Messages in this thread Patch in this message | | | Date | Mon, 27 Oct 2014 18:15:17 +0530 | Subject | Re: [PATCH] arm64: kgdb: fix single stepping | From | Vijay Kilari <> |
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Hi Akashi,
I could not reproduce this with my simulator. It would be good if you could post result of KGDB test suite.
From sysfs you can launch using below command:
echo V1F1000 > /sys/module/kgdbts/parameters/kgdbts or enable boot test.
Other than that it is OK.
On Mon, Oct 27, 2014 at 4:04 PM, Will Deacon <will.deacon@arm.com> wrote: > On Tue, Oct 21, 2014 at 06:07:30AM +0100, AKASHI Takahiro wrote: >> 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 and stops at the next instruction just >> after enable_dbg 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") >> >> This patch >> (1) moves kgdb_disable_single_step() from 'c' command handling to single >> step handler. >> This makes sure that single stepping gets effective at every 's' command. >> Please note that, under the current implementation, single step bit in >> spsr, which is cleared by the first single stepping, will not be set >> again for the consecutive 's' commands because single step bit in mdscr >> is still kept on (that is, kernel_active_single_step() in >> kgdb_arch_handle_exception() is true). >> (2) re-implements kgdb_roundup_cpus() because the current implementation >> enabled interrupts naively. See below. >> (3) removes 'enable_dbg' in el1_dbg. >> Single step bit in mdscr is turned on in do_handle_exception()-> >> kgdb_handle_expection() before returning to debugged context, and if >> debug exception is enabled in el1_dbg, we will see unexpected single- >> stepping in el1_dbg. >> Since v3.18, the following patch does the same: >> commit 1059c6bf8534 ("arm64: debug: don't re-enable debug exceptions >> on return from el1_dbg) >> (4) masks interrupts while single-stepping one instruction. >> If an interrupt is caught during processing a single-stepping, debug >> exception is unintentionally enabled by el1_irq's 'enable_dbg' before >> returning to debugged context. >> Thus, like in (2), we will see unexpected single-stepping in el1_irq. >> >> Basically (1) and (2) are for v3.15, (3) and (4) for v3.1[67]. > > (3) was CC'd for stable, so I don't think you need to mention that here. > > I'd like an ack from a KGDB person before I take this via the arm64 tree. > > Will > >> >> * issue fixed by (2): >> Without (2), we would see another problem if a breakpoint is set at >> interrupt-sensible places, like gic_handle_irq(): >> >> KGDB: re-enter error: breakpoint removed ffffffc000081258 >> ------------[ cut here ]------------ >> WARNING: CPU: 0 PID: 650 at kernel/debug/debug_core.c:435 >> kgdb_handle_exception+0x1dc/0x1f4() >> Modules linked in: >> CPU: 0 PID: 650 Comm: sh Not tainted 3.17.0-rc2+ #177 >> Call trace: >> [<ffffffc000087fac>] dump_backtrace+0x0/0x130 >> [<ffffffc0000880ec>] show_stack+0x10/0x1c >> [<ffffffc0004d683c>] dump_stack+0x74/0xb8 >> [<ffffffc0000ab824>] warn_slowpath_common+0x8c/0xb4 >> [<ffffffc0000ab90c>] warn_slowpath_null+0x14/0x20 >> [<ffffffc000121bfc>] kgdb_handle_exception+0x1d8/0x1f4 >> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28 >> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8 >> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac >> Exception stack(0xffffffc07e027650 to 0xffffffc07e027770) >> ... >> [<ffffffc000083cac>] el1_dbg+0x14/0x68 >> [<ffffffc00012178c>] kgdb_cpu_enter+0x464/0x5c0 >> [<ffffffc000121bb4>] kgdb_handle_exception+0x190/0x1f4 >> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28 >> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8 >> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac >> Exception stack(0xffffffc07e027ac0 to 0xffffffc07e027be0) >> ... >> [<ffffffc000083cac>] el1_dbg+0x14/0x68 >> [<ffffffc00032e4b4>] __handle_sysrq+0x11c/0x190 >> [<ffffffc00032e93c>] write_sysrq_trigger+0x4c/0x60 >> [<ffffffc0001e7d58>] proc_reg_write+0x54/0x84 >> [<ffffffc000192fa4>] vfs_write+0x98/0x1c8 >> [<ffffffc0001939b0>] SyS_write+0x40/0xa0 >> >> Once some interrupt occurs, a breakpoint at