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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 21 Sep 2023 13:46:34 +0530

Aditya Gupta <[email-masked]> writes:

> On Wed, Sep 20, 2023 at 05:45:36PM +0530, Aneesh Kumar K.V wrote:
>> 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?
>> 
>
> I could not find another way to get any other flag for RADIX vs HASH in
> makedumpfile. And currently I don't know of any other way.
>
> Both makedumpfile and crash look for '0x40' flag set in
> 'cur_cpu_spec.mmu_features', so only requirement for 'MMU_FTR_TYPE_RADIX' is to
> be '0x40', or we will need to change the value accordingly in the tools.
>

Instead of exporting cur_cpu_spec.mmu_feature, you could do
coreinfo_mmu_features that does

if (radix_enabled())
   coreinfo_mmu_feature = VMCORE_INFO_RADIX_TRANSLATION;

-aneesh