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

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Message-ID: <20160527152845.GB11721@potion>
Date:	Fri, 27 May 2016 17:28:46 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	kmeaw@...dex-team.ru
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, gleb@...nel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH] KVM: Handle MSR_IA32_PERF_CTL

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
> @@ -983,6 +983,7 @@ static u32 emulated_msrs[] = {
>         MSR_IA32_MCG_STATUS,
>         MSR_IA32_MCG_CTL,
>         MSR_IA32_SMBASE,
> +       MSR_IA32_PERF_CTL,

The MSR value is always 0, so there is no point in putting it here.