Re: [PATCH] microblaze: restore the page alignment of swapper_pg_dir

[Michal Simek <monstr@monstr.eu>]

On 7/27/26 23:58, Ramin Moussavi wrote:
> 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)

hm. This is the first time I get information about this regression.

Guenter: I know you had Microblaze in your testing. Is it still running?

Thanks,
Michal