Re: [9/8] KVM: x86: limit MSR_IA32_SPEC_CTRL access based on CPUID availability

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Hello Paolo,

I've back ported this patch on 4.1, after adding the per-vcpu MSR bitmap. Also enabled the SPEC_CTRL_MSR intercept if qemu instructed so [1].

Reviewed-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>

[1]
+++ b/arch/x86/kvm/vmx.c
@@ -8391,6 +8391,16 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id) vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_PRED_CMD, MSR_TYPE_R | MSR_TYPE_W); vmx_disable_intercept_for_msr(msr_bitmap, MSR_IA32_BNDCFGS, MSR_TYPE_R | MSR_TYPE_W);

+       /*
+        * If the physical CPU or the vCPU of this VM doesn't
+        * support SPEC_CTRL feature, catch each access to it.
+        */
+       if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+            !guest_cpuid_has_spec_ctrl(&vmx->vcpu))
+               vmx_enable_intercept_for_msr(
+                       msr_bitmap,
+                       MSR_IA32_SPEC_CTRL,
+                       MSR_TYPE_R | MSR_TYPE_W);

        /*
* If PML is turned on, failure on enabling PML just results in failure


On 09.01.2018 14:03, Paolo Bonzini wrote:
MSR_IA32_SPEC_CTRL is not available unless CPU[7,0].EDX[26] is 1.
Check that against host CPUID or guest CPUID, respectively for
host-initiated and guest-initiated accesses.

Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
	This is for after X86_FEATURE_SPEC_CTRL is added to Linux, but
	I still wanted to ack Jim's improvement.

  arch/x86/kvm/svm.c | 8 ++++++++
  arch/x86/kvm/vmx.c | 8 ++++++++
  2 files changed, 16 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 97126c2bd663..3a646580d7c5 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -3648,6 +3648,10 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
  		msr_info->data = svm->nested.vm_cr_msr;
  		break;
  	case MSR_IA32_SPEC_CTRL:
+		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+	    	    (!msr_info->host_initiated &&
+       		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
+			return 1;
  		msr_info->data = svm->spec_ctrl;
  		break;
  	case MSR_IA32_UCODE_REV:
@@ -3806,6 +3810,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
  		vcpu_unimpl(vcpu, "unimplemented wrmsr: 0x%x data 0x%llx\n", ecx, data);
  		break;
  	case MSR_IA32_SPEC_CTRL:
+		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+	    	    (!msr_info->host_initiated &&
+       		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
+			return 1;
  		svm->spec_ctrl = data;
  		break;
  	case MSR_IA32_APICBASE:
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 49b4a2d61603..42bc7ee293e4 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3368,6 +3368,10 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
  		msr_info->data = guest_read_tsc(vcpu);
  		break;
  	case MSR_IA32_SPEC_CTRL:
+		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+		    (!msr_info->host_initiated &&
+		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
+			return 1;
  		msr_info->data = to_vmx(vcpu)->spec_ctrl;
  		break;
  	case MSR_IA32_SYSENTER_CS:
@@ -3510,6 +3514,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
  		kvm_write_tsc(vcpu, msr_info);
  		break;
  	case MSR_IA32_SPEC_CTRL:
+		if (!static_cpu_has(X86_FEATURE_SPEC_CTRL) ||
+		    (!msr_info->host_initiated &&
+		     !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL)))
+			return 1;
  		to_vmx(vcpu)->spec_ctrl = data;
  		break;
  	case MSR_IA32_CR_PAT:





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