Re: [RFC] KVM: x86: Allow userspace exit on HLT and MWAIT, else yield on MWAIT

[Peter Zijlstra]

On Mon, Sep 18, 2023 at 01:59:50PM +0200, Alexander Graf wrote:
> The problem with MWAIT is that you don't really know when it's done.

This isn't really a problem. MWAIT is allowed (expected even) to return
early.

REP;NOP is a valid implementation of MWAIT.

MWAIT must not delay waking (much) after either:

 - write to monitored address
 - interrupt pending

But it doesn't say anything about not waking up sooner.

Now, obviously on real hardware you prefer if MWAIT were to also do the
whole C-state thing and safe your some actual power, but this is virt,
real hardware is not a concern and wakeup-timeliness also not much.


IIRC the ARM64 WFE thing has a 10khz timer or something it wakes from if
nothing else. So I suppose what I'm saying is that: nanosleep(100000)
might be a suitable MWAIT implementation.

It's virt, it sucks anyway :-)