linux-kernel - Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

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Message-ID: <20140618185939.GF1695@ERROL.INI.CMU.EDU>
Date:	Wed, 18 Jun 2014 14:59:39 -0400
From:	"Gabriel L. Somlo" <gsomlo@...il.com>
To:	Eric Northup <digitaleric@...gle.com>
Cc:	Nadav Amit <nadav.amit@...il.com>,
	Nadav Amit <namit@...technion.ac.il>,
	Paolo Bonzini <pbonzini@...hat.com>, gleb@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>, joro@...tes.org, mst@...hat.com,
	agraf@...e.de
Subject: Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

On Wed, Jun 18, 2014 at 11:30:07AM -0700, Eric Northup wrote:
> Quoting Gabriel's post http://www.spinics.net/lists/kvm/msg103792.html :
> 
> [...]
> 
> > E.g., OS X 10.5 *does* check CPUID, and panics if it doesn't find it.
> > It needs the MONITOR cpuid flag to be on, *and* the actual
> > instructions to work.

That was an argument in favor of finding a mechanism to allow (qemu)
users to enable an otherwise default-off monitor cpuid flag.

We definitely don't want to advertise monitor/mwait availability to
guests which would otherwise sanely fail back to a hlt-based idle loop
when cpuid tells them monitor/mwait are not available :)

However, check my earlier proposal of backing out of monitor/mwait
entirely (as it turns out, there's a kernel command line to tell OS X
not to use monitor/mwait, which is IMHO vastly preferable to creating
"undocumented" KVM hacks :)

Thanks much,
--Gabriel
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