/4] x86, mwaitt: introduce mwaitx idle with a configurable timer

Andy Lutomirski
2015-05-27T01:01:27Z

On Wed, May 20, 2015 at 10:48 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On May 20, 2015 6:34 PM, "Andy Lutomirski" <luto@kernel.org> wrote:
>> If we did that *and* we had a non-crappy mwaitx, then we could apply an optimization: when going idle, we could turn off the TSC deadline timer and use mwaitx instead. This would about an interrupt if the event that wakes us is our timer.
>>
>
> Hey, Intel, want to document your secret "Timed MWAIT" feature? It
> causes a transition to C0 when the deadline expires (see 4.2.4 of the
> Desktop 4th Generation Intel Core Processor Family Datasheet Volume 1,
> order number 328897-001) and it even has an erratum (HSD63 / BDM32),
> but the instruction itself doesn't appear to be documented.
>

Found more:

https://chromium-review.googlesource.com/#/c/205161/

Oddly, Coreboot seems to have mis-spelled that MSR. It's
MSR_PKG_CST_CONFIG_CONTROL, and bit 31 isn't defined in the SDM
(unsurprisingly).

--Andy