linux-kernel - Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

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Date:   Tue, 21 Mar 2017 10:29:32 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "Gabriel L. Somlo" <gsomlo@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests


> On Mar 21, 2017, at 9:58 AM, Radim Krčmář <rkrcmar@...hat.com> wrote:

> In '-smp 2', the writing VCPU always does 10000 wakeups by writing into
> monitored memory, but the mwaiting VCPU can be also woken up by host
> interrupts, which might add a few exits depending on timing.
> 
> I didn't spend much time in making the PASS/FAIL mean much, or ensuring
> that we only get 10000 wakeups ... it is nothing to be worried about.
> 
> Hint 240 behaves as nop even on my system, so I still don't find
> anything insane on that machine (if OS X is exluded) ...

From my days in Intel (10 years ago), I can say that MWAIT wakes for many
microarchitecural events beside interrupts.

Out of curiosity, aren’t you worried that on OS X the wbinvd causes an exit
after the monitor and before the mwait?