gic_handle_irq() triggers kgdb. >> Kgdb then calls kgdb_roundup_cpus() to sync with other cpus. >> Current kgdb_roundup_cpus() unmasks interrupts temporarily to >> use smp_call_function(). >> This eventually allows another interrupt to occur and likely results in >> hitting a breakpoint at gic_handle_irq() again since debug exception is >> always enabled in el1_irq. >> >> We can avoid this issue by specifying "nokgdbroundup" in kernel parameter, >> but this will also leave other cpus be in unknown state in terms of kgdb, >> and may result in interfering with kgdb activity. >> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> >> --- >> arch/arm64/kernel/kgdb.c | 60 +++++++++++++++++++++++++++++++++++----------- >> 1 file changed, 46 insertions(+), 14 deletions(-) >> >> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c >> index a0d10c5..81b5910 100644 >> --- a/arch/arm64/kernel/kgdb.c >> +++ b/arch/arm64/kernel/kgdb.c >> @@ -19,9 +19,13 @@ >> * along with this program. If not, see <http://www.gnu.org/licenses/>. >> */ >> >> +#include <linux/cpumask.h> >> #include <linux/irq.h> >> +#include <linux/irq_work.h> >> #include <linux/kdebug.h> >> #include <linux/kgdb.h> >> +#include <linux/percpu.h> >> +#include <asm/ptrace.h> >> #include <asm/traps.h> >> >> struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = { >> @@ -95,6 +99,9 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = { >> { "fpcr", 4, -1 }, >> }; >> >> +static DEFINE_PER_CPU(unsigned int, kgdb_pstate); >> +static DEFINE_PER_CPU(struct irq_work, kgdb_irq_work); >> + >> char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs) >> { >> if (regno >= DBG_MAX_REG_NUM || regno < 0) >> @@ -176,18 +183,14 @@ 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; >> case 's': >> + /* mask interrupts while single stepping */ >> + __this_cpu_write(kgdb_pstate, linux_regs->pstate); >> + linux_regs->pstate |= PSR_I_BIT; >> + >> /* >> * Update step address value with address passed >> * with step packet. >> @@ -198,8 +201,6 @@ 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 >> */ >> @@ -229,6 +230,18 @@ 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) >> { >> + unsigned int pstate; >> + >> + kernel_disable_single_step(); >> + atomic_set(&kgdb_cpu_doing_single_step, -1); >> + >> + /* restore interrupt mask status */ >> + pstate = __this_cpu_read(kgdb_pstate); >> + if (pstate & PSR_I_BIT) >> + regs->pstate |= PSR_I_BIT; >> + else >> + regs->pstate &= ~PSR_I_BIT; >> + >> kgdb_handle_exception(1, SIGTRAP, 0, regs); >> return 0; >> } >> @@ -249,16 +262,27 @@ static struct step_hook kgdb_step_hook = { >> .fn = kgdb_step_brk_fn >> }; >> >> -static void kgdb_call_nmi_hook(void *ignored) >> +static void kgdb_roundup_hook(struct irq_work *work) >> { >> kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs()); >> } >> >> void kgdb_roundup_cpus(unsigned long flags) >> { >> - local_irq_enable(); >> - smp_call_function(kgdb_call_nmi_hook, NULL, 0); >> - local_irq_disable(); >> + int cpu; >> + struct cpumask mask; >> + struct irq_work *work; >> + >> + mask = *cpu_online_mask; >> + cpumask_clear_cpu(smp_processor_id(), &mask); >> + cpu = cpumask_first(&mask); >> + if (cpu >= nr_cpu_ids) >> + return; >> + >> + for_each_cpu(cpu, &mask) { >> + work = per_cpu_ptr(&kgdb_irq_work, cpu); >> + irq_work_queue_on(work, cpu); >> + } >> } >> >> static int __kgdb_notify(struct die_args *args, unsigned long cmd) >> @@ -299,6 +323,8 @@ static struct notifier_block kgdb_notifier = { >> int kgdb_arch_init(void) >> { >> int ret = register_die_notifier(&kgdb_notifier); >> + int cpu; >> + struct irq_work *work; >> >> if (ret != 0) >> return ret; >> @@ -306,6 +332,12 @@ int kgdb_arch_init(void) >> register_break_hook(&kgdb_brkpt_hook); >> register_break_hook(&kgdb_compiled_brkpt_hook); >> register_step_hook(&kgdb_step_hook); >> + >> + for_each_possible_cpu(cpu) { >> + work = per_cpu_ptr(&kgdb_irq_work, cpu); >> + init_irq_work(work, kgdb_roundup_hook); >> + } >> + >> return 0; >> } >> >> -- >> 1.7.9.5 >>
Acked-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Regards Vijay
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