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

[Mihai Carabas]

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: