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

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Date:   Wed, 15 Mar 2017 20:29:23 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     "Gabriel L. Somlo" <gsomlo@...il.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        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,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH] kvm: better MWAIT emulation for guests

On Wed, Mar 15, 2017 at 02:14:26PM -0400, Gabriel L. Somlo wrote:
> Michael,
> 
> I tested this on OS X 10.7 (Lion), the last version that doesn't check
> CPUID for MWAIT support.
> 
> I used the latest kvm from git://git.kernel.org/pub/scm/virt/kvm/kvm.git
> first as-is, then with your v2 MWAIT patch applied.
> 
> Single-(V)CPU guest works as expected (but then again, single-vcpu
> guests worked even back when I tried emulating MWAIT the same as HLT).
> 
> When I try starting a SMP guest (with "-smp 4,cores=2"), the guest OS
> hangs after generating some output in text/verbose boot mode -- I gave
> up waiting for it after about 5 minutes. Works fine before your patch,
> which leads me to suspect that, as I feared, MWAIT doesn't wake
> immediately upon another VCPU writing to the MONITOR-ed memory location.
> 
> Tangentially, I remember back in the days of OS X 10.7, the
> alternative to exiting guest mode and emulating MWAIT and MONITOR as
> NOPs was to allow them both to run in guest mode.
> 
> While poorly documented by Intel at the time, MWAIT at L>0 effectively
> behaves as a NOP (i.e., doesn't actually put the physical core into
> low-power mode, because doing that would allow a guest to effectively
> DOS the host hardware).

Thanks for the testing, interesting.
Testing with Linux guest seems to show it works.
This could be an interrupt thing not a monitor thing.
Question: does your host CPU have this in its MWAIT leaf?
	Bit 01: Supports treating interrupts as break-event for MWAIT, even when interrupts disabled

We really should check that before enabling,
I'll add that.

> 
> Given how unusual it is for a guest to use MONITOR/MWAIT in the first
> place, what's wrong with leaving it all as is (i.e., emulated as NOP)?
> 
> Thanks,
> --Gabriel

I'm really looking into ways to use mwait within Linux guests,
this is just a building block that should help Mac OSX
as a side effect (and we do not want it broken if at all possible).

-- 
MST