linux-kernel - [RFC PATCH v2 1/4] KVM: SEV: Publish supported SEV-SNP policy bits

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Message-ID: <f48af9278ab89a27e521930c8ae02adfd2819bcc.1758308408.git.thomas.lendacky@amd.com>
Date: Fri, 19 Sep 2025 14:00:05 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<linux-crypto@...r.kernel.org>
CC: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
	<seanjc@...gle.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen
	<dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, "Thomas
 Gleixner" <tglx@...utronix.de>, Michael Roth <michael.roth@....com>, "Ashish
 Kalra" <ashish.kalra@....com>, Herbert Xu <herbert@...dor.apana.org.au>,
	"David Miller" <davem@...emloft.net>
Subject: [RFC PATCH v2 1/4] KVM: SEV: Publish supported SEV-SNP policy bits

Define the set of policy bits that KVM currently knows as not requiring
any implementation support within KVM. Provide this value to userspace
via the KVM_GET_DEVICE_ATTR ioctl.

Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
---
 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/svm/sev.c          | 12 ++++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 0f15d683817d..90e9c4551fa6 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -468,6 +468,7 @@ struct kvm_sync_regs {
 /* vendor-specific groups and attributes for system fd */
 #define KVM_X86_GRP_SEV			1
 #  define KVM_X86_SEV_VMSA_FEATURES	0
+#  define KVM_X86_SNP_POLICY_BITS	1
 
 struct kvm_vmx_nested_state_data {
 	__u8 vmcs12[KVM_STATE_NESTED_VMX_VMCS_SIZE];
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 01345b73f879..65bb2515ffb7 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -81,6 +81,8 @@ module_param_named(ciphertext_hiding_asids, nr_ciphertext_hiding_asids, uint, 04
 					 SNP_POLICY_MASK_DEBUG		| \
 					 SNP_POLICY_MASK_SINGLE_SOCKET)
 
+static u64 snp_supported_policy_bits __ro_after_init;
+
 #define INITIAL_VMSA_GPA 0xFFFFFFFFF000
 
 static u8 sev_enc_bit;
@@ -2134,6 +2136,10 @@ int sev_dev_get_attr(u32 group, u64 attr, u64 *val)
 		*val = sev_supported_vmsa_features;
 		return 0;
 
+	case KVM_X86_SNP_POLICY_BITS:
+		*val = snp_supported_policy_bits;
+		return 0;
+
 	default:
 		return -ENXIO;
 	}
@@ -2198,7 +2204,7 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
 	if (params.flags)
 		return -EINVAL;
 
-	if (params.policy & ~SNP_POLICY_MASK_VALID)
+	if (params.policy & ~snp_supported_policy_bits)
 		return -EINVAL;
 
 	/* Check for policy bits that must be set */
@@ -3084,8 +3090,10 @@ void __init sev_hardware_setup(void)
 		else if (sev_snp_supported)
 			sev_snp_supported = is_sev_snp_initialized();
 
-		if (sev_snp_supported)
+		if (sev_snp_supported) {
+			snp_supported_policy_bits = SNP_POLICY_MASK_VALID;
 			nr_ciphertext_hiding_asids = init_args.max_snp_asid;
+		}
 
 		/*
 		 * If ciphertext hiding is enabled, the joint SEV-ES/SEV-SNP
-- 
2.46.2