LoongArch: KVM: Add software breakpoint support

When VM runs in kvm mode, system will not exit to host mode when
executing a general software breakpoint instruction such as INSN_BREAK,
trap exception happens in guest mode rather than host mode. In order to
debug guest kernel on host side, one mechanism should be used to let VM
exit to host mode.

Here a hypercall instruction with a special code is used for software
breakpoint usage. VM exits to host mode and kvm hypervisor identifies
the special hypercall code and sets exit_reason with KVM_EXIT_DEBUG. And
then let qemu handle it.

Idea comes from ppc kvm, one api KVM_REG_LOONGARCH_DEBUG_INST is added
to get the hypercall code. VMM needs get sw breakpoint instruction with
this api and set the corresponding sw break point for guest kernel.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

3 files changed, 27 insertions, 3 deletions

diff --git a/arch/loongarch/kvm/exit.c b/arch/loongarch/kvm/exit.c
index 5ace3e8b4e80..43cf34f31c10 100644
--- a/arch/loongarch/kvm/exit.c
+++ b/arch/loongarch/kvm/exit.c
@@ -760,25 +760,37 @@ static void kvm_handle_service(struct kvm_vcpu *vcpu)
 
 static int kvm_handle_hypercall(struct kvm_vcpu *vcpu)
 {
+	int ret;
 	larch_inst inst;
 	unsigned int code;
 
 	inst.word = vcpu->arch.badi;
 	code = inst.reg0i15_format.immediate;
-	update_pc(&vcpu->arch);
+	ret = RESUME_GUEST;
 
 	switch (code) {
 	case KVM_HCALL_SERVICE:
 		vcpu->stat.hypercall_exits++;
 		kvm_handle_service(vcpu);
 		break;
+	case KVM_HCALL_SWDBG:
+		/* KVM_HCALL_SWDBG only in effective when SW_BP is enabled */
+		if (vcpu->guest_debug & KVM_GUESTDBG_SW_BP_MASK) {
+			vcpu->run->exit_reason = KVM_EXIT_DEBUG;
+			ret = RESUME_HOST;
+			break;
+		}
+		fallthrough;
 	default:
 		/* Treat it as noop intruction, only set return value */
 		kvm_write_reg(vcpu, LOONGARCH_GPR_A0, KVM_HCALL_INVALID_CODE);
 		break;
 	}
 
-	return RESUME_GUEST;
+	if (ret == RESUME_GUEST)
+		update_pc(&vcpu->arch);
+
+	return ret;
 }
 
 /*
diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
index 97a7aa508e96..9e8030d45129 100644
--- a/arch/loongarch/kvm/vcpu.c
+++ b/arch/loongarch/kvm/vcpu.c
@@ -248,7 +248,15 @@ int kvm_arch_vcpu_ioctl_set_mpstate(struct kvm_vcpu *vcpu,
 int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
 					struct kvm_guest_debug *dbg)
 {
-	return -EINVAL;
+	if (dbg->control & ~KVM_GUESTDBG_VALID_MASK)
+		return -EINVAL;
+
+	if (dbg->control & KVM_GUESTDBG_ENABLE)
+		vcpu->guest_debug = dbg->control;
+	else
+		vcpu->guest_debug = 0;
+
+	return 0;
 }
 
 static inline int kvm_set_cpuid(struct kvm_vcpu *vcpu, u64 val)
@@ -499,6 +507,9 @@ static int kvm_get_one_reg(struct kvm_vcpu *vcpu,
 		case KVM_REG_LOONGARCH_COUNTER:
 			*v = drdtime() + vcpu->kvm->arch.time_offset;
 			break;
+		case KVM_REG_LOONGARCH_DEBUG_INST:
+			*v = INSN_HVCL | KVM_HCALL_SWDBG;
+			break;
 		default:
 			ret = -EINVAL;
 			break;
diff --git a/arch/loongarch/kvm/vm.c b/arch/loongarch/kvm/vm.c
index 191d82309a1e..6b2e4f66ad26 100644
--- a/arch/loongarch/kvm/vm.c
+++ b/arch/loongarch/kvm/vm.c
@@ -76,6 +76,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_IMMEDIATE_EXIT:
 	case KVM_CAP_IOEVENTFD:
 	case KVM_CAP_MP_STATE:
+	case KVM_CAP_SET_GUEST_DEBUG:
 		r = 1;
 		break;
 	case KVM_CAP_NR_VCPUS: