Subject: [PATCH v26 2/7] arm64: kdump: implement machine_crash_shutdown()

From: [email-masked] (James Morse)
Date: Fri, 16 Sep 2016 15:49:31 +0100

On 16/09/16 04:21, AKASHI Takahiro wrote:
> On Wed, Sep 14, 2016 at 07:09:33PM +0100, James Morse wrote:
>> On 07/09/16 05:29, AKASHI Takahiro wrote:
>>> Primary kernel calls machine_crash_shutdown() to shut down non-boot cpus
>>> and save registers' status in per-cpu ELF notes before starting crash
>>> dump kernel. See kernel_kexec().
>>> Even if not all secondary cpus have shut down, we do kdump anyway.
>>>
>>> As we don't have to make non-boot(crashed) cpus offline (to preserve
>>> correct status of cpus at crash dump) before shutting down, this patch
>>> also adds a variant of smp_send_stop().

>>> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
>>> index 04744dc..a908958 100644
>>> --- a/arch/arm64/include/asm/kexec.h
>>> +++ b/arch/arm64/include/asm/kexec.h
>>> @@ -40,7 +40,46 @@
>>>  static inline void crash_setup_regs(struct pt_regs *newregs,
>>>  				    struct pt_regs *oldregs)
>>>  {
>>> -	/* Empty routine needed to avoid build errors. */
>>> +	if (oldregs) {
>>> +		memcpy(newregs, oldregs, sizeof(*newregs));
>>> +	} else {
>>> +		u64 tmp1, tmp2;
>>> +
>>> +		__asm__ __volatile__ (
>>> +			"stp	 x0,   x1, [%2, #16 *  0]\n"
>>> +			"stp	 x2,   x3, [%2, #16 *  1]\n"
>>> +			"stp	 x4,   x5, [%2, #16 *  2]\n"
>>> +			"stp	 x6,   x7, [%2, #16 *  3]\n"
>>> +			"stp	 x8,   x9, [%2, #16 *  4]\n"
>>> +			"stp	x10,  x11, [%2, #16 *  5]\n"
>>> +			"stp	x12,  x13, [%2, #16 *  6]\n"
>>> +			"stp	x14,  x15, [%2, #16 *  7]\n"
>>> +			"stp	x16,  x17, [%2, #16 *  8]\n"
>>> +			"stp	x18,  x19, [%2, #16 *  9]\n"
>>> +			"stp	x20,  x21, [%2, #16 * 10]\n"
>>> +			"stp	x22,  x23, [%2, #16 * 11]\n"
>>> +			"stp	x24,  x25, [%2, #16 * 12]\n"
>>> +			"stp	x26,  x27, [%2, #16 * 13]\n"
>>> +			"stp	x28,  x29, [%2, #16 * 14]\n"
>>> +			"mov	 %0,  sp\n"
>>> +			"stp	x30,  %0,  [%2, #16 * 15]\n"
>>> +
>>> +			"/* faked current PSTATE */\n"
>>> +			"mrs	 %0, CurrentEL\n"
>>> +			"mrs	 %1, DAIF\n"
>>> +			"orr	 %0, %0, %1\n"
>>> +			"mrs	 %1, NZCV\n"
>>> +			"orr	 %0, %0, %1\n"
>>> +
>>
>> What about SPSEL? While we don't use it, it is correctly preserved for
>> everything except a CPU that calls panic()...
> 
> My comment above might be confusing, but what I want to fake
> here is "spsr" as pt_regs.pstate is normally set based on spsr_el1.
> So there is no corresponding field of SPSEL in spsr.

Here is my logic, I may have missed something obvious, see what you think:

SPSR_EL{1,2} shows the CPU mode 'M' in bits 0-4, From aarch64 bit 4 is always 0.
>>From the register definitions in the ARM-ARM C5.2, likely values in 0-3 are:
0100 EL1t
0101 EL1h
1000 EL2t
1001 EL2h

I'm pretty sure this least significant bit is what SPSEL changes, so it does get
implicitly recorded in SPSR.
CurrentEL returns a value in bits 0-3, of which 0-1 are RES0, so we lose the
difference between EL?t and EL?h.

> 
>>
>>> +			/* pc */
>>> +			"adr	 %1, 1f\n"
>>> +		"1:\n"
>>> +			"stp	 %1, %0,   [%2, #16 * 16]\n"
>>> +			: "=r" (tmp1), "=r" (tmp2), "+r" (newregs)
>>> +			:
>>> +			: "memory"

Do you need the memory clobber? This asm only modifies values in newregs.

>>> +		);
>>> +	}
>>>  }
>>>  
>>>  #endif /* __ASSEMBLY__ */

>>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c

>>> +#ifdef CONFIG_KEXEC_CORE
>>> +void smp_send_crash_stop(void)
>>> +{
>>> +	cpumask_t mask;
>>> +	unsigned long timeout;
>>> +
>>> +	if (num_online_cpus() == 1)
>>> +		return;
>>> +
>>> +	cpumask_copy(&mask, cpu_online_mask);
>>> +	cpumask_clear_cpu(smp_processor_id(), &mask);
>>> +
>>> +	atomic_set(&waiting_for_crash_ipi, num_online_cpus() - 1);
>>> +
>>> +	pr_crit("SMP: stopping secondary CPUs\n");
>>> +	smp_cross_call(&mask, IPI_CPU_CRASH_STOP);
>>> +
>>> +	/* Wait up to one second for other CPUs to stop */
>>> +	timeout = USEC_PER_SEC;
>>> +	while ((atomic_read(&waiting_for_crash_ipi) > 0) && timeout--)
>>> +		udelay(1);
>>> +
>>> +	if (atomic_read(&waiting_for_crash_ipi) > 0)
>>> +		pr_warning("SMP: failed to stop secondary CPUs %*pbl\n",
>>> +			   cpumask_pr_args(cpu_online_mask));
>>> +}
>>> +#endif
>>
>> This is very similar to smp_send_stop() which also has the timeout. Is it
>> possible to merge them? You could use in_crash_kexec to choose the IPI type.
> 
> Yeah, we could merge them along with ipi_cpu_(crash_)stop().
> But the resulting code would be quite noisy if each line
> is switched by "if (in_crash_kexec)."
> Otherwise, we may have one big "if" like:
> void smp_send_stop(void)
> {
>     if (in_crash_kexec)
>         ...
>     else
>         ...
> }
> It seems to me that it is not much different from the current code.
> What do you think?

Hmm, yes, its too fiddly to keep the existing behaviour of both.

The problems are ipi_cpu_stop() doesn't call cpu_die(), (I can't see a good
reason for this, but more archaeology is needed), and ipi_cpu_crash_stop()
doesn't modify the online cpu mask.

I don't suggest we do this yet, but it could be future cleanup if it's proved to
be safe:
smp_send_stop() is only called from:  machine_halt(), machine_power_off(),
machine_restart() and panic(). In all those cases the CPUs are never expected to
come back, so we can probably merge the IPIs.  This involves modifying the
online cpu mask during kdump, (which I think is fine as it uses the atomic
bitops so we won't get blocked on a lock), and promoting in_crash_kexec to some
atomic type.

But I think we should leave it as it is for now,

Thanks,

James