Skip to contentSkip to conversation

SEV-SNP guest policy bit support updates

RFC PATCH Superseded Tom Lendacky 1 participant 5 messages Sep 19, 2025

Subsystems (6)
x86 KVM Crypto API KVM x86 +2 more subsystems

v2 of 4 Superseded Latest is v4 →

lore
Share & tools
lore

Tom Lendacky

author maintainer
#0 lore raw

v2 cover SEV-SNP guest policy bit support updates

This series aims to allow more flexibility in specifying SEV-SNP policy bits by improving discoverability of supported policy bits from userspace and enabling support for newer policy bits.

- The first patch adds a new KVM_X86_GRP_SEV attribute group, KVM_X86_SNP_POLICY_BITS, that can be used to return the supported SEV-SNP policy bits. The initial support for this attribute will return the current KVM supported policy bitmask.

- The next 3 patches provide for adding to the known SEV-SNP policy bits. Since some policy bits are dependent on specific levels of SEV firmware support, the CCP driver is updated to provide an API to return the supported policy bits.

The supported policy bits bitmask used by KVM is generated by taking the policy bitmask returned by the CCP driver and ANDing it with the KVM supported policy bits. KVM supported policy bits are policy bits that do not require any specific implementation support from KVM to allow.

This series has a prereq against the ciphertext hiding patches and so it is based on the ciphertext branch of the kvm-x86 repo.

The series is based off of:
https://github.com/kvm-x86/linux.git ciphertext

---

Changes for v2:
- Marked the KVM supported policy bits as read-only after init.

Tom Lendacky (4):
KVM: SEV: Publish supported SEV-SNP policy bits
KVM: SEV: Consolidate the SEV policy bits in a single header file
crypto: ccp - Add an API to return the supported SEV-SNP policy bits
KVM: SEV: Add known supported SEV-SNP policy bits

 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/svm/sev.c          | 45 ++++++++++++++++++++-------------
 arch/x86/kvm/svm/svm.h          |  3 ---
 drivers/crypto/ccp/sev-dev.c    | 37 +++++++++++++++++++++++++++
 include/linux/psp-sev.h         | 39 ++++++++++++++++++++++++++++
 5 files changed, 105 insertions(+), 20 deletions(-)

base-commit: 6c7c620585c6537dd5dcc75f972b875caf00f773

Signature · 1 line
2.46.2
author maintainer
#1 lore raw

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 <[email protected]>
---
 arch/x86/include/uapi/asm/kvm.h |  1 +
 arch/x86/kvm/svm/sev.c          | 12 ++++++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)
arch/x86/include/uapi/asm/kvm.h +1 -0
index 0f15d683817d..90e9c4551fa6 100644
@@ -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];
arch/x86/kvm/svm/sev.c +10 -2
index 01345b73f879..65bb2515ffb7 100644
@@ -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
Signature · 1 line
2.46.2
author maintainer
#2 lore raw

RFC PATCH v2 2/4 KVM: SEV: Consolidate the SEV policy bits in a single header file

In reply to Tom Lendacky

Consolidate SEV policy bit definitions into a single file. Use include/linux/psp-sev.h to hold the definitions and remove the current definitions from the arch/x86/kvm/svm/sev.c and arch/x86/include/svm.h files.

No functional change intended.

Signed-off-by Tom Lendacky <[email protected]>
---
 arch/x86/kvm/svm/sev.c  | 16 ++++------------
 arch/x86/kvm/svm/svm.h  |  3 ---
 include/linux/psp-sev.h | 19 +++++++++++++++++++
 3 files changed, 23 insertions(+), 15 deletions(-)
arch/x86/kvm/svm/sev.c +4 -12
index 65bb2515ffb7..e63f2ee57204 100644
@@ -66,15 +66,7 @@ module_param_named(ciphertext_hiding_asids, nr_ciphertext_hiding_asids, uint, 04
 #define AP_RESET_HOLD_NAE_EVENT		1
 #define AP_RESET_HOLD_MSR_PROTO		2
 
