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Date:   Sun, 10 May 2020 05:41:59 +0800
From:   Wei Li <liwei391@...wei.com>
To:     <daniel.thompson@...aro.org>, <jason.wessel@...driver.com>,
        <dianders@...omium.org>, <maz@...nel.org>, <mark.rutland@....com>,
        <mhiramat@...nel.org>, <davem@...emloft.net>, <will@...nel.org>,
        <catalin.marinas@....com>
CC:     <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <liwei1412@....com>
Subject: [PATCH 4/4] arm64: kgdb: Set PSTATE.SS to 1 to reenable single-step

After fixing wrongly single-stepping into the irq handler, when we execute
single-step in kdb/kgdb, we can see only the first step can work.

Refer to the ARM Architecture Reference Manual (ARM DDI 0487E.a) D2.12,
i think PSTATE.SS=1 should be set each step for 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:
* kdb:
Entering kdb (current=0xffff8000119d2dc0, pid 0) on processor 0 due to Keyboard Entry
[0]kdb> bp printk
Instruction(i) BP #0 at 0xffff80001014874c (printk)
    is enabled   addr at ffff80001014874c, hardtype=0 installed=0

[0]kdb> g

/ # echo h > /proc/sysrq-trigger

Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to Breakpoint @ 0xffff80001014874c
[3]kdb> ss

Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
[3]kdb> ss

Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
[3]kdb> ss

Entering kdb (current=0xffff0000fa6948c0, pid 265) on processor 3 due to SS trap @ 0xffff800010148750
[3]kdb>

* kgdb:
(gdb) target remote 127.1:23002
Remote debugging using 127.1:23002
arch_kgdb_breakpoint () at /home/liwei/main_code/linux/arch/arm64/include/asm/kgdb.h:21
21		asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
(gdb) b printk
Breakpoint 1 at 0xffff80001014874c: file /home/liwei/main_code/linux/kernel/printk/printk.c, line 2076.
(gdb) c
Continuing.
[New Thread 277]
[Switching to Thread 276]

Thread 171 hit Breakpoint 1, printk (fmt=0xffff8000112fc130 "\001\066sysrq: HELP : ")
    at /home/liwei/main_code/linux/kernel/printk/printk.c:2076
2076	{
(gdb) stepi
0xffff800010148750	2076	{
(gdb) stepi
0xffff800010148750	2076	{
(gdb) stepi
0xffff800010148750	2076	{
(gdb)

After this patch:
* kdb:
Entering kdb (current=0xffff8000119d2dc0, pid 0) on processor 0 due to Keyboard Entry
[0]kdb> bp printk
Instruction(i) BP #0 at 0xffff80001014874c (printk)
    is enabled   addr at ffff80001014874c, hardtype=0 installed=0

[0]kdb> g

/ # echo h > /proc/sysrq-trigger

Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to Breakpoint @ 0xffff80001014874c
[2]kdb> ss

Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148750
[2]kdb> ss

Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148754
[2]kdb> ss

Entering kdb (current=0xffff0000fa800040, pid 264) on processor 2 due to SS trap @ 0xffff800010148758
[2]kdb>

* kgdb:
(gdb) target remote 127.1:23002
Remote debugging using 127.1:23002
arch_kgdb_breakpoint () at /home/liwei/main_code/linux/arch/arm64/include/asm/kgdb.h:21
21		asm ("brk %0" : : "I" (KGDB_COMPILED_DBG_BRK_IMM));
(gdb) b printk
Breakpoint 1 at 0xffff80001014874c: file /home/liwei/main_code/linux/kernel/printk/printk.c, line 2076.
(gdb) c
Continuing.
[New Thread 281]
[New Thread 280]
[Switching to Thread 281]

Thread 174 hit Breakpoint 1, printk (fmt=0xffff8000112fc138 "\001\066sysrq: HELP : ")
    at /home/liwei/main_code/linux/kernel/printk/printk.c:2076
2076	{
(gdb) stepi
0xffff800010148750	2076	{
(gdb) stepi
2080		va_start(args, fmt);
(gdb) stepi
0xffff800010148758	2080		va_start(args, fmt);
(gdb)

Fixes: 44679a4f142b ("arm64: KGDB: Add step debugging support")
Signed-off-by: Wei Li <liwei391@...wei.com>
---
 arch/arm64/include/asm/debug-monitors.h | 2 ++
 arch/arm64/kernel/debug-monitors.c      | 2 +-
 arch/arm64/kernel/kgdb.c                | 3 ++-
 3 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/debug-monitors.h b/arch/arm64/include/asm/debug-monitors.h
index b62469f3475b..a48b507c89ee 100644
--- a/arch/arm64/include/asm/debug-monitors.h
+++ b/arch/arm64/include/asm/debug-monitors.h
@@ -78,6 +78,8 @@ struct step_hook {
 	int (*fn)(struct pt_regs *regs, unsigned int esr);
 };
 
+void set_regs_spsr_ss(struct pt_regs *regs);
+
 void register_user_step_hook(struct step_hook *hook);
 void unregister_user_step_hook(struct step_hook *hook);
 
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 25ce6b5a52d2..7a58233677de 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -141,7 +141,7 @@ postcore_initcall(debug_monitors_init);
 /*
  * Single step API and exception handling.
  */
-static void set_regs_spsr_ss(struct pt_regs *regs)
+void set_regs_spsr_ss(struct pt_regs *regs)
 {
 	regs->pstate |= DBG_SPSR_SS;
 }
diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index 3910ac06c261..093ad9d2e5e6 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -230,7 +230,8 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
 			kernel_prepare_single_step(&per_cpu(kgdb_ss_flags,
 					raw_smp_processor_id()), linux_regs);
 			kernel_enable_single_step(linux_regs);
-		}
+		} else
+			set_regs_spsr_ss(linux_regs);
 
 		err = 0;
 		break;
-- 
2.17.1

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