linux-kernel - [PATCH] KVM: Handle MSR_IA32_PERF

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Date:	Tue, 31 May 2016 17:38:24 +0300
From:	Dmitry Bilunov <kmeaw@...dex-team.ru>
To:	rkrcmar@...hat.com
Cc:	pbonzini@...hat.com, gsomlo@...il.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, gleb@...nel.org,
	kmeaw@...dex-team.ru
Subject: [PATCH] KVM: Handle MSR_IA32_PERF_CTL

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.

Signed-off-by: Dmitry Bilunov <kmeaw@...dex-team.ru>
---
 arch/x86/kvm/x86.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c805cf4..d0a5b4b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2314,6 +2314,7 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_AMD64_NB_CFG:
 	case MSR_FAM10H_MMIO_CONF_BASE:
 	case MSR_AMD64_BU_CFG2:
+	case MSR_IA32_PERF_CTL:
 		msr_info->data = 0;
 		break;
 	case MSR_K7_EVNTSEL0 ... MSR_K7_EVNTSEL3:
-- 
2.8.2