microblaze handles TLB misses in software, and the handler builds the
address of the L1 entry by ORing the index into the page directory base
instead of adding it (hw_exception_handler.S):
bsrli r5, r3, PGDIR_SHIFT - 2
andi r5, r5, PAGE_SIZE - 4
/* Assume pgdir aligned on 4K boundary, no need for
"andi r4,r4,0xfffff003" */
or r4, r4, r5
lwi r4, r4, 0 /* Get L1 entry */
The index is masked to the low 12 bits, so the OR only works if those bits
of the base are zero -- which is exactly the assumption the comment states
and the reason the masking of the base can be skipped.
swapper_pg_dir had no alignment directive of its own. It was aligned
because it followed empty_zero_page in head.S, and that one carried the
.align 12:
.section .data
.global empty_zero_page
.align 12
empty_zero_page:
.space PAGE_SIZE
.global swapper_pg_dir
swapper_pg_dir:
.space PAGE_SIZE
Commit 6215d9f447 ("arch, mm: consolidate empty_zero_page") removed
empty_zero_page from head.S, and with it the .align 12 that -- despite
sitting next to empty_zero_page -- was what page aligned swapper_pg_dir.
Since then swapper_pg_dir lands wherever .data happens to put it, its low
bits are no longer zero, and every kernel TLB miss ORs the index into a
base with a nonzero offset. The resulting L1 lookups read the wrong
words, no valid translation is ever installed, and the kernel spins in
exceptions long before it can print anything. On qemu-system-microblazeel
(petalogix-s3adsp1800) the console stays completely silent at 100% CPU;
there is no oops and no guest error reported by qemu, which makes this
awkward to diagnose.
Give swapper_pg_dir the alignment it requires, rather than relying on a
neighbour to provide it.
microblaze has no noMMU variant left in mainline -- CONFIG_MMU is def_bool
y and mmu_defconfig is the only defconfig -- so this is not a corner case:
every mainline microblaze kernel since v7.1-rc1 fails to boot, including
the v7.1 release.
v7.0: swapper_pg_dir = 0xc05fd000 (aligned)
v7.1-rc1: swapper_pg_dir = 0xc0603140 (offset 320)
v7.1-rc1 + this fix: swapper_pg_dir = 0xc0604000 (aligned)
next-20260726: swapper_pg_dir = 0xc0615140 (offset 320)
next-20260726 + this fix: swapper_pg_dir = 0xc0616000 (aligned)
Verified on qemu-system-microblazeel (petalogix-s3adsp1800) with
mmu_defconfig and microblazeel gcc 12.5.0: v7.1-rc1 and next-20260726 both
print nothing at all without the fix, and both boot to userspace with it.
Link: https://lore.kernel.org/20260727215823.1422701-1-ramin.moussavi@yacoub.de
Fixes: 6215d9f447 ("arch, mm: consolidate empty_zero_page")
Signed-off-by: Ramin Moussavi <ramin.moussavi@yacoub.de>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
c2689266e5