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

[Aditya Gupta]

On Wed, Sep 20, 2023 at 05:45:36PM +0530, Aneesh Kumar K.V wrote:
> Aditya Gupta <adityag at linux.ibm.com> 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 <sachinp at linux.ibm.com>
> > Tested-by: Sachin Sant <sachinp at linux.ibm.com>
> > Signed-off-by: Aditya Gupta <adityag at linux.ibm.com>
> >
> > ---
> > Corresponding makedumpfile patches to fix address translation, in Radix
> > MMU case:
> >
> > Link: https://lore.kernel.org/kexec/B5F0F00E-F2B1-47D7-A143-5683D10DC29A@linux.ibm.com/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.

Thanks,
- Aditya Gupta