Subject: [PATCH v12 12/16] arm64: kdump: implement machine_crash_shutdown()

From: [email-masked] (AKASHI Takahiro)
Date: Thu, 10 Dec 2015 21:55:20 +0900

On 12/10/2015 08:44 PM, Marc Zyngier wrote:
> On 10/12/15 11:34, AKASHI Takahiro wrote:
>> Marc,
>>
>> I was back from my vacation.
>>
>> On 11/27/2015 11:39 PM, Marc Zyngier wrote:
>>> On 24/11/15 22:25, Geoff Levand wrote:
>>>> From: AKASHI Takahiro <[email-masked]>
>>>>
>>>> kdump calls machine_crash_shutdown() to shut down non-boot cpus and
>>>> save registers' status in per-cpu ELF notes before starting the crash
>>>> dump kernel. See kernel_kexec().
>>>>
>>>> ipi_cpu_stop() is a bit modified and used to support this behavior.
>>>>
>>>> Signed-off-by: AKASHI Takahiro <[email-masked]>
>>>> ---
>>>>    arch/arm64/include/asm/kexec.h    | 34 +++++++++++++++++++++++++++++++++-
>>>>    arch/arm64/kernel/machine_kexec.c | 31 +++++++++++++++++++++++++++++--
>>>>    arch/arm64/kernel/smp.c           | 16 ++++++++++++++--
>>>>    3 files changed, 76 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/include/asm/kexec.h b/arch/arm64/include/asm/kexec.h
>>>> index 46d63cd..555a955 100644
>>>> --- a/arch/arm64/include/asm/kexec.h
>>>> +++ b/arch/arm64/include/asm/kexec.h
>>>> @@ -30,6 +30,8 @@
>>>>
>>>>    #if !defined(__ASSEMBLY__)
>>>>
>>>> +extern bool in_crash_kexec;
>>>> +
>>>>    /**
>>>>     * crash_setup_regs() - save registers for the panic kernel
>>>>     *
>>>> @@ -40,7 +42,37 @@
>>>>    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 {
>>>> +		__asm__ __volatile__ (
>>>> +			"stp	 x0,   x1, [%3, #16 *  0]\n"
>>>> +			"stp	 x2,   x3, [%3, #16 *  1]\n"
>>>> +			"stp	 x4,   x5, [%3, #16 *  2]\n"
>>>> +			"stp	 x6,   x7, [%3, #16 *  3]\n"
>>>> +			"stp	 x8,   x9, [%3, #16 *  4]\n"
>>>> +			"stp	x10,  x11, [%3, #16 *  5]\n"
>>>> +			"stp	x12,  x13, [%3, #16 *  6]\n"
>>>> +			"stp	x14,  x15, [%3, #16 *  7]\n"
>>>> +			"stp	x16,  x17, [%3, #16 *  8]\n"
>>>> +			"stp	x18,  x19, [%3, #16 *  9]\n"
>>>> +			"stp	x20,  x21, [%3, #16 * 10]\n"
>>>> +			"stp	x22,  x23, [%3, #16 * 11]\n"
>>>> +			"stp	x24,  x25, [%3, #16 * 12]\n"
>>>> +			"stp	x26,  x27, [%3, #16 * 13]\n"
>>>> +			"stp	x28,  x29, [%3, #16 * 14]\n"
>>>> +			"str	x30,	   [%3, #16 * 15]\n"
>>>> +			"mov	%0, sp\n"
>>>> +			"adr	%1, 1f\n"
>>>> +			"mrs	%2, spsr_el1\n"
>>>> +		"1:"
>>>> +			: "=r" (newregs->sp),
>>>> +			  "=r" (newregs->pc),
>>>> +			  "=r" (newregs->pstate)
>>>> +			: "r"  (&newregs->regs)
>>>> +			: "memory"
>>>> +		);
>>>
>>> I wonder how useful this thing is, given that it starts by corrupting
>>> whatever register is holding newregs->regs. Maybe this is not supposed
>>> to be accurate anyway...
>>
>> I'm not quite sure about what part of my code you're mentioning here, but
>> crash_setup_regs() is solely called by crash_kexec(), and panic() is
>> the only caller of crash_kexec() with NULL argument which, in turn, is
>> used as 'oldregs' in crash_setup_regs().
>
> You have this assembly sequence:
>
> stp	 x0,   x1, [%3, #16 *  0]
> [...]
>
> where %3 itself is one of the x[0..30] registers.

Not only %3, but also

> So you are saving
> things that have already been corrupted by the saving procedure. Not
> sure how useful that is, but as I said, maybe it is not supposed to be
> completely accurate.

x0, x1 ... are the current values in panic(), and not the exact cpu contexts
at the place we are really interested in.
We have no way here in panic() to know them, but sp and pc would still be useful
for back-tracing in later investigation of dump file.

Please note that the same problem exists on arm (and x86) implementation.

