x86/kvm/hyper-v: Add support for synthetic debugger via hypercalls

There is another mode for the synthetic debugger which uses hypercalls
to send/recv network data instead of the MSR interface.

This interface is much slower and less recommended since you might get
a lot of VMExits while KDVM polling for new packets to recv, rather
than simply checking the pending page to see if there is data avialble
and then request.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Jon Doron <arilou@gmail.com>
Message-Id: <20200529134543.1127440-6-arilou@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -1835,6 +1835,34 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		}
 		ret = kvm_hv_send_ipi(vcpu, ingpa, outgpa, true, false);
 		break;
+	case HVCALL_POST_DEBUG_DATA:
+	case HVCALL_RETRIEVE_DEBUG_DATA:
+		if (unlikely(fast)) {
+			ret = HV_STATUS_INVALID_PARAMETER;
+			break;
+		}
+		fallthrough;
+	case HVCALL_RESET_DEBUG_SESSION: {
+		struct kvm_hv_syndbg *syndbg = vcpu_to_hv_syndbg(vcpu);
+
+		if (!kvm_hv_is_syndbg_enabled(vcpu)) {
+			ret = HV_STATUS_INVALID_HYPERCALL_CODE;
+			break;
+		}
+
+		if (!(syndbg->options & HV_X64_SYNDBG_OPTION_USE_HCALLS)) {
+			ret = HV_STATUS_OPERATION_DENIED;
+			break;
+		}
+		vcpu->run->exit_reason = KVM_EXIT_HYPERV;
+		vcpu->run->hyperv.type = KVM_EXIT_HYPERV_HCALL;
+		vcpu->run->hyperv.u.hcall.input = param;
+		vcpu->run->hyperv.u.hcall.params[0] = ingpa;
+		vcpu->run->hyperv.u.hcall.params[1] = outgpa;
+		vcpu->arch.complete_userspace_io =
+				kvm_hv_hypercall_complete_userspace;
+		return 0;
+	}
 	default:
 		ret = HV_STATUS_INVALID_HYPERCALL_CODE;
 		break;