KVM: VMX: Nop emulation of MSR_IA32_POWER_CTL

Since commits 668fffa3 ("kvm: better MWAIT emulation for guestsâ€)
and 4d5422ce ("KVM: X86: Provide a capability to disable MWAIT interceptsâ€),
KVM was modified to allow an admin to configure certain guests to execute
MONITOR/MWAIT inside guest without being intercepted by host.

This is useful in case admin wishes to allocate a dedicated logical
processor for each vCPU thread. Thus, making it safe for guest to
completely control the power-state of the logical processor.

The ability to use this new KVM capability was introduced to QEMU by
commits 6f131f13e68d ("kvm: support -overcommit cpu-pm=on|offâ€) and
2266d4431132 ("i386/cpu: make -cpu host support monitor/mwaitâ€).

However, exposing MONITOR/MWAIT to a Linux guest may cause it's intel_idle
kernel module to execute c1e_promotion_disable() which will attempt to
RDMSR/WRMSR from/to MSR_IA32_POWER_CTL to manipulate the "C1E Enable"
bit. This behaviour was introduced by commit
32e95180 ("intel_idle: export both C1 and C1Eâ€).

Becuase KVM doesn't emulate this MSR, running KVM with ignore_msrs=0
will cause the above guest behaviour to raise a #GP which will cause
guest to kernel panic.

Therefore, add support for nop emulation of MSR_IA32_POWER_CTL to
avoid #GP in guest in this scenario.

Future commits can optimise emulation further by reflecting guest
MSR changes to host MSR to provide guest with the ability to
fine-tune the dedicated logical processor power-state.
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/vmx.c

@@ -1692,6 +1692,9 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_IA32_SYSENTER_ESP:
 		msr_info->data = vmcs_readl(GUEST_SYSENTER_ESP);
 		break;
+	case MSR_IA32_POWER_CTL:
+		msr_info->data = vmx->msr_ia32_power_ctl;
+		break;
 	case MSR_IA32_BNDCFGS:
 		if (!kvm_mpx_supported() ||
 		    (!msr_info->host_initiated &&
@@ -1822,6 +1825,9 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_IA32_SYSENTER_ESP:
 		vmcs_writel(GUEST_SYSENTER_ESP, data);
 		break;
+	case MSR_IA32_POWER_CTL:
+		vmx->msr_ia32_power_ctl = data;
+		break;
 	case MSR_IA32_BNDCFGS:
 		if (!kvm_mpx_supported() ||
 		    (!msr_info->host_initiated &&

arch/x86/kvm/vmx/vmx.h

@@ -257,6 +257,8 @@ struct vcpu_vmx {
 
 	unsigned long host_debugctlmsr;
 
+	u64 msr_ia32_power_ctl;
+
 	/*
 	 * Only bits masked by msr_ia32_feature_control_valid_bits can be set in
 	 * msr_ia32_feature_control. FEATURE_CONTROL_LOCKED is always included

arch/x86/kvm/x86.c

@@ -1170,6 +1170,7 @@ static u32 emulated_msrs[] = {
 	MSR_PLATFORM_INFO,
 	MSR_MISC_FEATURES_ENABLES,
 	MSR_AMD64_VIRT_SPEC_CTRL,
+	MSR_IA32_POWER_CTL,
 };
 
 static unsigned num_emulated_msrs;