[PATCH] ppc64: add mmu information to vmcoreinfo

Aditya Gupta adityag at linux.ibm.com
Mon Oct 23 18:26:12 AEDT 2023

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.

Also, even along with `cur_cpu_spec->mmu_features` makedumpfile has to
depend on the 'MMU_FTR_TYPE_RADIX' flag in mmu_features, implying kernel
developers need to be cautious of changes to 'MMU_FTR_*' defines.

A more stable approach was suggested in the below thread by contributors:

The suggestion was to add whether 'RADIX_MMU' is enabled in vmcoreinfo

This patch also implements the suggestion, by adding 'RADIX_MMU' in
vmcoreinfo, which makedumpfile can use to get whether the crashed system
had RADIX MMU (in which case 'NUMBER(RADIX_MMU)=1') or not (in which

Fixes: 368a0590d954 ("powerpc/book3s64/vmemmap: switch radix to use a different vmemmap handling function")
Reported-by: Sachin Sant <sachinp at linux.ibm.com>
Signed-off-by: Aditya Gupta <adityag at linux.ibm.com>
 arch/powerpc/kexec/core.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kexec/core.c b/arch/powerpc/kexec/core.c
index de64c7962991..005269ac3244 100644
--- a/arch/powerpc/kexec/core.c
+++ b/arch/powerpc/kexec/core.c
@@ -74,6 +74,9 @@ void arch_crash_save_vmcoreinfo(void)
 	VMCOREINFO_OFFSET(mmu_psize_def, shift);
+	VMCOREINFO_SYMBOL(cur_cpu_spec);
+	VMCOREINFO_OFFSET(cpu_spec, mmu_features);
+	vmcoreinfo_append_str("NUMBER(RADIX_MMU)=%d\n", early_radix_enabled());
 	vmcoreinfo_append_str("KERNELOFFSET=%lx\n", kaslr_offset());

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