2021-02-08 07:21:13

by Christophe Leroy

Subject: [PATCH] powerpc/32: Preserve cr1 in exception prolog stack check to fix build error

THREAD_ALIGN_SHIFT = THREAD_SHIFT + 1 = PAGE_SHIFT + 1
Maximum PAGE_SHIFT is 18 for 256k pages so
THREAD_ALIGN_SHIFT is 19 at the maximum.

No need to clobber cr1, it can be preserved when moving r1
into CR when we check stack overflow.

This reduces the number of instructions in Machine Check Exception
prolog and fixes a build failure reported by the kernel test robot
on v5.10 stable when building with RTAS + VMAP_STACK + KVM. That
build failure is due to too many instructions in the prolog hence
not fitting between 0x200 and 0x300. Allthough the problem doesn't
show up in mainline, it is still worth the change.

Reported-by: kernel test robot <[email protected]>
Fixes: 98bf2d3f4970 ("powerpc/32s: Fix RTAS machine check with VMAP stack")
Cc: [email protected]
Signed-off-by: Christophe Leroy <[email protected]>
---
arch/powerpc/kernel/head_32.h | 2 +-
arch/powerpc/kernel/head_book3s_32.S | 6 ------
2 files changed, 1 insertion(+), 7 deletions(-)

diff --git a/arch/powerpc/kernel/head_32.h b/arch/powerpc/kernel/head_32.h
index a2f72c966baf..abc7b603ab65 100644
--- a/arch/powerpc/kernel/head_32.h
+++ b/arch/powerpc/kernel/head_32.h
@@ -47,7 +47,7 @@
lwz r1,TASK_STACK-THREAD(r1)
addi r1, r1, THREAD_SIZE - INT_FRAME_SIZE
1:
- mtcrf 0x7f, r1
+ mtcrf 0x3f, r1
bt 32 - THREAD_ALIGN_SHIFT, stack_overflow
#else
subi r11, r1, INT_FRAME_SIZE /* use r1 if kernel */
diff --git a/arch/powerpc/kernel/head_book3s_32.S b/arch/powerpc/kernel/head_book3s_32.S
index 54140f4927e5..10e6aa88b1ff 100644
--- a/arch/powerpc/kernel/head_book3s_32.S
+++ b/arch/powerpc/kernel/head_book3s_32.S
@@ -278,12 +278,6 @@ MachineCheck:
7: EXCEPTION_PROLOG_2
addi r3,r1,STACK_FRAME_OVERHEAD
#ifdef CONFIG_PPC_CHRP
-#ifdef CONFIG_VMAP_STACK
- mfspr r4, SPRN_SPRG_THREAD
- tovirt(r4, r4)
- lwz r4, RTAS_SP(r4)
- cmpwi cr1, r4, 0
-#endif
beq cr1, machine_check_tramp
twi 31, 0, 0
#else
--
2.25.0

2021-02-12 00:41:26

by Michael Ellerman

Subject: Re: [PATCH] powerpc/32: Preserve cr1 in exception prolog stack check to fix build error

On Mon, 8 Feb 2021 07:17:40 +0000 (UTC), Christophe Leroy wrote:
> THREAD_ALIGN_SHIFT = THREAD_SHIFT + 1 = PAGE_SHIFT + 1
> Maximum PAGE_SHIFT is 18 for 256k pages so
> THREAD_ALIGN_SHIFT is 19 at the maximum.
>
> No need to clobber cr1, it can be preserved when moving r1
> into CR when we check stack overflow.
>
> [...]

Applied to powerpc/next.

[1/1] powerpc/32: Preserve cr1 in exception prolog stack check to fix build error
https://git.kernel.org/powerpc/c/3642eb21256a317ac14e9ed560242c6d20cf06d9

cheers