Mwait usage on AMD processors

[Huang Rui]

Hi Len, Rafael, and all,

AMD proposed a new instruction named mwaitx. This is an extension of
mwait with a configurable timer (mwaitx = mwait + timer). And mwaitx
will act as mwait if timer is disabled. However, mwait/mwaitx cannot
let cpu core go to C1 state at current AMD processors, but has less
power consumption even at C0 while core is waiting.
As you know, mwait/mwaitx would have better performance than halt. So
could we propose an implementation to use mwaitx at idle call in boot
phase and cpuidle driver after boot phase. And the mwaitx idle is
exposed to user as an optional kernel parameter(idle=...), and decided
by user.

Any comments are warm for me :)

Thanks,
Rui
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