Re: [PATCH] RISC-V: Implement kgdb_roundup_cpus() to enable IPI KGDB Roundup

[Andrew Jones <ajones@ventanamicro.com>]

On Fri, Jul 19, 2024 at 04:12:10PM GMT, Jinjie Ruan wrote:
> Until now, the generic weak kgdb_roundup_cpus() has been used for kgdb on
> RISCV. A custom one allows to debug CPUs that are stuck with interrupts
> disabled.

This confuses me since we're using an IPI for the roundup. How does that
work on CPUs stuck with interrupts disabled? I think this text comes
from the commit message for the arm64 patch, but arm64 does the roundup
with a pseudo-NMI.

> And using an IPI is better than the generic one since it avoids
> the potential situation described in the generic kgdb_call_nmi_hook().

This is true.

> 
> After this patch, the kgdb test show that:
> 	# echo g > /proc/sysrq-trigger
> 	[2]kdb> btc
> 	btc: cpu status: Currently on cpu 2
> 	Available cpus: 0-1(-), 2, 3(-)
> 	Stack traceback for pid 0
> 	0xffffffff81c13a40        0        0  1    0   -  0xffffffff81c14510  swapper/0
> 	CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.10.0-g3120273055b6-dirty #51
> 	Hardware name: riscv-virtio,qemu (DT)
> 	Call Trace:
> 	[<ffffffff80006c48>] dump_backtrace+0x28/0x30
> 	[<ffffffff80fceb38>] show_stack+0x38/0x44
> 	[<ffffffff80fe6a04>] dump_stack_lvl+0x58/0x7a
> 	[<ffffffff80fe6a3e>] dump_stack+0x18/0x20
> 	[<ffffffff801143fa>] kgdb_cpu_enter+0x682/0x6b2
> 	[<ffffffff801144ca>] kgdb_nmicallback+0xa0/0xac
> 	[<ffffffff8000a392>] handle_IPI+0x9c/0x120
> 	[<ffffffff800a2baa>] handle_percpu_devid_irq+0xa4/0x1e4
> 	[<ffffffff8009cca8>] generic_handle_domain_irq+0x28/0x36
> 	[<ffffffff800a9e5c>] ipi_mux_process+0xe8/0x110
> 	[<ffffffff806e1e30>] imsic_handle_irq+0xf8/0x13a
> 	[<ffffffff8009cca8>] generic_handle_domain_irq+0x28/0x36
> 	[<ffffffff806dff12>] riscv_intc_aia_irq+0x2e/0x40
> 	[<ffffffff80fe6ab0>] handle_riscv_irq+0x54/0x86
> 	[<ffffffff80ff2e4a>] call_on_irq_stack+0x32/0x40
> 
> Rebased on Ryo Takakura's "RISC-V: Enable IPI CPU Backtrace" patch.
> 
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>  arch/riscv/kernel/smp.c | 27 +++++++++++++++++++++++++--
>  1 file changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/kernel/smp.c b/arch/riscv/kernel/smp.c
> index 9b047899791c..c180a647a30e 100644
> --- a/arch/riscv/kernel/smp.c
> +++ b/arch/riscv/kernel/smp.c
> @@ -13,6 +13,7 @@
>  #include <linux/interrupt.h>
>  #include <linux/module.h>
>  #include <linux/kexec.h>
> +#include <linux/kgdb.h>
>  #include <linux/percpu.h>
>  #include <linux/profile.h>
>  #include <linux/smp.h>
> @@ -35,6 +36,7 @@ enum ipi_message_type {
>  	IPI_IRQ_WORK,
>  	IPI_TIMER,
>  	IPI_CPU_BACKTRACE,
> +	IPI_KGDB_ROUNDUP,
>  	IPI_MAX
>  };
>  
> @@ -115,6 +117,7 @@ void arch_irq_work_raise(void)
>  
>  static irqreturn_t handle_IPI(int irq, void *data)
>  {
> +	unsigned int cpu = smp_processor_id();
>  	int ipi = irq - ipi_virq_base;
>  
>  	switch (ipi) {
> @@ -128,7 +131,7 @@ static irqreturn_t handle_IPI(int irq, void *data)
>  		ipi_stop();
>  		break;
>  	case IPI_CPU_CRASH_STOP:
> -		ipi_cpu_crash_stop(smp_processor_id(), get_irq_regs());
> +		ipi_cpu_crash_stop(cpu, get_irq_regs());
>  		break;
>  	case IPI_IRQ_WORK:
>  		irq_work_run();
> @@ -141,8 +144,11 @@ static irqreturn_t handle_IPI(int irq, void *data)
>  	case IPI_CPU_BACKTRACE:
>  		nmi_cpu_backtrace(get_irq_regs());
>  		break;
> +	case IPI_KGDB_ROUNDUP:
> +		kgdb_nmicallback(cpu, get_irq_regs());
> +		break;
>  	default:
> -		pr_warn("CPU%d: unhandled IPI%d\n", smp_processor_id(), ipi);
> +		pr_warn("CPU%d: unhandled IPI%d\n", cpu, ipi);
>  		break;
>  	}
>  
> @@ -209,6 +215,7 @@ static const char * const ipi_names[] = {
>  	[IPI_IRQ_WORK]		= "IRQ work interrupts",
>  	[IPI_TIMER]		= "Timer broadcast interrupts",
>  	[IPI_CPU_BACKTRACE]     = "CPU backtrace interrupts",
> +	[IPI_KGDB_ROUNDUP]	= "KGDB roundup interrupts",
>  };
>  
>  void show_ipi_stats(struct seq_file *p, int prec)
> @@ -339,3 +346,19 @@ void arch_trigger_cpumask_backtrace(const cpumask_t *mask, int exclude_cpu)
>  {
>  	nmi_trigger_cpumask_backtrace(mask, exclude_cpu, riscv_backtrace_ipi);
>  }
> +
> +#ifdef CONFIG_KGDB
> +void kgdb_roundup_cpus(void)
> +{
> +	int this_cpu = raw_smp_processor_id();
> +	int cpu;
> +
> +	for_each_online_cpu(cpu) {
> +		/* No need to roundup ourselves */
> +		if (cpu == this_cpu)
> +			continue;
> +
> +		send_ipi_single(cpu, IPI_KGDB_ROUNDUP);
> +	}
> +}
> +#endif
> -- 
> 2.34.1
>

Other than the commit message, the patch looks good to me, so

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

but I guess we should extend this and the CPU backtrace support to use
NMIs when we have support for them.

Thanks,
drew

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