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

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Message-ID: <20230919090446.GC21729@noisy.programming.kicks-ass.net>
Date:   Tue, 19 Sep 2023 11:04:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Graf <graf@...zon.de>
Cc:     David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Nicolas Saenz Julienne <nsaenz@...zon.es>,
        "Griffoul, Fred" <fgriffo@...zon.com>
Subject: Re: [RFC] KVM: x86: Allow userspace exit on HLT and MWAIT, else
 yield on MWAIT

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 :-)