[PATCH v2 04/11] KVM: SEV: publish supported VMSA features

[Paolo Bonzini <pbonzini@redhat.com>]

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>
Message-Id: <20240209183743.22030-5-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 .../virt/kvm/x86/amd-memory-encryption.rst    | 12 ++++++++++
 arch/x86/include/uapi/asm/kvm.h               |  1 +
 arch/x86/kvm/svm/sev.c                        | 23 +++++++++++++++++--
 arch/x86/kvm/svm/svm.c                        |  1 +
 arch/x86/kvm/svm/svm.h                        |  1 +
 5 files changed, 36 insertions(+), 2 deletions(-)

diff --git a/Documentation/virt/kvm/x86/amd-memory-encryption.rst b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
index 37c5c37f4f6e..5ed11bc16b96 100644
--- a/Documentation/virt/kvm/x86/amd-memory-encryption.rst
+++ b/Documentation/virt/kvm/x86/amd-memory-encryption.rst
@@ -424,6 +424,18 @@ 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.
+
+Currently only one attribute is implemented:
+
+* group 0, attribute ``KVM_X86_SEV_VMSA_FEATURES``: return the set of all
+  bits that are accepted in the ``vmsa_features`` of ``KVM_SEV_INIT2``.
+
 Firmware Management
 ===================
 
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index b305daff056e..cccaa5ff6d01 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -459,6 +459,7 @@ struct kvm_sync_regs {
 
 /* attributes for system fd (group 0) */
 #define KVM_X86_XCOMP_GUEST_SUPP	0
+#define KVM_X86_SEV_VMSA_FEATURES	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 f760106c31f8..53e958805ab9 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -59,10 +59,12 @@ module_param_named(sev_es, sev_es_enabled, bool, 0444);
 /* enable/disable SEV-ES DebugSwap support */
 static bool sev_es_debug_swap_enabled = true;
 module_param_named(debug_swap, sev_es_debug_swap_enabled, bool, 0444);
+static u64 sev_supported_vmsa_features;
 #else
 #define sev_enabled false
 #define sev_es_enabled false
 #define sev_es_debug_swap_enabled false
+#define sev_supported_vmsa_features 0
 #endif /* CONFIG_KVM_AMD_SEV */
 
 static u8 sev_enc_bit;
@@ -612,8 +614,7 @@ 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)
-		save->sev_features |= SVM_SEV_FEAT_DEBUG_SWAP;
+	save->sev_features = sev_supported_vmsa_features;
 
 	pr_debug("Virtual Machine Save Area (VMSA):\n");
 	print_hex_dump_debug("", DUMP_PREFIX_NONE, 16, 1, save, sizeof(*save), false);
@@ -1849,6 +1850,20 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	return ret;
 }
 
+int sev_dev_get_attr(u64 attr, u64 *val)
+{
+	switch (attr) {
+#ifdef CONFIG_KVM_AMD_SEV
+	case KVM_X86_SEV_VMSA_FEATURES:
+		*val = sev_supported_vmsa_features;
+		return 0;
+#endif
+
+	default:
+		return -ENXIO;
+	}
+}
+
 int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp)
 {
 	struct kvm_sev_cmd sev_cmd;
@@ -2276,6 +2291,10 @@ void __init sev_hardware_setup(void)
 	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;
 #endif
 }
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index e90b429c84f1..aa1792f402ab 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5014,6 +5014,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
 	.enable_smi_window = svm_enable_smi_window,
 #endif
 
+	.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,
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 8ef95139cd24..d630026b23b0 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -671,6 +671,7 @@ void avic_refresh_virtual_apic_mode(struct kvm_vcpu *vcpu);
 extern unsigned int max_sev_asid;
 
 void sev_vm_destroy(struct kvm *kvm);
+int sev_dev_get_attr(u64 attr, u64 *val);
 int sev_mem_enc_ioctl(struct kvm *kvm, void __user *argp);
 int sev_mem_enc_register_region(struct kvm *kvm,
 				struct kvm_enc_region *range);
-- 
2.39.1