>> Given this fact, I think that the values saved in newregs as indicated above
>> will be the best estimate of current cpu contexts.
>>
>> The other caller of crash_kexec() is die() in traps.c, but here we call
>> it with explicit cpu contexts at exception.
>>
>>>
>>>> +	}
>>>>    }
>>>>
>>>>    #endif /* !defined(__ASSEMBLY__) */
>>>> diff --git a/arch/arm64/kernel/machine_kexec.c b/arch/arm64/kernel/machine_kexec.c
>>>> index da28a26..d2d7e90 100644
>>>> --- a/arch/arm64/kernel/machine_kexec.c
>>>> +++ b/arch/arm64/kernel/machine_kexec.c
>>>> @@ -9,6 +9,7 @@
>>>>     * published by the Free Software Foundation.
>>>>     */
>>>>
>>>> +#include <linux/kernel.h>
>>>>    #include <linux/kexec.h>
>>>>    #include <linux/of_fdt.h>
>>>>    #include <linux/slab.h>
>>>> @@ -23,6 +24,7 @@
>>>>    extern const unsigned char arm64_relocate_new_kernel[];
>>>>    extern const unsigned long arm64_relocate_new_kernel_size;
>>>>
>>>> +bool in_crash_kexec;
>>>>    static unsigned long kimage_start;
>>>>
>>>>    /**
>>>> @@ -203,13 +205,38 @@ void machine_kexec(struct kimage *kimage)
>>>>    	 */
>>>>
>>>>    	cpu_soft_restart(virt_to_phys(cpu_reset),
>>>> -		is_hyp_mode_available(),
>>>> +		in_crash_kexec ? 0 : is_hyp_mode_available(),
>>>>    		reboot_code_buffer_phys, kimage->head, kimage_start);
>>>>
>>>>    	BUG(); /* Should never get here. */
>>>>    }
>>>>
>>>> +/**
>>>> + * machine_crash_shutdown - shutdown non-boot cpus and save registers
>>>> + */
>>>>    void machine_crash_shutdown(struct pt_regs *regs)
>>>>    {
>>>> -	/* Empty routine needed to avoid build errors. */
>>>> +	struct pt_regs dummy_regs;
>>>> +	int cpu;
>>>> +
>>>> +	local_irq_disable();
>>>> +
>>>> +	in_crash_kexec = true;
>>>> +
>>>> +	/*
>>>> +	 * clear and initialize the per-cpu info. This is necessary
>>>> +	 * because, otherwise, slots for offline cpus would never be
>>>> +	 * filled up. See smp_send_stop().
>>>> +	 */
>>>> +	memset(&dummy_regs, 0, sizeof(dummy_regs));
>>>> +	for_each_possible_cpu(cpu)
>>>> +		crash_save_cpu(&dummy_regs, cpu);
>>>> +
>>>> +	/* shutdown non-boot cpus */
>>>> +	smp_send_stop();
>>>> +
>>>> +	/* for boot cpu */
>>>> +	crash_save_cpu(regs, smp_processor_id());
>>>> +
>>>> +	pr_info("Starting crashdump kernel...\n");
>>>>    }
>>>> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
>>>> index b1adc51..15aabef 100644
>>>> --- a/arch/arm64/kernel/smp.c
>>>> +++ b/arch/arm64/kernel/smp.c
>>>> @@ -37,6 +37,7 @@
>>>>    #include <linux/completion.h>
>>>>    #include <linux/of.h>
>>>>    #include <linux/irq_work.h>
>>>> +#include <linux/kexec.h>
>>>>
>>>>    #include <asm/alternative.h>
>>>>    #include <asm/atomic.h>
>>>> @@ -54,6 +55,8 @@
>>>>    #include <asm/ptrace.h>
>>>>    #include <asm/virt.h>
>>>>
>>>> +#include "cpu-reset.h"
>>>> +
>>>>    #define CREATE_TRACE_POINTS
>>>>    #include <trace/events/ipi.h>
>>>>
>>>> @@ -683,8 +686,12 @@ static DEFINE_RAW_SPINLOCK(stop_lock);
>>>>    /*
>>>>     * ipi_cpu_stop - handle IPI from smp_send_stop()
>>>>     */
>>>> -static void ipi_cpu_stop(unsigned int cpu)
>>>> +static void ipi_cpu_stop(unsigned int cpu, struct pt_regs *regs)
>>>>    {
>>>> +#ifdef CONFIG_KEXEC
>>>> +	/* printing messages may slow down the shutdown. */
>>>> +	if (!in_crash_kexec)
>>>> +#endif
>>>>    	if (system_state == SYSTEM_BOOTING ||
>>>>    	    system_state == SYSTEM_RUNNING) {
>>>>    		raw_spin_lock(&stop_lock);
>>>
>>> Irrespective of how useful this change is, how about having a predicate
>>> instead? Something like:
>>>
>>> static inline bool is_in_crash_kexec(void)
>>> {
>>> #ifdef CONFIG_KEXEC
>>> 	return in_crash_kexec;
>>> #else
>>> 	return false;
>>> #endif
>>> }
>>
>> OK, I will take your idea.
>>
>>> located in machine_kexec.c (making the in_crash_kernel static), and then
>>
>> but cannot make in_crash_kernel static because it is also used in both smp.c
>> and machine_kexec.c.
>
> smp.c only reads from in_crash_kernel (at least from what I can see in
> this patch), so it should be able to use the accessor.

Only if we define the accessor as a real function, not an inline function in a header :)

-Takahiro AKASHI

> Thanks,
> 	M.