Re: [PATCH v2] arm64: kdump: Avoid to power off nonpanic CPUs

From: James Morse
Date: Tue Nov 21 2017 - 11:49:14 EST

Hi Leo Yan,

On 18/11/17 09:12, Leo Yan wrote:
> commit a88ce63b642c ("arm64: kexec: have own crash_smp_send_stop() for
> crash dump for nonpanic cores") introduces ARM64 architecture function

(This commit fixed a bug where the core-code version was used, this didn't save
the CPU registers, which made kdump useless.)

> crash_smp_send_stop() to replace the weak function, this results in
> the nonpanic CPUs to be hot-plugged out and CPUs are placed into low
> power state on ARM64 platforms with the flow:
> Panic CPU:
> machine_crash_shutdown()
> crash_smp_send_stop()
> smp_cross_call(&mask, IPI_CPU_CRASH_STOP)
> Nonpanic CPUs:
> handle_IPI()
> ipi_cpu_crash_stop()
> cpu_ops[cpu]->cpu_die()
> The upper patch has no issue if enabled crash dump only; but if enabled
> crash dump and Coresight debug module for panic dumping at the meantime,
> nonpanic CPUs are powered off in crash dump flow, later this may
> introduce conflicts with the Coresight debug module because Coresight
> debug registers dumping requires the CPU must be powered on for some
> platforms (e.g. Hi6220 on Hikey board).

Is it just Hikey with this problem?

> If we cannot keep the CPUs
> powered on, we can see the hardware lockup issue when access Coresight
> debug registers.

By 'hardware lockup issue' do you mean you want to use the Coresight debug
registers to inspect what caused the panic()=>kdump in the first place?
You mention 'dumping requires the CPU [to] be powered on', I assume it loses
state when powered off.

...or does the CPU hang if you use PSCI to power it off while the Coresight
debug is running?

> To fix this issue, this commit bypasses CPU hotplug operation in func
> crash_smp_send_stop() when coresight CPU debug module has been enabled
> and let CPUs to run into WFE/WFI states so CPUs can still be powered on
> after crash dump. This finally is more safe for Coresight debug module
> to dump registers and avoid hardware lockup.

Ah, there is a hardware-lockup.

Wouldn't the same thing happen if I poke the sysfs cpu online/offline interface
while this thing is running? (Not to mention cpu-idle)

Shouldn't this be fixed in firmware? If EL3 can see the Coresight debug is running,
it can hold the CPU in WFE instead of trying to actually power off. Firmware can
know if the debug hardware and the CPU are powered together, (which I guess is
why this is a problem on Hikey).



> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 9f7195a..31dab1f 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -856,7 +856,7 @@ static void ipi_cpu_crash_stop(unsigned int cpu, struct pt_regs *regs)
> local_irq_disable();
> if (cpu_ops[cpu]->cpu_die)
> cpu_ops[cpu]->cpu_die(cpu);
> #endif