-/* As defined by SEV-SNP Firmware ABI, under "Guest Policy". */
-#define SNP_POLICY_MASK_API_MINOR	GENMASK_ULL(7, 0)
-#define SNP_POLICY_MASK_API_MAJOR	GENMASK_ULL(15, 8)
-#define SNP_POLICY_MASK_SMT		BIT_ULL(16)
-#define SNP_POLICY_MASK_RSVD_MBO	BIT_ULL(17)
-#define SNP_POLICY_MASK_DEBUG		BIT_ULL(19)
-#define SNP_POLICY_MASK_SINGLE_SOCKET	BIT_ULL(20)
-
-#define SNP_POLICY_MASK_VALID		(SNP_POLICY_MASK_API_MINOR	| \
+#define KVM_SNP_POLICY_MASK_VALID	(SNP_POLICY_MASK_API_MINOR	| \
 					 SNP_POLICY_MASK_API_MAJOR	| \
 					 SNP_POLICY_MASK_SMT		| \
 					 SNP_POLICY_MASK_RSVD_MBO	| \

@@ -3091,7 +3083,7 @@ void __init sev_hardware_setup(void)
 			sev_snp_supported = is_sev_snp_initialized();
 
 		if (sev_snp_supported) {
-			snp_supported_policy_bits = SNP_POLICY_MASK_VALID;
+			snp_supported_policy_bits = KVM_SNP_POLICY_MASK_VALID;
 			nr_ciphertext_hiding_asids = init_args.max_snp_asid;
 		}
 

@@ -5040,10 +5032,10 @@ struct vmcb_save_area *sev_decrypt_vmsa(struct kvm_vcpu *vcpu)
 
 	/* Check if the SEV policy allows debugging */
 	if (sev_snp_guest(vcpu->kvm)) {
-		if (!(sev->policy & SNP_POLICY_DEBUG))
+		if (!(sev->policy & SNP_POLICY_MASK_DEBUG))
 			return NULL;
 	} else {
-		if (sev->policy & SEV_POLICY_NODBG)
+		if (sev->policy & SEV_POLICY_MASK_NODBG)
 			return NULL;
 	}
arch/x86/kvm/svm/svm.h +0 -3
index 58b9d168e0c8..61911a2b78c3 100644
@@ -113,9 +113,6 @@ struct kvm_sev_info {
 	cpumask_var_t have_run_cpus; /* CPUs that have done VMRUN for this VM. */
 };
 
-#define SEV_POLICY_NODBG	BIT_ULL(0)
-#define SNP_POLICY_DEBUG	BIT_ULL(19)
-
 struct kvm_svm {
 	struct kvm kvm;
include/linux/psp-sev.h +19 -0
index e0dbcb4b4fd9..27c92543bf38 100644
@@ -14,6 +14,25 @@
 
 #include <uapi/linux/psp-sev.h>
 
+/* As defined by SEV API, under "Guest Policy". */
+#define SEV_POLICY_MASK_NODBG			BIT(0)
+#define SEV_POLICY_MASK_NOKS			BIT(1)
+#define SEV_POLICY_MASK_ES			BIT(2)
+#define SEV_POLICY_MASK_NOSEND			BIT(3)
+#define SEV_POLICY_MASK_DOMAIN			BIT(4)
+#define SEV_POLICY_MASK_SEV			BIT(5)
+#define SEV_POLICY_MASK_API_MAJOR		GENMASK(23, 16)
+#define SEV_POLICY_MASK_API_MINOR		GENMASK(31, 24)
+
+/* As defined by SEV-SNP Firmware ABI, under "Guest Policy". */
+#define SNP_POLICY_MASK_API_MINOR		GENMASK_ULL(7, 0)
+#define SNP_POLICY_MASK_API_MAJOR		GENMASK_ULL(15, 8)
+#define SNP_POLICY_MASK_SMT			BIT_ULL(16)
+#define SNP_POLICY_MASK_RSVD_MBO		BIT_ULL(17)
+#define SNP_POLICY_MASK_MIGRATE_MA		BIT_ULL(18)
+#define SNP_POLICY_MASK_DEBUG			BIT_ULL(19)
+#define SNP_POLICY_MASK_SINGLE_SOCKET		BIT_ULL(20)
+
 #define SEV_FW_BLOB_MAX_SIZE	0x4000	/* 16KB */
 
 /**
Signature · 1 line
2.46.2
author maintainer
#3 lore raw

RFC PATCH v2 3/4 crypto: ccp - Add an API to return the supported SEV-SNP policy bits

In reply to Tom Lendacky

Supported policy bits are dependent on the level of SEV firmware that is currently running. Create an API to return the supported policy bits for a given level of firmware. KVM will AND that value with the KVM supported policy bits to generate the actual supported policy bits.

