[PATCH v6 2/2] arm64: kgdb: Set PSTATE.SS to 1 to re-enable single-step

[Sumit Garg]

Currently only the first attempt to single-step has any effect. After
that all further stepping remains "stuck" at the same program counter
value.

Refer to the ARM Architecture Reference Manual (ARM DDI 0487E.a) D2.12,
PSTATE.SS=1 should be set at each step before transferring the PE to the
'Active-not-pending' state. The problem here is PSTATE.SS=1 is not set
since the second single-step.

After the first single-step, the PE transferes to the 'Inactive' state,
with PSTATE.SS=0 and MDSCR.SS=1, thus PSTATE.SS won't be set to 1 due to
kernel_active_single_step()=true. Then the PE transferes to the
'Active-pending' state when ERET and returns to the debugger by step
exception.

Before this patch:
==================
Entering kdb (current=0xffff3376039f0000, pid 1) on processor 0 due to Keyboard Entry
[0]kdb>

[0]kdb>
[0]kdb> bp write_sysrq_trigger
Instruction(i) BP #0 at 0xffffa45c13d09290 (write_sysrq_trigger)
    is enabled   addr at ffffa45c13d09290, hardtype=0 installed=0

[0]kdb> go
$ echo h > /proc/sysrq-trigger

Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to Breakpoint @ 0xffffad651a309290
[1]kdb> ss

Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to SS trap @ 0xffffad651a309294
[1]kdb> ss

Entering kdb (current=0xffff4f7e453f8000, pid 175) on processor 1 due to SS trap @ 0xffffad651a309294
[1]kdb>

After this patch:
=================
Entering kdb (current=0xffff6851c39f0000, pid 1) on processor 0 due to Keyboard Entry
[0]kdb> bp write_sysrq_trigger
Instruction(i) BP #0 at 0xffffc02d2dd09290 (write_sysrq_trigger)
    is enabled   addr at ffffc02d2dd09290, hardtype=0 installed=0

[0]kdb> go
$ echo h > /proc/sysrq-trigger

Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to Breakpoint @ 0xffffc02d2dd09290
[1]kdb> ss

Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd09294
[1]kdb> ss

Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd09298
[1]kdb> ss

Entering kdb (current=0xffff6851c53c1840, pid 174) on processor 1 due to SS trap @ 0xffffc02d2dd0929c
[1]kdb>

Fixes: 44679a4f142b ("arm64: KGDB: Add step debugging support")
Co-developed-by: Wei Li <liwei391 at huawei.com>
Signed-off-by: Wei Li <liwei391 at huawei.com>
Signed-off-by: Sumit Garg <sumit.garg at linaro.org>
Tested-by: Douglas Anderson <dianders at chromium.org>
Acked-by: Daniel Thompson <daniel.thompson at linaro.org>
Tested-by: Daniel Thompson <daniel.thompson at linaro.org>
---
 arch/arm64/include/asm/debug-monitors.h | 1 +
 arch/arm64/kernel/debug-monitors.c      | 5 +++++
 arch/arm64/kernel/kgdb.c                | 2 ++
 3 files changed, 8 insertions(+)

diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
index 7b7e05c02691..13d437bcbf58 100644
--- a/arch/arm64/include/asm/debug-monitors.h
+++ b/arch/arm64/include/asm/debug-monitors.h
@@ -104,6 +104,7 @@ void user_regs_reset_single_step(struct user_pt_regs *regs,
 void kernel_enable_single_step(struct pt_regs *regs);
 void kernel_disable_single_step(void);
 int kernel_active_single_step(void);
+void kernel_rewind_single_step(struct pt_regs *regs);
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
 int reinstall_suspended_bps(struct pt_regs *regs);
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 3da09778267e..64f2ecbdfe5c 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -438,6 +438,11 @@ int kernel_active_single_step(void)
 }
 NOKPROBE_SYMBOL(kernel_active_single_step);
 
+void kernel_rewind_single_step(struct pt_regs *regs)
+{
+	set_regs_spsr_ss(regs);
+}
+
 /* ptrace API */
 void user_enable_single_step(struct task_struct *task)
 {
diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index cda9c1e9864f..4e1f983df3d1 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -224,6 +224,8 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
 		 */
 		if (!kernel_active_single_step())
 			kernel_enable_single_step(linux_regs);
+		else
+			kernel_rewind_single_step(linux_regs);
 		err = 0;
 		break;
 	default:
-- 
2.34.1