With KVM virtualization, debug exception is passthrough to guest kernel
rather than host mode. Here hypercall instruction with special code is used
for sw breakpoint usage.

Now only software breakpoint is supported, and it is allowed to
insert/remove software breakpoint. We can debug guest kernel with gdb
method after kernel is loaded, hardware breakpoint will be added in later.

Signed-off-by: Bibo Mao <maobibo@loongson.cn>
---
v1 ... v2:
  1. Enable TARGET_KVM_HAVE_GUEST_DEBUG on loongarch64 platform
---
 configs/targets/loongarch64-softmmu.mak |  1 +
 target/loongarch/kvm/kvm.c              | 76 +++++++++++++++++++++++++
 2 files changed, 77 insertions(+)

diff --git a/configs/targets/loongarch64-softmmu.mak b/configs/targets/loongarch64-softmmu.mak
index 84beb19b90..65b65e0c34 100644
--- a/configs/targets/loongarch64-softmmu.mak
+++ b/configs/targets/loongarch64-softmmu.mak
@@ -1,5 +1,6 @@
 TARGET_ARCH=loongarch64
 TARGET_BASE_ARCH=loongarch
+TARGET_KVM_HAVE_GUEST_DEBUG=y
 TARGET_SUPPORTS_MTTCG=y
 TARGET_XML_FILES= gdb-xml/loongarch-base32.xml gdb-xml/loongarch-base64.xml gdb-xml/loongarch-fpu.xml
 # all boards require libfdt
diff --git a/target/loongarch/kvm/kvm.c b/target/loongarch/kvm/kvm.c
index bc75552d0f..0a94784276 100644
--- a/target/loongarch/kvm/kvm.c
+++ b/target/loongarch/kvm/kvm.c
@@ -28,6 +28,7 @@
 #include "trace.h"
 
 static bool cap_has_mp_state;
+static unsigned int brk_insn;
 const KVMCapabilityInfo kvm_arch_required_capabilities[] = {
     KVM_CAP_LAST_INFO
 };
@@ -658,7 +659,14 @@ static void kvm_loongarch_vm_stage_change(void *opaque, bool running,
 
 int kvm_arch_init_vcpu(CPUState *cs)
 {
+    uint64_t val;
+
     qemu_add_vm_change_state_handler(kvm_loongarch_vm_stage_change, cs);
+
+    if (!kvm_get_one_reg(cs, KVM_REG_LOONGARCH_DEBUG_INST, &val)) {
+        brk_insn = val;
+    }
+
     return 0;
 }
 
@@ -733,6 +741,67 @@ bool kvm_arch_stop_on_emulation_error(CPUState *cs)
     return true;
 }
 
+void kvm_arch_update_guest_debug(CPUState *cpu, struct kvm_guest_debug *dbg)
+{
+    if (kvm_sw_breakpoints_active(cpu)) {
+        dbg->control |= KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP;
+    }
+}
+
+int kvm_arch_insert_sw_breakpoint(CPUState *cs, struct kvm_sw_breakpoint *bp)
+{
+    if (cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 4, 0) ||
+        cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&brk_insn, 4, 1)) {
+        error_report("%s failed", __func__);
+        return -EINVAL;
+    }
+    return 0;
+}
+
+int kvm_arch_remove_sw_breakpoint(CPUState *cs, struct kvm_sw_breakpoint *bp)
+{
+    static uint32_t brk;
+
+    if (cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&brk, 4, 0) ||
+        brk != brk_insn ||
+        cpu_memory_rw_debug(cs, bp->pc, (uint8_t *)&bp->saved_insn, 4, 1)) {
+        error_report("%s failed", __func__);
+        return -EINVAL;
+    }
+    return 0;
+}
+
+int kvm_arch_insert_hw_breakpoint(vaddr addr, vaddr len, int type)
+{
+    return -ENOSYS;
+}
+
+int kvm_arch_remove_hw_breakpoint(vaddr addr, vaddr len, int type)
+{
+    return -ENOSYS;
+}
+
+void kvm_arch_remove_all_hw_breakpoints(void)
+{
+}
+
+static bool kvm_loongarch_handle_debug(CPUState *cs, struct kvm_run *run)
+{
+    LoongArchCPU *cpu = LOONGARCH_CPU(cs);
+    CPULoongArchState *env = &cpu->env;
+
+    kvm_cpu_synchronize_state(cs);
+    if (cs->singlestep_enabled) {
+        return true;
+    }
+
+    if (kvm_find_sw_breakpoint(cs, env->pc)) {
+        return true;
+    }
+
+    return false;
+}
+
 int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
 {
     int ret = 0;
@@ -751,6 +820,13 @@ int kvm_arch_handle_exit(CPUState *cs, struct kvm_run *run)
                          run->iocsr_io.len,
                          run->iocsr_io.is_write);
         break;
+
+    case KVM_EXIT_DEBUG:
+        if (kvm_loongarch_handle_debug(cs, run)) {
+            ret = EXCP_DEBUG;
+        }
+        break;
+
     default:
         ret = -1;
         warn_report("KVM: unknown exit reason %d", run->exit_reason);

base-commit: 922582ace2df59572a671f5c0c5c6c5c706995e5
-- 
2.39.3