Signed-off-by Tom Lendacky <[email protected]>
---
 arch/x86/kvm/svm/sev.c       |  3 ++-
 drivers/crypto/ccp/sev-dev.c | 37 ++++++++++++++++++++++++++++++++++++
 include/linux/psp-sev.h      | 20 +++++++++++++++++++
 3 files changed, 59 insertions(+), 1 deletion(-)
arch/x86/kvm/svm/sev.c +2 -1
index e63f2ee57204..f77da22200fb 100644
@@ -3083,7 +3083,8 @@ void __init sev_hardware_setup(void)
 			sev_snp_supported = is_sev_snp_initialized();
 
 		if (sev_snp_supported) {
-			snp_supported_policy_bits = KVM_SNP_POLICY_MASK_VALID;
+			snp_supported_policy_bits = sev_get_snp_policy_bits();
+			snp_supported_policy_bits &= KVM_SNP_POLICY_MASK_VALID;
 			nr_ciphertext_hiding_asids = init_args.max_snp_asid;
 		}
drivers/crypto/ccp/sev-dev.c +37 -0
index 334405461657..d4159cec12a0 100644
@@ -2583,6 +2583,43 @@ void sev_platform_shutdown(void)
 }
 EXPORT_SYMBOL_GPL(sev_platform_shutdown);
 
+u64 sev_get_snp_policy_bits(void)
+{
+	struct psp_device *psp = psp_master;
+	struct sev_device *sev;
+	u64 policy_bits;
+
+	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
+		return 0;
+
+	if (!psp || !psp->sev_data)
+		return 0;
+
+	sev = psp->sev_data;
+
+	policy_bits = SNP_POLICY_MASK_BASE;
+
+	if (sev->snp_plat_status.feature_info) {
+		if (sev->snp_feat_info_0.ecx & SNP_RAPL_DISABLE_SUPPORTED)
+			policy_bits |= SNP_POLICY_MASK_RAPL_DIS;
+
+		if (sev->snp_feat_info_0.ecx & SNP_CIPHER_TEXT_HIDING_SUPPORTED)
+			policy_bits |= SNP_POLICY_MASK_CIPHERTEXT_HIDING_DRAM;
+
+		if (sev->snp_feat_info_0.ecx & SNP_AES_256_XTS_POLICY_SUPPORTED)
+			policy_bits |= SNP_POLICY_MASK_MEM_AES_256_XTS;
+
+		if (sev->snp_feat_info_0.ecx & SNP_CXL_ALLOW_POLICY_SUPPORTED)
+			policy_bits |= SNP_POLICY_MASK_CXL_ALLOW;
+
+		if (sev_version_greater_or_equal(1, 58))
+			policy_bits |= SNP_POLICY_MASK_PAGE_SWAP_DISABLE;
+	}
+
+	return policy_bits;
+}
+EXPORT_SYMBOL_GPL(sev_get_snp_policy_bits);
+
 void sev_dev_destroy(struct psp_device *psp)
 {
 	struct sev_device *sev = psp->sev_data;
include/linux/psp-sev.h +20 -0
index 27c92543bf38..1b4c68ec5c65 100644
@@ -32,6 +32,20 @@
 #define SNP_POLICY_MASK_MIGRATE_MA		BIT_ULL(18)
 #define SNP_POLICY_MASK_DEBUG			BIT_ULL(19)
 #define SNP_POLICY_MASK_SINGLE_SOCKET		BIT_ULL(20)
+#define SNP_POLICY_MASK_CXL_ALLOW		BIT_ULL(21)
+#define SNP_POLICY_MASK_MEM_AES_256_XTS		BIT_ULL(22)
+#define SNP_POLICY_MASK_RAPL_DIS		BIT_ULL(23)
+#define SNP_POLICY_MASK_CIPHERTEXT_HIDING_DRAM	BIT_ULL(24)
+#define SNP_POLICY_MASK_PAGE_SWAP_DISABLE	BIT_ULL(25)
+
+/* Base SEV-SNP policy bitmask for minimum supported SEV firmware version */
+#define SNP_POLICY_MASK_BASE	(SNP_POLICY_MASK_API_MINOR		| \
+				 SNP_POLICY_MASK_API_MAJOR		| \
+				 SNP_POLICY_MASK_SMT			| \
+				 SNP_POLICY_MASK_RSVD_MBO		| \
+				 SNP_POLICY_MASK_MIGRATE_MA		| \
+				 SNP_POLICY_MASK_DEBUG			| \
+				 SNP_POLICY_MASK_SINGLE_SOCKET)
 
 #define SEV_FW_BLOB_MAX_SIZE	0x4000	/* 16KB */
 

@@ -868,7 +882,10 @@ struct snp_feature_info {
 	u32 edx;
 } __packed;
 
