microblaze: restore the ABI argument home area below pt_regs (PTO)

microblaze: restore the ABI argument home area below pt_regs (PTO)

The MicroBlaze ABI has the caller reserve stack space for the arguments
it passes in registers: REG_PARM_STACK_SPACE is 24 and
OUTGOING_REG_PARM_STACK_SPACE is 1 in the gcc backend, so a callee may
write to [caller_sp + 4, caller_sp + 28).  The kernel calls C functions
from entry.S with r1 pointing at pt_regs, handing the callee license to
spill its incoming arguments over the saved registers -- the syscall
dispatch is the worst case, where the first argument slot is PT_R1, the
saved user stack pointer.

This was latent until GCC 15: since 3b9b8d6cfdf5 ("ira: Scale
save/restore costs of callee save registers with block frequency") the
allocator prefers spilling incoming arguments over copying them into
callee-saved registers, and a kernel built with gcc >= 15 (without the
TARGET_CALLEE_SAVE_COST workaround some distributions carry) corrupts
PT_R1 on the first syscall: init takes SIGSEGV and the kernel panics.

The kernel had exactly this reservation until 2011:
commit 6e83557c38b4 ("microblaze: Remove r0_ram pointer and PTO alignment")
removed STATE_SAVE_ARG_SPACE and with it the PTO offset, as part of
cleaning up what was thought to be copied-from-v850 leftovers.  Restore
it: the frame is STATE_SAVE_SIZE = PT_SIZE + PTO, r1 stays at the frame
base through every asm-to-C call, and the saved registers are reached
at r1 + PTO + PT_*.  PTO is 28 rather than the historic 24, which was
one word short: FIRST_PARM_OFFSET is 4 and REG_PARM_STACK_SPACE is 24,
so the area spans [sp+4, sp+28) and needs 28 bytes.  With 24 the last
argument slot overlapped pt_regs' r0 -- harmless only because r0 is the
constant-zero register.  STACK_BOUNDARY is 32 bits, so 28 needs no
further rounding.

Two places deliberately keep their mainline addressing.
hw_exception_handler.S needs no offset change: its real-mode handler
saves into the standalone pt_pool_space buffer rather than a stack
frame, and _unaligned_data_exception works through a pointer to pt_regs
in r7, where plain PT_* offsets are already right.  And the user-SP
reload after popping the frame keeps PT_R1 - PT_SIZE because PTO
cancels there: (PTO + PT_R1) - (PT_SIZE + PTO) = PT_R1 - PT_SIZE.  Both
are exactly as the pre-2011 code had them.

The instruction count is unchanged -- the same instructions with
different immediates, no per-call reservation and no trampolines.  The
cost is 28 bytes more kernel stack per saved frame.

Fixes: 6e83557c38b4 ("microblaze: Remove r0_ram pointer and PTO alignment")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: Ramin Moussavi <ramin.moussavi@yacoub.de>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>
Link: https://patch.msgid.link/20260821151809.1233057-5-ramin.moussavi@yacoub.de
Signed-off-by: Michal Simek <michal.simek@amd.com>