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

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Message-ID: <20140619112356.GB429@minantech.com>
Date:	Thu, 19 Jun 2014 14:23:56 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Nadav Amit <nadav.amit@...il.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	"Gabriel L. Somlo" <gsomlo@...il.com>,
	Eric Northup <digitaleric@...gle.com>,
	Nadav Amit <namit@...technion.ac.il>,
	Paolo Bonzini <pbonzini@...hat.com>,
	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, agraf@...e.de
Subject: Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

On Thu, Jun 19, 2014 at 01:53:36PM +0300, Nadav Amit wrote:
> 
> On Jun 19, 2014, at 1:18 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> 
> > On Wed, Jun 18, 2014 at 02:46:01PM -0400, Gabriel L. Somlo wrote:
> >> On Wed, Jun 18, 2014 at 10:59:14AM -0700, Eric Northup wrote:
> >>> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@...technion.ac.il> wrote:
> >>>> mwait and monitor are currently handled as nop. Considering this behavior, they
> >>>> should still be handled correctly, i.e., check execution conditions and generate
> >>>> exceptions when required. mwait and monitor may also be executed in real-mode
> >>>> and are not handled in that case.  This patch performs the emulation of
> >>>> monitor-mwait according to Intel SDM (other than checking whether interrupt can
> >>>> be used as a break event).
> >>>> 
> >>>> Signed-off-by: Nadav Amit <namit@...technion.ac.il>
> >> 
> >> How about this instead (details in the commit log below) ? Please let
> >> me know what you think, and if you'd prefer me to send it out as a
> >> separate patch rather than a reply to this thread.
> >> 
> >> Thanks,
> >> --Gabriel
> > 
> > If there's an easy workaround, I'm inclined to agree.
> > We can always go back to Gabriel's patch (and then we'll need
> > Nadav's one too) but if we release a kernel with this
> > support it becomes an ABI and we can't go back.
> > 
> > So let's be careful here, and revert the hack for 3.16.
> > 
> > 
> > Acked-by: Michael S. Tsirkin <mst@...hat.com>
> > 
> Personally, I got a custom guest which requires mwait for executing correctly.
Can you elaborate on this guest a little bit. With nop implementation
for mwait the guest will hog a host cpu. Do you consider this to be
"executing correctly?"

--
			Gleb.
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