+#define SNP_RAPL_DISABLE_SUPPORTED		BIT(2)
 #define SNP_CIPHER_TEXT_HIDING_SUPPORTED	BIT(3)
+#define SNP_AES_256_XTS_POLICY_SUPPORTED	BIT(4)
+#define SNP_CXL_ALLOW_POLICY_SUPPORTED		BIT(5)
 
 #ifdef CONFIG_CRYPTO_DEV_SP_PSP
 

@@ -1014,6 +1031,7 @@ void *snp_alloc_firmware_page(gfp_t mask);
 void snp_free_firmware_page(void *addr);
 void sev_platform_shutdown(void);
 bool sev_is_snp_ciphertext_hiding_supported(void);
+u64 sev_get_snp_policy_bits(void);
 
 #else	/* !CONFIG_CRYPTO_DEV_SP_PSP */
 

@@ -1052,6 +1070,8 @@ static inline void sev_platform_shutdown(void) { }
 
 static inline bool sev_is_snp_ciphertext_hiding_supported(void) { return false; }
 
+static inline u64 sev_get_snp_policy_bits(void) { return 0; }
+
 #endif	/* CONFIG_CRYPTO_DEV_SP_PSP */
 
 #endif	/* __PSP_SEV_H__ */
Signature · 1 line
2.46.2
author maintainer
#4 lore raw

RFC PATCH v2 4/4 KVM: SEV: Add known supported SEV-SNP policy bits

In reply to Tom Lendacky

Add to the known supported SEV-SNP policy bits that don't require any implementation support from KVM in order to successfully use them.

At this time, this includes:
- CXL_ALLOW
- MEM_AES_256_XTS
- RAPL_DIS
- CIPHERTEXT_HIDING_DRAM
- PAGE_SWAP_DISABLE

Arguably, RAPL_DIS and CIPHERTEXT_HIDING_DRAM require KVM and the CCP driver to enable these features in order for the setting of the policy bits to be successfully handled. But, a guest owner may not wish their guest to run on a system that doesn't provide support for those features, so allowing the specification of these bits accomplishes that. Whether or not the bit is supported by SEV firmware, a system that doesn't support these features will either fail during the KVM validation of supported policy bits before issuing the LAUNCH_START or fail during the LAUNCH_START.

Signed-off-by Tom Lendacky <[email protected]>
---
 arch/x86/kvm/svm/sev.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)
arch/x86/kvm/svm/sev.c +16 -6
index f77da22200fb..2385c9a0befe 100644
@@ -66,12 +66,22 @@ module_param_named(ciphertext_hiding_asids, nr_ciphertext_hiding_asids, uint, 04
 #define AP_RESET_HOLD_NAE_EVENT		1
 #define AP_RESET_HOLD_MSR_PROTO		2
 
-#define KVM_SNP_POLICY_MASK_VALID	(SNP_POLICY_MASK_API_MINOR	| \
-					 SNP_POLICY_MASK_API_MAJOR	| \
-					 SNP_POLICY_MASK_SMT		| \
-					 SNP_POLICY_MASK_RSVD_MBO	| \
-					 SNP_POLICY_MASK_DEBUG		| \
-					 SNP_POLICY_MASK_SINGLE_SOCKET)
+/*
+ * SEV-SNP policy bits that can be supported by KVM. These include policy bits
+ * that have implementation support within KVM or policy bits that do not rely
+ * on any implementation support within KVM.
+ */
+#define KVM_SNP_POLICY_MASK_VALID	(SNP_POLICY_MASK_API_MINOR		| \
+					 SNP_POLICY_MASK_API_MAJOR		| \
+					 SNP_POLICY_MASK_SMT			| \
+					 SNP_POLICY_MASK_RSVD_MBO		| \
+					 SNP_POLICY_MASK_DEBUG			| \
+					 SNP_POLICY_MASK_SINGLE_SOCKET		| \
+					 SNP_POLICY_MASK_CXL_ALLOW		| \
+					 SNP_POLICY_MASK_MEM_AES_256_XTS	| \
+					 SNP_POLICY_MASK_RAPL_DIS		| \
+					 SNP_POLICY_MASK_CIPHERTEXT_HIDING_DRAM	| \
+					 SNP_POLICY_MASK_PAGE_SWAP_DISABLE)
 
 static u64 snp_supported_policy_bits __ro_after_init;
Signature · 1 line
2.46.2