Re: [PATCH 08/30] powerpc/setup: Refactor/untangle panic notifiers

[Hari Bathini <hbathini@linux.ibm.com>]

On 28/04/22 4:19 am, Guilherme G. Piccoli wrote:
> The panic notifiers infrastructure is a bit limited in the scope of
> the callbacks - basically every kind of functionality is dropped
> in a list that runs in the same point during the kernel panic path.
> This is not really on par with the complexities and particularities
> of architecture / hypervisors' needs, and a refactor is ongoing.
> 
> As part of this refactor, it was observed that powerpc has 2 notifiers,
> with mixed goals: one is just a KASLR offset dumper, whereas the other
> aims to hard-disable IRQs (necessary on panic path), warn firmware of
> the panic event (fadump) and run low-level platform-specific machinery
> that might stop kernel execution and never come back.
> 
> Clearly, the 2nd notifier has opposed goals: disable IRQs / fadump
> should run earlier while low-level platform actions should
> run late since it might not even return. Hence, this patch decouples
> the notifiers splitting them in three:
> 
> - First one is responsible for hard-disable IRQs and fadump,
> should run early;
> 
> - The kernel KASLR offset dumper is really an informative notifier,
> harmless and may run at any moment in the panic path;
> 
> - The last notifier should run last, since it aims to perform
> low-level actions for specific platforms, and might never return.
> It is also only registered for 2 platforms, pseries and ps3.
> 
> The patch better documents the notifiers and clears the code too,
> also removing a useless header.
> 
> Currently no functionality change should be observed, but after
> the planned panic refactor we should expect more panic reliability
> with this patch.
> 
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Hari Bathini <hbathini@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Paul Mackerras <paulus@samba.org>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

The change looks good. I have tested it on an LPAR (ppc64).

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>

> ---
> 
> We'd like to thanks specially the MiniCloud infrastructure [0] maintainers,
> that allow us to test PowerPC code in a very complete, functional and FREE
> environment (there's no need even for adding a credit card, like many "free"
> clouds require ¬¬ ).
> 
> [0] https://openpower.ic.unicamp.br/minicloud
> 
>   arch/powerpc/kernel/setup-common.c | 74 ++++++++++++++++++++++--------
>   1 file changed, 54 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
> index 518ae5aa9410..52f96b209a96 100644
> --- a/arch/powerpc/kernel/setup-common.c
> +++ b/arch/powerpc/kernel/setup-common.c
> @@ -23,7 +23,6 @@
>   #include <linux/console.h>
>   #include <linux/screen_info.h>
>   #include <linux/root_dev.h>
> -#include <linux/notifier.h>
>   #include <linux/cpu.h>
>   #include <linux/unistd.h>
>   #include <linux/serial.h>
> @@ -680,8 +679,25 @@ int check_legacy_ioport(unsigned long base_port)
>   }
>   EXPORT_SYMBOL(check_legacy_ioport);
>   
> -static int ppc_panic_event(struct notifier_block *this,
> -                             unsigned long event, void *ptr)
> +/*
> + * Panic notifiers setup
> + *
> + * We have 3 notifiers for powerpc, each one from a different "nature":
> + *
> + * - ppc_panic_fadump_handler() is a hypervisor notifier, which hard-disables
> + *   IRQs and deal with the Firmware-Assisted dump, when it is configured;
> + *   should run early in the panic path.
> + *
> + * - dump_kernel_offset() is an informative notifier, just showing the KASLR
> + *   offset if we have RANDOMIZE_BASE set.
> + *
> + * - ppc_panic_platform_handler() is a low-level handler that's registered
> + *   only if the platform wishes to perform final actions in the panic path,
> + *   hence it should run late and might not even return. Currently, only
> + *   pseries and ps3 platforms register callbacks.
> + */
> +static int ppc_panic_fadump_handler(struct notifier_block *this,
> +				    unsigned long event, void *ptr)
>   {
>   	/*
>   	 * panic does a local_irq_disable, but we really
> @@ -691,45 +707,63 @@ static int ppc_panic_event(struct notifier_block *this,
>   
>   	/*
>   	 * If firmware-assisted dump has been registered then trigger
> -	 * firmware-assisted dump and let firmware handle everything else.
> +	 * its callback and let the firmware handles everything else.
>   	 */
>   	crash_fadump(NULL, ptr);
> -	if (ppc_md.panic)
> -		ppc_md.panic(ptr);  /* May not return */
> +
>   	return NOTIFY_DONE;
>   }
>   
> -static struct notifier_block ppc_panic_block = {
> -	.notifier_call = ppc_panic_event,
> -	.priority = INT_MIN /* may not return; must be done last */
> -};
> -
> -/*
> - * Dump out kernel offset information on panic.
> - */
>   static int dump_kernel_offset(struct notifier_block *self, unsigned long v,
>   			      void *p)
>   {
>   	pr_emerg("Kernel Offset: 0x%lx from 0x%lx\n",
>   		 kaslr_offset(), KERNELBASE);
>   
> -	return 0;
> +	return NOTIFY_DONE;
>   }
>   
> +static int ppc_panic_platform_handler(struct notifier_block *this,
> +				      unsigned long event, void *ptr)
> +{
> +	/*
> +	 * This handler is only registered if we have a panic callback
> +	 * on ppc_md, hence NULL check is not needed.
> +	 * Also, it may not return, so it runs really late on panic path.
> +	 */
> +	ppc_md.panic(ptr);
> +
> +	return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block ppc_fadump_block = {
> +	.notifier_call = ppc_panic_fadump_handler,
> +	.priority = INT_MAX, /* run early, to notify the firmware ASAP */
> +};
> +
>   static struct notifier_block kernel_offset_notifier = {
> -	.notifier_call = dump_kernel_offset
> +	.notifier_call = dump_kernel_offset,
> +};
> +
> +static struct notifier_block ppc_panic_block = {
> +	.notifier_call = ppc_panic_platform_handler,
> +	.priority = INT_MIN, /* may not return; must be done last */
>   };
>   
>   void __init setup_panic(void)
>   {
> +	/* Hard-disables IRQs + deal with FW-assisted dump (fadump) */
> +	atomic_notifier_chain_register(&panic_notifier_list,
> +				       &ppc_fadump_block);
> +
>   	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && kaslr_offset() > 0)
>   		atomic_notifier_chain_register(&panic_notifier_list,
>   					       &kernel_offset_notifier);
>   
> -	/* PPC64 always does a hard irq disable in its panic handler */
> -	if (!IS_ENABLED(CONFIG_PPC64) && !ppc_md.panic)
> -		return;
> -	atomic_notifier_chain_register(&panic_notifier_list, &ppc_panic_block);
> +	/* Low-level platform-specific routines that should run on panic */
> +	if (ppc_md.panic)
> +		atomic_notifier_chain_register(&panic_notifier_list,
> +					       &ppc_panic_block);
>   }
>   
>   #ifdef CONFIG_CHECK_CACHE_COHERENCY