Re: [RFC PATCH v3 07/10] KVM: x86: Advertise ARCH_CAP_VIRTUAL_ENUM support

From: Jim Mattson
Date: Fri Apr 12 2024 - 00:23:12 EST

On Wed, Apr 10, 2024 at 8:08 AM Chao Gao <chao.gao@xxxxxxxxx> wrote:
>
> From: Zhang Chen <chen.zhang@xxxxxxxxx>
>
> Bit 63 of IA32_ARCH_CAPABILITIES MSR indicates availablility of the
> VIRTUAL_ENUMERATION_MSR (index 0x50000000) which enumerates features
> like e.g., mitigation enumeration that in turn is used for the guest to
> report software mitigations it is using.
>
> 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@xxxxxxxxx>
> Co-developed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> ---
> arch/x86/kvm/svm/svm.c | 1 +
> arch/x86/kvm/vmx/vmx.c | 19 +++++++++++++++++++
> arch/x86/kvm/vmx/vmx.h | 2 ++
> arch/x86/kvm/x86.c | 16 +++++++++++++++-
> 4 files changed, 37 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d1a9f9951635..e3406971a8b7 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4288,6 +4288,7 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index)
> {
> switch (index) {
> case MSR_IA32_MCG_EXT_CTL:
> + case MSR_VIRTUAL_ENUMERATION:
> case KVM_FIRST_EMULATED_VMX_MSR ... KVM_LAST_EMULATED_VMX_MSR:
> return false;
> case MSR_IA32_SMBASE:
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index cdfcc1290d82..dcb06406fd09 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1955,6 +1955,8 @@ 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) {
> @@ -1962,6 +1964,9 @@ 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;
> }
> @@ -2113,6 +2118,12 @@ 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);
> @@ -2457,6 +2468,14 @@ 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 a4dfe538e5a8..0519cf6187ac 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -294,6 +294,8 @@ struct vcpu_vmx {
> u64 force_spec_ctrl_mask;
> u64 force_spec_ctrl_value;
>
> + u64 msr_virtual_enumeration;
> +
> u32 msr_ia32_umwait_control;
>
> /*
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9a59b5a93d0e..4721b6fe7641 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1564,6 +1564,7 @@ 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)];
> @@ -1579,6 +1580,7 @@ static const u32 msr_based_features_all_except_vmx[] = {
> 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_except_vmx) +
> @@ -1621,7 +1623,8 @@ static bool kvm_is_immutable_feature_msr(u32 msr)
> 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_GDS_NO | \
> - ARCH_CAP_RFDS_NO | ARCH_CAP_RFDS_CLEAR | ARCH_CAP_BHI_NO)
> + ARCH_CAP_RFDS_NO | ARCH_CAP_RFDS_CLEAR | ARCH_CAP_BHI_NO | \
> + ARCH_CAP_VIRTUAL_ENUM)
>
> static u64 kvm_get_arch_capabilities(void)
> {
> @@ -1635,6 +1638,17 @@ 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.
> + */

Virtualize IA32_SPEC_CTRL has been an SVM feature for years. See
https://lore.kernel.org/kvm/160738054169.28590.5171339079028237631.stgit@bmoger-ubuntu/.

> + 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.
> --
> 2.39.3
>
>