KVM: SEV: publish supported VMSA features

Compute the set of features to be stored in the VMSA when KVM is
initialized; move it from there into kvm_sev_info when SEV is initialized,
and then into the initial VMSA.

The new variable can then be used to return the set of supported features
to userspace, via the KVM_GET_DEVICE_ATTR ioctl.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-ID: <20240404121327.3107131-6-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/x86/amd-memory-encryption.rst

@@ -425,6 +425,18 @@ issued by the hypervisor to make the guest ready for execution.
 
 Returns: 0 on success, -negative on error
 
+Device attribute API
+====================
+
+Attributes of the SEV implementation can be retrieved through the
+``KVM_HAS_DEVICE_ATTR`` and ``KVM_GET_DEVICE_ATTR`` ioctls on the ``/dev/kvm``
+device node, using group ``KVM_X86_GRP_SEV``.
+
+Currently only one attribute is implemented:
+
+* ``KVM_X86_SEV_VMSA_FEATURES``: return the set of all bits that
+  are accepted in the ``vmsa_features`` of ``KVM_SEV_INIT2``.
+
 Firmware Management
 ===================
 

arch/x86/include/uapi/asm/kvm.h

@@ -457,8 +457,13 @@ struct kvm_sync_regs {
 
 #define KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE	0x00000001
 
-/* attributes for system fd (group 0) */
+/* vendor-independent attributes for system fd (group 0) */
-#define KVM_X86_XCOMP_GUEST_SUPP	0
+#define KVM_X86_GRP_SYSTEM		0
+#  define KVM_X86_XCOMP_GUEST_SUPP	0
+
+/* vendor-specific groups and attributes for system fd */
+#define KVM_X86_GRP_SEV			1
+#  define KVM_X86_SEV_VMSA_FEATURES	0
 
 struct kvm_vmx_nested_state_data {
 	__u8 vmcs12[KVM_STATE_NESTED_VMX_VMCS_SIZE];

arch/x86/kvm/svm/sev.c

@@ -46,6 +46,7 @@ module_param_named(sev_es, sev_es_enabled, bool, 0444);
 /* enable/disable SEV-ES DebugSwap support */
 static bool sev_es_debug_swap_enabled = false;
 module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
+static u64 sev_supported_vmsa_features;
 
 static u8 sev_enc_bit;
 static DECLARE_RWSEM(sev_deactivate_lock);
@@ -603,8 +604,8 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
 	save->xss  = svm->vcpu.arch.ia32_xss;
 	save->dr6  = svm->vcpu.arch.dr6;
 
-	if (sev_es_debug_swap_enabled) {
+	if (sev_supported_vmsa_features) {
-		save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
+		save->sev_features = sev_supported_vmsa_features;
 		pr_warn_once("Enabling DebugSwap with KVM_SEV_ES_INIT. "
 			     "This will not work starting with Linux 6.10\n");
 	}
@@ -1843,6 +1844,21 @@ out_fput:
 	return ret;
 }
 
+int sev_dev_get_attr(u32 group, u64 attr, u64 *val)
+{
+	if (group != KVM_X86_GRP_SEV)
+		return -ENXIO;
+
+	switch (attr) {
+	case KVM_X86_SEV_VMSA_FEATURES:
+		*val = sev_supported_vmsa_features;
+		return 0;
+
+	default:
+		return -ENXIO;
+	}
+}
+
 int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
 {
 	struct kvm_sev_cmd sev_cmd;
@@ -2275,6 +2291,10 @@ out:
 	if (!sev_es_enabled || !cpu_feature_enabled(X86_FEATURE_DEBUG_SWAP) ||
 	    !cpu_feature_enabled(X86_FEATURE_NO_NESTED_DATA_BP))
 		sev_es_debug_swap_enabled = false;
+
+	sev_supported_vmsa_features = 0;
+	if (sev_es_debug_swap_enabled)
+		sev_supported_vmsa_features |= SVM_SEV_FEAT_DEBUG_SWAP;
 }
 
 void sev_hardware_unsetup(void)

arch/x86/kvm/svm/svm.c

@@ -5026,6 +5026,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
 #endif
 
 #ifdef CONFIG_KVM_AMD_SEV
+	.dev_get_attr = sev_dev_get_attr,
 	.mem_enc_ioctl = sev_mem_enc_ioctl,
 	.mem_enc_register_region = sev_mem_enc_register_region,
 	.mem_enc_unregister_region = sev_mem_enc_unregister_region,

arch/x86/kvm/svm/svm.h

@@ -692,6 +692,7 @@ void __init sev_set_cpu_caps(void);
 void __init sev_hardware_setup(void);
 void sev_hardware_unsetup(void);
 int sev_cpu_init(struct svm_cpu_data *sd);
+int sev_dev_get_attr(u32 group, u64 attr, u64 *val);
 extern unsigned int max_sev_asid;
 #else
 static inline struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu) {
@@ -704,6 +705,7 @@ static inline void __init sev_set_cpu_caps(void) {}
 static inline void __init sev_hardware_setup(void) {}
 static inline void sev_hardware_unsetup(void) {}
 static inline int sev_cpu_init(struct svm_cpu_data *sd) { return 0; }
+static inline int sev_dev_get_attr(u32 group, u64 attr, u64 *val) { return -ENXIO; }
 #define max_sev_asid 0
 #endif