KVM: SEV: Allow per-guest configuration of GHCB protocol version

The GHCB protocol version may be different from one guest to the next.
Add a field to track it for each KVM instance and extend KVM_SEV_INIT2
to allow it to be configured by userspace.

Now that all SEV-ES support for GHCB protocol version 2 is in place, go
ahead and default to it when creating SEV-ES guests through the new
KVM_SEV_INIT2 interface. Keep the older KVM_SEV_ES_INIT interface
restricted to GHCB protocol version 1.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michael Roth <michael.roth@amd.com>
Message-ID: <20240501071048.2208265-5-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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

@@ -95,13 +95,19 @@ Returns: 0 on success, -negative on error
         struct kvm_sev_init {
                 __u64 vmsa_features;  /* initial value of features field in VMSA */
                 __u32 flags;          /* must be 0 */
-                __u32 pad[9];
+                __u16 ghcb_version;   /* maximum guest GHCB version allowed */
+                __u16 pad1;
+                __u32 pad2[8];
         };
 
 It is an error if the hypervisor does not support any of the bits that
 are set in ``flags`` or ``vmsa_features``.  ``vmsa_features`` must be
 0 for SEV virtual machines, as they do not have a VMSA.
 
+``ghcb_version`` must be 0 for SEV virtual machines, as they do not issue GHCB
+requests. If ``ghcb_version`` is 0 for any other guest type, then the maximum
+allowed guest GHCB protocol will default to version 2.
+
 This command replaces the deprecated KVM_SEV_INIT and KVM_SEV_ES_INIT commands.
 The commands did not have any parameters (the ```data``` field was unused) and
 only work for the KVM_X86_DEFAULT_VM machine type (0).
@@ -112,7 +118,8 @@ They behave as if:
   KVM_SEV_ES_INIT
 
 * the ``flags`` and ``vmsa_features`` fields of ``struct kvm_sev_init`` are
-  set to zero
+  set to zero, and ``ghcb_version`` is set to 0 for KVM_SEV_INIT and 1 for
+  KVM_SEV_ES_INIT.
 
 If the ``KVM_X86_SEV_VMSA_FEATURES`` attribute does not exist, the hypervisor only
 supports KVM_SEV_INIT and KVM_SEV_ES_INIT.  In that case, note that KVM_SEV_ES_INIT

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

@@ -711,7 +711,9 @@ struct kvm_sev_cmd {
 struct kvm_sev_init {
 	__u64 vmsa_features;
 	__u32 flags;
-	__u32 pad[9];
+	__u16 ghcb_version;
+	__u16 pad1;
+	__u32 pad2[8];
 };
 
 struct kvm_sev_launch_start {

arch/x86/kvm/svm/sev.c

@@ -33,7 +33,8 @@
 #include "cpuid.h"
 #include "trace.h"
 
-#define GHCB_VERSION_MAX	1ULL
+#define GHCB_VERSION_MAX	2ULL
+#define GHCB_VERSION_DEFAULT	2ULL
 #define GHCB_VERSION_MIN	1ULL
 
 #define GHCB_HV_FT_SUPPORTED	GHCB_HV_FT_SNP
@@ -268,12 +269,24 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
 	if (data->vmsa_features & ~valid_vmsa_features)
 		return -EINVAL;
 
+	if (data->ghcb_version > GHCB_VERSION_MAX || (!es_active && data->ghcb_version))
+		return -EINVAL;
+
 	if (unlikely(sev->active))
 		return -EINVAL;
 
 	sev->active = true;
 	sev->es_active = es_active;
 	sev->vmsa_features = data->vmsa_features;
+	sev->ghcb_version = data->ghcb_version;
+
+	/*
+	 * Currently KVM supports the full range of mandatory features defined
+	 * by version 2 of the GHCB protocol, so default to that for SEV-ES
+	 * guests created via KVM_SEV_INIT2.
+	 */
+	if (sev->es_active && !sev->ghcb_version)
+		sev->ghcb_version = GHCB_VERSION_DEFAULT;
 
 	ret = sev_asid_new(sev);
 	if (ret)
@@ -307,6 +320,7 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
 {
 	struct kvm_sev_init data = {
 		.vmsa_features = 0,
+		.ghcb_version = 0,
 	};
 	unsigned long vm_type;
 
@@ -314,6 +328,14 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
 		return -EINVAL;
 
 	vm_type = (argp->id == KVM_SEV_INIT ? KVM_X86_SEV_VM : KVM_X86_SEV_ES_VM);
+
+	/*
+	 * KVM_SEV_ES_INIT has been deprecated by KVM_SEV_INIT2, so it will
+	 * continue to only ever support the minimal GHCB protocol version.
+	 */
+	if (vm_type == KVM_X86_SEV_ES_VM)
+		data.ghcb_version = GHCB_VERSION_MIN;
+
 	return __sev_guest_init(kvm, argp, &data, vm_type);
 }
 
@@ -2897,6 +2919,7 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
 	struct kvm_vcpu *vcpu = &svm->vcpu;
+	struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info;
 	u64 ghcb_info;
 	int ret = 1;
 
@@ -2907,7 +2930,7 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 
 	switch (ghcb_info) {
 	case GHCB_MSR_SEV_INFO_REQ:
-		set_ghcb_msr(svm, GHCB_MSR_SEV_INFO(GHCB_VERSION_MAX,
+		set_ghcb_msr(svm, GHCB_MSR_SEV_INFO((__u64)sev->ghcb_version,
 						    GHCB_VERSION_MIN,
 						    sev_enc_bit));
 		break;
@@ -3268,11 +3291,14 @@ void sev_init_vmcb(struct vcpu_svm *svm)
 
 void sev_es_vcpu_reset(struct vcpu_svm *svm)
 {
+	struct kvm_vcpu *vcpu = &svm->vcpu;
+	struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info;
+
 	/*
 	 * Set the GHCB MSR value as per the GHCB specification when emulating
 	 * vCPU RESET for an SEV-ES guest.
 	 */
-	set_ghcb_msr(svm, GHCB_MSR_SEV_INFO(GHCB_VERSION_MAX,
+	set_ghcb_msr(svm, GHCB_MSR_SEV_INFO((__u64)sev->ghcb_version,
 					    GHCB_VERSION_MIN,
 					    sev_enc_bit));
 }

arch/x86/kvm/svm/svm.h

@@ -87,6 +87,7 @@ struct kvm_sev_info {
 	struct list_head regions_list;  /* List of registered regions */
 	u64 ap_jump_table;	/* SEV-ES AP Jump Table address */
 	u64 vmsa_features;
+	u16 ghcb_version;	/* Highest guest GHCB protocol version allowed */
 	struct kvm *enc_context_owner; /* Owner of copied encryption context */
 	struct list_head mirror_vms; /* List of VMs mirroring */
 	struct list_head mirror_entry; /* Use as a list entry of mirrors */