KVM: SEV: Add support to handle MSR based Page State Change VMGEXIT

SEV-SNP VMs can ask the hypervisor to change the page state in the RMP
table to be private or shared using the Page State Change MSR protocol
as defined in the GHCB specification.

When using gmem, private/shared memory is allocated through separate
pools, and KVM relies on userspace issuing a KVM_SET_MEMORY_ATTRIBUTES
KVM ioctl to tell the KVM MMU whether or not a particular GFN should be
backed by private memory or not.

Forward these page state change requests to userspace so that it can
issue the expected KVM ioctls. The KVM MMU will handle updating the RMP
entries when it is ready to map a private page into a guest.

Use the existing KVM_HC_MAP_GPA_RANGE hypercall format to deliver these
requests to userspace via KVM_EXIT_HYPERCALL.
Signed-off-by: Michael Roth <michael.roth@amd.com>
Co-developed-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Message-ID: <20240501085210.2213060-10-michael.roth@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/sev-common.h

@@ -101,11 +101,17 @@ enum psc_op {
 	/* GHCBData[11:0] */				\
 	GHCB_MSR_PSC_REQ)
 
+#define GHCB_MSR_PSC_REQ_TO_GFN(msr) (((msr) & GENMASK_ULL(51, 12)) >> 12)
+#define GHCB_MSR_PSC_REQ_TO_OP(msr) (((msr) & GENMASK_ULL(55, 52)) >> 52)
+
 #define GHCB_MSR_PSC_RESP		0x015
 #define GHCB_MSR_PSC_RESP_VAL(val)			\
 	/* GHCBData[63:32] */				\
 	(((u64)(val) & GENMASK_ULL(63, 32)) >> 32)
 
+/* Set highest bit as a generic error response */
+#define GHCB_MSR_PSC_RESP_ERROR (BIT_ULL(63) | GHCB_MSR_PSC_RESP)
+
 /* GHCB Hypervisor Feature Request/Response */
 #define GHCB_MSR_HV_FT_REQ		0x080
 #define GHCB_MSR_HV_FT_RESP		0x081

arch/x86/kvm/svm/sev.c

@@ -3461,6 +3461,48 @@ static void set_ghcb_msr(struct vcpu_svm *svm, u64 value)
 	svm->vmcb->control.ghcb_gpa = value;
 }
 
+static int snp_complete_psc_msr(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+
+	if (vcpu->run->hypercall.ret)
+		set_ghcb_msr(svm, GHCB_MSR_PSC_RESP_ERROR);
+	else
+		set_ghcb_msr(svm, GHCB_MSR_PSC_RESP);
+
+	return 1; /* resume guest */
+}
+
+static int snp_begin_psc_msr(struct vcpu_svm *svm, u64 ghcb_msr)
+{
+	u64 gpa = gfn_to_gpa(GHCB_MSR_PSC_REQ_TO_GFN(ghcb_msr));
+	u8 op = GHCB_MSR_PSC_REQ_TO_OP(ghcb_msr);
+	struct kvm_vcpu *vcpu = &svm->vcpu;
+
+	if (op != SNP_PAGE_STATE_PRIVATE && op != SNP_PAGE_STATE_SHARED) {
+		set_ghcb_msr(svm, GHCB_MSR_PSC_RESP_ERROR);
+		return 1; /* resume guest */
+	}
+
+	if (!(vcpu->kvm->arch.hypercall_exit_enabled & (1 << KVM_HC_MAP_GPA_RANGE))) {
+		set_ghcb_msr(svm, GHCB_MSR_PSC_RESP_ERROR);
+		return 1; /* resume guest */
+	}
+
+	vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
+	vcpu->run->hypercall.nr = KVM_HC_MAP_GPA_RANGE;
+	vcpu->run->hypercall.args[0] = gpa;
+	vcpu->run->hypercall.args[1] = 1;
+	vcpu->run->hypercall.args[2] = (op == SNP_PAGE_STATE_PRIVATE)
+				       ? KVM_MAP_GPA_RANGE_ENCRYPTED
+				       : KVM_MAP_GPA_RANGE_DECRYPTED;
+	vcpu->run->hypercall.args[2] |= KVM_MAP_GPA_RANGE_PAGE_SZ_4K;
+
+	vcpu->arch.complete_userspace_io = snp_complete_psc_msr;
+
+	return 0; /* forward request to userspace */
+}
+
 static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
@@ -3566,6 +3608,12 @@ static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 				  GHCB_MSR_INFO_POS);
 		break;
 	}
+	case GHCB_MSR_PSC_REQ:
+		if (!sev_snp_guest(vcpu->kvm))
+			goto out_terminate;
+
+		ret = snp_begin_psc_msr(svm, control->ghcb_gpa);
+		break;
 	case GHCB_MSR_TERM_REQ: {
 		u64 reason_set, reason_code;