LoongArch: ptrace: Add hardware single step support

Use the generic ptrace_resume code for PTRACE_SYSCALL, PTRACE_CONT,
PTRACE_KILL and PTRACE_SINGLESTEP handling. This implies defining
arch_has_single_step() and implementing the user_enable_single_step()
and user_disable_single_step() functions.

LoongArch cannot do hardware single-stepping per se, the hardware
single-stepping it is achieved by configuring the instruction fetch
watchpoints (FWPS) and specifies that the next instruction must trigger
the watch exception by setting the mask bit. In some scenarios
CSR.FWPS.Skip is used to ignore the next hit result, avoid endless
repeated triggering of the same watchpoint without canceling it.

Signed-off-by: Qing Zhang <zhangqing@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

1 files changed, 41 insertions, 6 deletions

diff --git a/arch/loongarch/kernel/traps.c b/arch/loongarch/kernel/traps.c
index ba67d787f0d7..1d47747e2154 100644
--- a/arch/loongarch/kernel/traps.c
+++ b/arch/loongarch/kernel/traps.c
@@ -513,14 +513,49 @@ asmlinkage void noinstr do_watch(struct pt_regs *regs)
 {
 	irqentry_state_t state = irqentry_enter(regs);
 
-#ifdef CONFIG_HAVE_HW_BREAKPOINT
-	breakpoint_handler(regs);
-	watchpoint_handler(regs);
-	force_sig(SIGTRAP);
-#else
+#ifndef CONFIG_HAVE_HW_BREAKPOINT
 	pr_warn("Hardware watch point handler not implemented!\n");
-#endif
+#else
+	if (test_tsk_thread_flag(current, TIF_SINGLESTEP)) {
+		int llbit = (csr_read32(LOONGARCH_CSR_LLBCTL) & 0x1);
+		unsigned long pc = instruction_pointer(regs);
+		union loongarch_instruction *ip = (union loongarch_instruction *)pc;
+
+		if (llbit) {
+			/*
+			 * When the ll-sc combo is encountered, it is regarded as an single
+			 * instruction. So don't clear llbit and reset CSR.FWPS.Skip until
+			 * the llsc execution is completed.
+			 */
+			csr_write32(CSR_FWPC_SKIP, LOONGARCH_CSR_FWPS);
+			csr_write32(CSR_LLBCTL_KLO, LOONGARCH_CSR_LLBCTL);
+			goto out;
+		}
 
+		if (pc == current->thread.single_step) {
+			/*
+			 * Certain insns are occasionally not skipped when CSR.FWPS.Skip is
+			 * set, such as fld.d/fst.d. So singlestep needs to compare whether
+			 * the csr_era is equal to the value of singlestep which last time set.
+			 */
+			if (!is_self_loop_ins(ip, regs)) {
+				/*
+				 * Check if the given instruction the target pc is equal to the
+				 * current pc, If yes, then we should not set the CSR.FWPS.SKIP
+				 * bit to break the original instruction stream.
+				 */
+				csr_write32(CSR_FWPC_SKIP, LOONGARCH_CSR_FWPS);
+				goto out;
+			}
+		}
+	} else {
+		breakpoint_handler(regs);
+		watchpoint_handler(regs);
+	}
+
+	force_sig(SIGTRAP);
+out:
+#endif
 	irqentry_exit(regs, state);
 }