Re: [RFC PATCH v2 06/11] KVM: x86: Advertise ARCH_CAP_VIRTUAL_ENUM support

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 4/14/2023 2:25 PM, Chao Gao wrote:
> From: Zhang Chen <chen.zhang@intel.com>
> 
> Bit 63 of IA32_ARCH_CAPABILITIES MSR indicates availablility of the
> VIRTUAL_ENUMERATION_MSR (index 0x50000000) that enumerates features like
> e.g., mitigation enumeration which is used for guest to hint VMMs the
> software mitigations in use.
> 
> Advertise ARCH_CAP_VIRTUAL_ENUM support for VMX and emulate read/write
> of the VIRTUAL_ENUMERATION_MSR. Now VIRTUAL_ENUMERATION_MSR is always 0.
> 
> Signed-off-by: Zhang Chen <chen.zhang@intel.com>
> Co-developed-by: Chao Gao <chao.gao@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> Tested-by: Jiaan Lu <jiaan.lu@intel.com>
> ---
>   arch/x86/kvm/svm/svm.c |  1 +
>   arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
>   arch/x86/kvm/vmx/vmx.h |  1 +
>   arch/x86/kvm/x86.c     | 16 +++++++++++++++-
>   4 files changed, 36 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 57f241c5a371..195d0cf9309a 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4093,6 +4093,7 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index)
>   {
>   	switch (index) {
>   	case MSR_IA32_MCG_EXT_CTL:
> +	case MSR_VIRTUAL_ENUMERATION:
>   	case MSR_IA32_VMX_BASIC ... MSR_IA32_VMX_VMFUNC:
>   		return false;
>   	case MSR_IA32_SMBASE:
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9f6919bec2b3..85419137decb 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1943,6 +1943,8 @@ static inline bool is_vmx_feature_control_msr_valid(struct vcpu_vmx *vmx,
>   	return !(msr->data & ~valid_bits);
>   }
>   
> +#define VIRTUAL_ENUMERATION_VALID_BITS	0ULL
> +
>   static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>   {
>   	switch (msr->index) {
> @@ -1950,6 +1952,9 @@ static int vmx_get_msr_feature(struct kvm_msr_entry *msr)
>   		if (!nested)
>   			return 1;
>   		return vmx_get_vmx_msr(&vmcs_config.nested, msr->index, &msr->data);
> +	case MSR_VIRTUAL_ENUMERATION:
> +		msr->data = VIRTUAL_ENUMERATION_VALID_BITS;
> +		return 0;
>   	default:
>   		return KVM_MSR_RET_INVALID;
>   	}
> @@ -2096,6 +2101,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   	case MSR_IA32_DEBUGCTLMSR:
>   		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
>   		break;
> +	case MSR_VIRTUAL_ENUMERATION:
> +		if (!msr_info->host_initiated &&
> +		    !(vcpu->arch.arch_capabilities & ARCH_CAP_VIRTUAL_ENUM))
> +			return 1;
> +		msr_info->data = vmx->msr_virtual_enumeration;
> +		break;
>   	default:
>   	find_uret_msr:
>   		msr = vmx_find_uret_msr(vmx, msr_info->index);
> @@ -2437,6 +2448,14 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		}
>   		ret = kvm_set_msr_common(vcpu, msr_info);
>   		break;
> +	case MSR_VIRTUAL_ENUMERATION:
> +		if (!msr_info->host_initiated)
> +			return 1;
> +		if (data & ~VIRTUAL_ENUMERATION_VALID_BITS)
> +			return 1;
> +
> +		vmx->msr_virtual_enumeration = data;
> +		break;
>   
>   	default:
>   	find_uret_msr:
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index 021d86b52e18..a7faaf9fdc26 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -292,6 +292,7 @@ struct vcpu_vmx {
>   
>   	u64		      spec_ctrl;
>   	u64		      guest_spec_ctrl;
> +	u64		      msr_virtual_enumeration;
>   	u32		      msr_ia32_umwait_control;
>   
>   	/*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3c58dbae7b4c..a1bc52bebdcc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1537,6 +1537,7 @@ static const u32 emulated_msrs_all[] = {
>   
>   	MSR_K7_HWCR,
>   	MSR_KVM_POLL_CONTROL,
> +	MSR_VIRTUAL_ENUMERATION,
>   };
>   
>   static u32 emulated_msrs[ARRAY_SIZE(emulated_msrs_all)];
> @@ -1570,6 +1571,7 @@ static const u32 msr_based_features_all[] = {
>   	MSR_IA32_UCODE_REV,
>   	MSR_IA32_ARCH_CAPABILITIES,
>   	MSR_IA32_PERF_CAPABILITIES,
> +	MSR_VIRTUAL_ENUMERATION,
>   };
>   
>   static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all)];
> @@ -1591,7 +1593,8 @@ static unsigned int num_msr_based_features;
>   	 ARCH_CAP_SKIP_VMENTRY_L1DFLUSH | ARCH_CAP_SSB_NO | ARCH_CAP_MDS_NO | \
>   	 ARCH_CAP_PSCHANGE_MC_NO | ARCH_CAP_TSX_CTRL_MSR | ARCH_CAP_TAA_NO | \
>   	 ARCH_CAP_SBDR_SSDP_NO | ARCH_CAP_FBSDP_NO | ARCH_CAP_PSDP_NO | \
> -	 ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO)
> +	 ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO | \
> +	 ARCH_CAP_VIRTUAL_ENUM)

We cannot do it.

Otherwise, an AMD L1 with X86_FEATURE_ARCH_CAPABILITIES configured is 
possible to expose MSR_VIRTUAL_ENUMERATION to L2 while no support for it.

>   
>   static u64 kvm_get_arch_capabilities(void)
>   {
> @@ -1610,6 +1613,17 @@ static u64 kvm_get_arch_capabilities(void)
>   	 */
>   	data |= ARCH_CAP_PSCHANGE_MC_NO;
>   
> +	/*
> +	 * Virtual enumeration is a paravirt feature. The only usage for now
> +	 * is to bridge the gap caused by microarchitecture changes between
> +	 * different Intel processors. And its usage is linked to "virtualize
> +	 * IA32_SPEC_CTRL" which is a VMX feature. Whether AMD SVM can benefit
> +	 * from the same usage and how to implement it is still unclear. Limit
> +	 * virtual enumeration to VMX.
> +	 */
> +	if (static_call(kvm_x86_has_emulated_msr)(NULL, MSR_VIRTUAL_ENUMERATION))
> +		data |= ARCH_CAP_VIRTUAL_ENUM;
> +
>   	/*
>   	 * If we're doing cache flushes (either "always" or "cond")
>   	 * we will do one whenever the guest does a vmlaunch/vmresume.