[PATCH] microblaze: restore the page alignment of swapper_pg_dir

[Ramin Moussavi]

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 6215d9f4470f ("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.

Fixes: 6215d9f4470f ("arch, mm: consolidate empty_zero_page")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Ramin Moussavi <ramin.moussavi@xxxxxxxxx>
---
 arch/microblaze/kernel/head.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/microblaze/kernel/head.S b/arch/microblaze/kernel/head.S
index 808019c3b7ac..9bd3e513c89b 100644
--- a/arch/microblaze/kernel/head.S
+++ b/arch/microblaze/kernel/head.S
@@ -39,6 +39,8 @@
 #include <asm/processor.h>
 
 .section .data
+/* The MMU requires a page aligned page directory. */
+.align 12
 .global swapper_pg_dir
 swapper_pg_dir:
 	.space	PAGE_SIZE
-- 
2.53.0