LoongArch: KVM: Add hypercall service support for usermode VMM

Some VMMs provides special hypercall service in usermode, KVM should not
handle the usermode hypercall service, thus pass it to usermode, let the
usermode VMM handle it.

Here a new code KVM_HCALL_CODE_USER_SERVICE is added for the user-mode
hypercall service, KVM lets all six registers visible to usermode VMM.

Signed-off-by: Bibo Mao <[e-mail]>
Signed-off-by: Huacai Chen <[e-mail]>

arch/loongarch/include/asm/kvm_host.h

···
 #define LOONGARCH_PV_FEAT_UPDATED	BIT_ULL(63)
 #define LOONGARCH_PV_FEAT_MASK		(BIT(KVM_FEATURE_IPI) |		\
 					 BIT(KVM_FEATURE_STEAL_TIME) |	\
+					 BIT(KVM_FEATURE_USER_HCALL) |	\
 					 BIT(KVM_FEATURE_VIRT_EXTIOI))
 
 struct kvm_vcpu_arch {

arch/loongarch/include/asm/kvm_para.h

···
 
 #define KVM_HCALL_CODE_SERVICE		0
 #define KVM_HCALL_CODE_SWDBG		1
+#define KVM_HCALL_CODE_USER_SERVICE	2
 
 #define KVM_HCALL_SERVICE		HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_SERVICE)
 #define  KVM_HCALL_FUNC_IPI		1
 #define  KVM_HCALL_FUNC_NOTIFY		2
 
 #define KVM_HCALL_SWDBG			HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_SWDBG)
+
+#define KVM_HCALL_USER_SERVICE		HYPERCALL_ENCODE(HYPERVISOR_KVM, KVM_HCALL_CODE_USER_SERVICE)
 
 /*
  * LoongArch hypercall return code

arch/loongarch/include/asm/kvm_vcpu.h

···
 int  kvm_emu_mmio_write(struct kvm_vcpu *vcpu, larch_inst inst);
 int  kvm_complete_mmio_read(struct kvm_vcpu *vcpu, struct kvm_run *run);
 int  kvm_complete_iocsr_read(struct kvm_vcpu *vcpu, struct kvm_run *run);
+int  kvm_complete_user_service(struct kvm_vcpu *vcpu, struct kvm_run *run);
 int  kvm_emu_idle(struct kvm_vcpu *vcpu);
 int  kvm_pending_timer(struct kvm_vcpu *vcpu);
 int  kvm_handle_fault(struct kvm_vcpu *vcpu, int fault);

arch/loongarch/include/uapi/asm/kvm_para.h

···
 #define  KVM_FEATURE_STEAL_TIME		2
 /* BIT 24 - 31 are features configurable by user space vmm */
 #define  KVM_FEATURE_VIRT_EXTIOI	24
+#define  KVM_FEATURE_USER_HCALL		25
 
 #endif /* _UAPI_ASM_KVM_PARA_H */

arch/loongarch/kvm/exit.c

···
 	return kvm_handle_rdwr_fault(vcpu, true);
 }
 
+int kvm_complete_user_service(struct kvm_vcpu *vcpu, struct kvm_run *run)
+{
+	update_pc(&vcpu->arch);
+	kvm_write_reg(vcpu, LOONGARCH_GPR_A0, run->hypercall.ret);
+
+	return 0;
+}
+
 /**
  * kvm_handle_fpu_disabled() - Guest used fpu however it is disabled at host
  * @vcpu:	Virtual CPU context.
···
 	case KVM_HCALL_SERVICE:
 		vcpu->stat.hypercall_exits++;
 		kvm_handle_service(vcpu);
+		break;
+	case KVM_HCALL_USER_SERVICE:
+		if (!kvm_guest_has_pv_feature(vcpu, KVM_FEATURE_USER_HCALL)) {
+			kvm_write_reg(vcpu, LOONGARCH_GPR_A0, KVM_HCALL_INVALID_CODE);
+			break;
+		}
+
+		vcpu->stat.hypercall_exits++;
+		vcpu->run->exit_reason = KVM_EXIT_HYPERCALL;
+		vcpu->run->hypercall.nr = KVM_HCALL_USER_SERVICE;
+		vcpu->run->hypercall.args[0] = kvm_read_reg(vcpu, LOONGARCH_GPR_A0);
+		vcpu->run->hypercall.args[1] = kvm_read_reg(vcpu, LOONGARCH_GPR_A1);
+		vcpu->run->hypercall.args[2] = kvm_read_reg(vcpu, LOONGARCH_GPR_A2);
+		vcpu->run->hypercall.args[3] = kvm_read_reg(vcpu, LOONGARCH_GPR_A3);
+		vcpu->run->hypercall.args[4] = kvm_read_reg(vcpu, LOONGARCH_GPR_A4);
+		vcpu->run->hypercall.args[5] = kvm_read_reg(vcpu, LOONGARCH_GPR_A5);
+		vcpu->run->hypercall.flags = 0;
+		/*
+		 * Set invalid return value by default, let user-mode VMM modify it.
+		 */
+		vcpu->run->hypercall.ret = KVM_HCALL_INVALID_CODE;
+		ret = RESUME_HOST;
 		break;
 	case KVM_HCALL_SWDBG:
 		/* KVM_HCALL_SWDBG only in effective when SW_BP is enabled */

arch/loongarch/kvm/vcpu.c

···
 		vcpu->mmio_needed = 0;
 	}
 
-	if (run->exit_reason == KVM_EXIT_LOONGARCH_IOCSR) {
+	switch (run->exit_reason) {
+	case KVM_EXIT_HYPERCALL:
+		kvm_complete_user_service(vcpu, run);
+		break;
+	case KVM_EXIT_LOONGARCH_IOCSR:
 		if (!run->iocsr_io.is_write)
 			kvm_complete_iocsr_read(vcpu, run);
+		break;
 	}
 
 	if (!vcpu->wants_to_run)