I tried to verify kgdb in vanilla kernel on fast model, but it seems that the single stepping with kgdb doesn't work correctly since its first appearance at v3.15.

On v3.15, 'stepi' command after breaking the kernel at some breakpoint steps forward to the next instruction, but the succeeding 'stepi' never goes beyond that. On v3.16, 'stepi' moves forward to the second instruction just after call of do_debug_execption() in el1_dbg, and never goes beyond that. This variance of behavior seems to come in with the following patch in v3.16:

commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault paths where possible")

I cannot deny the possibility that this migh be a bug of fast model, but this patch works for me better than vanilla, and hopefully will help you see what's happening behind this issue.

Signed-off-by: AKASHI Takahiro takahiro.akashi@linaro.org --- arch/arm64/include/asm/assembler.h | 7 +++++++ arch/arm64/kernel/entry.S | 4 ++-- arch/arm64/kernel/kgdb.c | 15 ++++++--------- 3 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h index 5901480..a345973 100644 --- a/arch/arm64/include/asm/assembler.h +++ b/arch/arm64/include/asm/assembler.h @@ -87,6 +87,13 @@ 9990: .endm

+ .macro enable_dbg_if_not_stepping, tmp + mrs \tmp, mdscr_el1 + tbnz \tmp, #0, 9990f + enable_dbg +9990: + .endm + /* * Enable both debug exceptions and interrupts. This is likely to be * faster than two daifclr operations, since writes to this register diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index d5eb447..b882f5d 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -325,7 +325,7 @@ el1_dbg: mrs x0, far_el1 mov x2, sp // struct pt_regs bl do_debug_exception - enable_dbg + enable_dbg_if_not_stepping x2 kernel_exit 1 el1_inv: // TODO: add support for undefined instructions in kernel mode @@ -339,7 +339,7 @@ ENDPROC(el1_sync) .align 6 el1_irq: kernel_entry 1 - enable_dbg + enable_dbg_if_not_stepping x2 #ifdef CONFIG_TRACE_IRQFLAGS bl trace_hardirqs_off #endif diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c index 75c9cf1..bbc085f 100644 --- a/arch/arm64/kernel/kgdb.c +++ b/arch/arm64/kernel/kgdb.c @@ -176,14 +176,6 @@ int kgdb_arch_handle_exception(int exception_vector, int signo, * over and over again. */ kgdb_arch_update_addr(linux_regs, remcom_in_buffer); - atomic_set(&kgdb_cpu_doing_single_step, -1); - kgdb_single_step = 0; - - /* - * Received continue command, disable single step - */ - if (kernel_active_single_step()) - kernel_disable_single_step();

err = 0; break; @@ -198,13 +190,13 @@ int kgdb_arch_handle_exception(int exception_vector, int signo, */ kgdb_arch_update_addr(linux_regs, remcom_in_buffer); atomic_set(&kgdb_cpu_doing_single_step, raw_smp_processor_id()); - kgdb_single_step = 1;

/* * Enable single step handling */ if (!kernel_active_single_step()) kernel_enable_single_step(linux_regs); + err = 0; break; default: @@ -229,7 +221,12 @@ static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned int esr)

static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr) { + kernel_disable_single_step(); + kgdb_handle_exception(1, SIGTRAP, 0, regs); + + atomic_set(&kgdb_cpu_doing_single_step, -1); + return 0; }