Subject: Re: [PATCH v5 1/3] powerpc: make fadump resilient with memory add/remove events

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 15 Nov 2023 10:14:00 +0530

Sourabh Jain <[email-masked]> writes:

....

> diff --git a/arch/powerpc/include/asm/fadump-internal.h b/arch/powerpc/include/asm/fadump-internal.h
> index 27f9e11eda28..7be3d8894520 100644
> --- a/arch/powerpc/include/asm/fadump-internal.h
> +++ b/arch/powerpc/include/asm/fadump-internal.h
> @@ -42,7 +42,25 @@ static inline u64 fadump_str_to_u64(const char *str)
>  
>  #define FADUMP_CPU_UNKNOWN		(~((u32)0))
>  
> -#define FADUMP_CRASH_INFO_MAGIC		fadump_str_to_u64("FADMPINF")
> +/*
> + * The introduction of new fields in the fadump crash info header has
> + * led to a change in the magic key, from `FADMPINF` to `FADMPSIG`.
> + * This alteration ensures backward compatibility, enabling the kernel
> + * with the updated fadump crash info to handle kernel dumps from older
> + * kernels.
> + *
> + * To prevent the need for further changes to the magic number in the
> + * event of future modifications to the fadump header, a version field
> + * has been introduced to track the fadump crash info header version.
> + *
> + * Historically, there was no connection between the magic number and
> + * the fadump crash info header version. However, moving forward, the
> + * `FADMPINF` magic number in header will be treated as version 0, while
> + * the `FADMPSIG` magic number in header will include a version field to
> + * determine its version.
> + */
> +#define FADUMP_CRASH_INFO_MAGIC		fadump_str_to_u64("FADMPSIG")
> +#define FADUMP_VERSION			1
>

Can we keep the old magic details as

#define FADUMP_CRASH_INFO_MAGIC_OLD		fadump_str_to_u64("FADMPINF")
#define FADUMP_CRASH_INFO_MAGIC		fadump_str_to_u64("FADMPSIG")

Also considering the struct need not be backward compatible, can we just
do

struct fadump_crash_info_header {
	u64		magic_number;
	u32		crashing_cpu;
	u64		elfcorehdr_addr;
	u64		elfcorehdr_size;
	u64		vmcoreinfo_raddr;
	u64		vmcoreinfo_size;
	struct pt_regs	regs;
	struct cpumask	cpu_mask;
};

static inline bool fadump_compatible(struct fadump_crash_info_header
*fdh)
{
	return (fdh->magic_number == FADUMP_CRASH_INFO_MAGIC)
}

and fail fadump if we find it not compatible?

-aneesh