Re: [PATCH v6 3/4] KVM: SVM: Add GUEST_TSC_FREQ MSR for Secure TSC enabled guests

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 08, 2025, Nikunj A Dadhania wrote:
> Introduce the read-only MSR GUEST_TSC_FREQ (0xc0010134) that returns
> guest's effective frequency in MHZ when Secure TSC is enabled for SNP
> guests. Disable interception of this MSR when Secure TSC is enabled. Note
> that GUEST_TSC_FREQ MSR is accessible only to the guest and not from the
> hypervisor context.
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> Tested-by: Vaishali Thakkar <vaishali.thakkar@suse.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
>  arch/x86/include/asm/svm.h |  1 +
>  arch/x86/kvm/svm/sev.c     |  3 +++
>  arch/x86/kvm/svm/svm.c     |  1 +
>  arch/x86/kvm/svm/svm.h     | 11 ++++++++++-
>  4 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
> index 9b7fa99ae951..6ab66b80e751 100644
> --- a/arch/x86/include/asm/svm.h
> +++ b/arch/x86/include/asm/svm.h
> @@ -290,6 +290,7 @@ static_assert((X2AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AVIC_
>  #define SVM_SEV_FEAT_RESTRICTED_INJECTION		BIT(3)
>  #define SVM_SEV_FEAT_ALTERNATE_INJECTION		BIT(4)
>  #define SVM_SEV_FEAT_DEBUG_SWAP				BIT(5)
> +#define SVM_SEV_FEAT_SECURE_TSC				BIT(9)
>  
>  struct vmcb_seg {
>  	u16 selector;
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 0bc708ee2788..50263b473f95 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -4504,6 +4504,9 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>  	/* Clear intercepts on selected MSRs */
>  	set_msr_interception(vcpu, svm->msrpm, MSR_EFER, 1, 1);
>  	set_msr_interception(vcpu, svm->msrpm, MSR_IA32_CR_PAT, 1, 1);
> +
> +	if (snp_secure_tsc_enabled(vcpu->kvm))
> +		set_msr_interception(vcpu, svm->msrpm, MSR_AMD64_GUEST_TSC_FREQ, 1, 1);
>  }
>  
>  void sev_init_vmcb(struct vcpu_svm *svm)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 8abeab91d329..e65721db1f81 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -143,6 +143,7 @@ static const struct svm_direct_access_msrs {
>  	{ .index = X2APIC_MSR(APIC_TMICT),		.always = false },
>  	{ .index = X2APIC_MSR(APIC_TMCCT),		.always = false },
>  	{ .index = X2APIC_MSR(APIC_TDCR),		.always = false },
> +	{ .index = MSR_AMD64_GUEST_TSC_FREQ,		.always = false },
>  	{ .index = MSR_INVALID,				.always = false },
>  };
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index d4490eaed55d..711e21b7a3d0 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -44,7 +44,7 @@ static inline struct page *__sme_pa_to_page(unsigned long pa)
>  #define	IOPM_SIZE PAGE_SIZE * 3
>  #define	MSRPM_SIZE PAGE_SIZE * 2
>  
> -#define MAX_DIRECT_ACCESS_MSRS	48
> +#define MAX_DIRECT_ACCESS_MSRS	49
>  #define MSRPM_OFFSETS	32
>  extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
>  extern bool npt_enabled;
> @@ -377,10 +377,19 @@ static __always_inline bool sev_snp_guest(struct kvm *kvm)
>  	return (sev->vmsa_features & SVM_SEV_FEAT_SNP_ACTIVE) &&
>  	       !WARN_ON_ONCE(!sev_es_guest(kvm));
>  }
> +
> +static inline bool snp_secure_tsc_enabled(struct kvm *kvm)

This is only ever used in sev.c, it has no business living in svm.c.  And there's
especially no reason to have a stub for CONFIG_KVM_AMD_SEV=n.
> +{
> +	struct kvm_sev_info *sev = to_kvm_sev_info(kvm);
> +
> +	return (sev->vmsa_features & SVM_SEV_FEAT_SECURE_TSC) &&
> +		!WARN_ON_ONCE(!sev_snp_guest(kvm));
> +}
>  #else
>  #define sev_guest(kvm) false
>  #define sev_es_guest(kvm) false
>  #define sev_snp_guest(kvm) false
> +#define snp_secure_tsc_enabled(kvm) false
>  #endif
>  
>  static inline bool ghcb_gpa_is_registered(struct vcpu_svm *svm, u64 val)
> -- 
> 2.43.0
>