This is a note to let you know that I've just added the patch titled
powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
powerpc-makefile-don-t-pass-mcpu-powerpc64-when-buil.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit d71160d39db7e5c803f59c543b434ce106c5070f
Author: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Date: Tue Feb 15 22:28:58 2022 +1100
powerpc/Makefile: Don't pass -mcpu=powerpc64 when building 32-bit
[ Upstream commit 2863dd2db23e0407f6c50b8ba5c0e55abef894f1 ]
When CONFIG_GENERIC_CPU=y (true for all our defconfigs) we pass
-mcpu=powerpc64 to the compiler, even when we're building a 32-bit
kernel.
This happens because we have an ifdef CONFIG_PPC_BOOK3S_64/else block in
the Makefile that was written before 32-bit supported GENERIC_CPU. Prior
to that the else block only applied to 64-bit Book3E.
The GCC man page says -mcpu=powerpc64 "[specifies] a pure ... 64-bit big
endian PowerPC ... architecture machine [type], with an appropriate,
generic processor model assumed for scheduling purposes."
It's unclear how that interacts with -m32, which we are also passing,
although obviously -m32 is taking precedence in some sense, as the
32-bit kernel only contains 32-bit instructions.
This was noticed by inspection, not via any bug reports, but it does
affect code generation. Comparing before/after code generation, there
are some changes to instruction scheduling, and the after case (with
-mcpu=powerpc64 removed) the compiler seems more keen to use r8.
Fix it by making the else case only apply to Book3E 64, which excludes
32-bit.
Fixes: 0e00a8c9fd92 ("powerpc: Allow CPU selection also on PPC32")
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20220215112858.304779-1-mpe@xxxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/powerpc/Makefile b/arch/powerpc/Makefile
index aa6808e70647..72610e2d6176 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -173,7 +173,7 @@ else
CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=power7,$(call cc-option,-mtune=power5))
CFLAGS-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mcpu=power5,-mcpu=power4)
endif
-else
+else ifdef CONFIG_PPC_BOOK3E_64
CFLAGS-$(CONFIG_GENERIC_CPU) += -mcpu=powerpc64
endif
