Re: [PATCH v4 5/5] KVM: SVM: Enable Secure TSC for SNP guests

[Tom Lendacky <thomas.lendacky@amd.com>]

On 3/10/25 01:45, Nikunj A Dadhania wrote:
> From: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
> 
> Add support for Secure TSC, allowing userspace to configure the Secure TSC
> feature for SNP guests. Use the SNP specification's desired TSC frequency
> parameter during the SNP_LAUNCH_START command to set the mean TSC
> frequency in KHz for Secure TSC enabled guests. If the frequency is not
> specified by the VMM, default to tsc_khz.
> 
> Signed-off-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
> Co-developed-by: Nikunj A Dadhania <nikunj@amd.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
>  arch/x86/include/uapi/asm/kvm.h |  3 ++-
>  arch/x86/kvm/svm/sev.c          | 19 +++++++++++++++++++
>  2 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
> index 460306b35a4b..075af0dcee25 100644
> --- a/arch/x86/include/uapi/asm/kvm.h
> +++ b/arch/x86/include/uapi/asm/kvm.h
> @@ -839,7 +839,8 @@ struct kvm_sev_snp_launch_start {
>  	__u64 policy;
>  	__u8 gosvw[16];
>  	__u16 flags;
> -	__u8 pad0[6];
> +	__u8 pad0[2];
> +	__u32 desired_tsc_khz;
>  	__u64 pad1[4];
>  };
>  
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 50263b473f95..b61d6bd75b37 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2205,6 +2205,20 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  
>  	start.gctx_paddr = __psp_pa(sev->snp_context);
>  	start.policy = params.policy;
> +
> +	if (snp_secure_tsc_enabled(kvm)) {
> +		u32 user_tsc_khz = params.desired_tsc_khz;
> +
> +		/* Use tsc_khz if the VMM has not provided the TSC frequency */
> +		if (!user_tsc_khz)
> +			user_tsc_khz = tsc_khz;
> +
> +		start.desired_tsc_khz = user_tsc_khz;

Do we need to perform any sanity checking against this value?

Thanks,
Tom

> +
> +		/* Set the arch default TSC for the VM*/
> +		kvm->arch.default_tsc_khz = user_tsc_khz;
> +	}
> +
>  	memcpy(start.gosvw, params.gosvw, sizeof(params.gosvw));
>  	rc = __sev_issue_cmd(argp->sev_fd, SEV_CMD_SNP_LAUNCH_START, &start, &argp->error);
>  	if (rc) {
> @@ -2927,6 +2941,8 @@ void __init sev_set_cpu_caps(void)
>  	if (sev_snp_enabled) {
>  		kvm_cpu_cap_set(X86_FEATURE_SEV_SNP);
>  		kvm_caps.supported_vm_types |= BIT(KVM_X86_SNP_VM);
> +
> +		kvm_cpu_cap_check_and_set(X86_FEATURE_SNP_SECURE_TSC);
>  	}
>  }
>  
> @@ -3059,6 +3075,9 @@ void __init sev_hardware_setup(void)
>  	sev_supported_vmsa_features = 0;
>  	if (sev_es_debug_swap_enabled)
>  		sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
> +
> +	if (sev_snp_enabled && cpu_feature_enabled(X86_FEATURE_SNP_SECURE_TSC))
> +		sev_supported_vmsa_features |= SVM_SEV_FEAT_SECURE_TSC;
>  }
>  
>  void sev_hardware_unsetup(void)