Re: [PATCH v4 2/5] x86/umwait: Initialize umwait control values

From: Peter Zijlstra
Date: Tue Jun 11 2019 - 04:55:27 EST

On Fri, Jun 07, 2019 at 03:00:34PM -0700, Fenghua Yu wrote:
> umwait or tpause allows processor to enter a light-weight
> power/performance optimized state (C0.1 state) or an improved
> power/performance optimized state (C0.2 state) for a period
> specified by the instruction or until the system time limit or until
> a store to the monitored address range in umwait.
> IA32_UMWAIT_CONTROL MSR register allows kernel to enable/disable C0.2
> on the processor and set maximum time the processor can reside in
> C0.1 or C0.2.
> By default C0.2 is enabled so the user wait instructions can enter the
> C0.2 state to save more power with slower wakeup time.
> Default maximum umwait time is 100000 cycles. A later patch provides
> a sysfs interface to adjust this value.
> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Reviewed-by: Ashok Raj <ashok.raj@xxxxxxxxx>
> Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> arch/x86/include/asm/msr-index.h | 4 +++
> arch/x86/power/Makefile | 1 +
> arch/x86/power/umwait.c | 56 ++++++++++++++++++++++++++++++++

Why is this in power/, this isn't in the least related to
suspend/hybernate. arch/x86/kernel/cpu/ might be a better place for
instruction support.