Subject: Re: [PATCH v2 1/2] powerpc: add `cur_cpu_spec` symbol to vmcoreinfo

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 20 Sep 2023 17:45:36 +0530

Aditya Gupta <[email-masked]> writes:

> Since below commit, address mapping for vmemmap has changed for Radix
> MMU, where address mapping is stored in kernel page table itself,
> instead of earlier used 'vmemmap_list'.
>
>     commit 368a0590d954 ("powerpc/book3s64/vmemmap: switch radix to use
>     a different vmemmap handling function")
>
> Hence with upstream kernel, in case of Radix MMU, makedumpfile fails to do
> address translation for vmemmap addresses, as it depended on vmemmap_list,
> which can now be empty.
>
> While fixing the address translation in makedumpfile, it was identified
> that currently makedumpfile cannot distinguish between Hash MMU and
> Radix MMU, unless VMLINUX is passed with -x flag to makedumpfile.
> And hence fails to assign offsets and shifts correctly (such as in L4 to
> PGDIR offset calculation in makedumpfile).
>
> For getting the MMU, makedumpfile uses `cur_cpu_spec.mmu_features`.
>
> Add `cur_cpu_spec` symbol and offset of `mmu_features` in the
> `cpu_spec` struct, to VMCOREINFO, so that makedumpfile can assign the
> offsets correctly, without needing a VMLINUX.
>
> Fixes: 368a0590d954 ("powerpc/book3s64/vmemmap: switch radix to use a different vmemmap handling function")
> Reported-by: Sachin Sant <[email-masked]>
> Tested-by: Sachin Sant <[email-masked]>
> Signed-off-by: Aditya Gupta <[email-masked]>
>
> ---
> Corresponding makedumpfile patches to fix address translation, in Radix
> MMU case:
>
> Link: https://lore.kernel.org/kexec/[email-masked]/T/#t
> ---
> ---
>  arch/powerpc/kexec/core.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/kexec/core.c b/arch/powerpc/kexec/core.c
> index de64c7962991..369b8334a4f0 100644
> --- a/arch/powerpc/kexec/core.c
> +++ b/arch/powerpc/kexec/core.c
> @@ -63,6 +63,8 @@ void arch_crash_save_vmcoreinfo(void)
>  #ifndef CONFIG_NUMA
>  	VMCOREINFO_SYMBOL(contig_page_data);
>  #endif
> +	VMCOREINFO_SYMBOL(cur_cpu_spec);
> +	VMCOREINFO_OFFSET(cpu_spec, mmu_features);
>  #if defined(CONFIG_PPC64) && defined(CONFIG_SPARSEMEM_VMEMMAP)
>  	VMCOREINFO_SYMBOL(vmemmap_list);
>  	VMCOREINFO_SYMBOL(mmu_vmemmap_psize);
>

That implies we now have to be careful when updating MMU_FTR_* #defines.
It is not bad considering other hacks we do in crash to identify kernel
changes tied to version number. But i am wondering if there another way
to identify radix vs hash?

-aneesh