linux-kernel - Re: [PATCH] KVM: Handle MSR_IA32_PERF

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Message-ID: <20160526204439.GF2186@HEDWIG.INI.CMU.EDU>
Date:	Thu, 26 May 2016 16:44:39 -0400
From:	"Gabriel L. Somlo" <gsomlo@...il.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	kmeaw@...dex-team.ru, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, gleb@...nel.org, pbonzini@...hat.com
Subject: Re: [PATCH] KVM: Handle MSR_IA32_PERF_CTL

On Thu, May 26, 2016 at 10:39:31PM +0200, Radim Krčmář wrote:
> 2016-05-26 10:32+0300, kmeaw@...dex-team.ru:
> > From: Dmitry Bilunov <kmeaw@...dex-team.ru>
> > 
> > Intel CPUs having Turbo Boost feature implement an MSR to provide a
> > control interface via rdmsr/wrmsr instructions. One could detect the
> > presence of this feature by issuing one of these instructions and
> > handling the #GP exception which is generated in case the referenced MSR
> > is not implemented by the CPU.
> > 
> > KVM's vCPU model behaves exactly as a real CPU in this case by injecting
> > a fault when MSR_IA32_PERF_CTL is called (which KVM does not support).
> > However, some operating systems use this register during an early boot
> > stage in which their kernel is not capable of handling #GP correctly,
> > causing #DP and finally a triple fault effectively resetting the vCPU.
> > 
> > This patch implements a dummy handler for MSR_IA32_PERF_CTL to avoid the
> > crashes. Most notably it fixes an issue with MacOS X 10.10 kernel.
> > 
> > Signed-off-by: Dmitry Bilunov <kmeaw@...dex-team.ru>
> > ---
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > @@ -2050,6 +2051,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> >         case MSR_VM_HSAVE_PA:
> >         case MSR_AMD64_PATCH_LOADER:
> >         case MSR_AMD64_BU_CFG2:
> > +       case MSR_IA32_PERF_CTL:
> >                 break;
> 
> Does MacOS X write it too?

After setting /sys/module/kvm/parameters/ignore_msrs, all I get in
dmesg after firing up OS X is:

	vcpu0 ignored rdmsr: 0x199

So no, I don't think it would try to write it.

HTH,
--Gabriel