[PATCH 0/2] Compiler Attributes, s390: Provide and use __uninitialized macro

Heiko Carstens
2024-02-05T15:48:53Z

With INIT_STACK_ALL_PATTERN or INIT_STACK_ALL_ZERO enabled the kernel will
be compiled with -ftrivial-auto-var-init=<...> which causes initialization
of stack variables at function entry time.

In order to avoid the performance impact that comes with this users can use
the "uninitialized" attribute to prevent such initialization.

In particular code sections in s390 specific kernel code which use floating
point or vector registers all come with a 520 byte stack variable to save
already in use registers, if required.

If the above named config options are enabled this stack variable will
always be initialized on function entry in addition to saving register
contents, which contradicts the intend (performance improvement) of such
code sections.

Therefore provide a generic __uninitialized macro and an s390 specific
DECLARE_KERNEL_FPU_ONSTACK() macro which provides a kernel fpu variable
with an __uninitialized attribute, and convert all existing code to use
this.

If people are ok which this approach, I'd like to carry this via the s390
tree to avoid potential merge conflicts, since there is a larger fpu code
rework pending

Thanks,
Heiko

Heiko Carstens (2):
Compiler Attributes: Add __uninitialized macro
s390/fpu: make use of __uninitialized macro

arch/s390/crypto/chacha-glue.c | 2 +-
arch/s390/crypto/crc32-vx.c | 2 +-
arch/s390/include/asm/fpu/types.h | 3 +++
arch/s390/kernel/sysinfo.c | 2 +-
include/linux/compiler_attributes.h | 12 ++++++++++++
lib/raid6/s390vx.uc | 4 ++--
6 files changed, 20 insertions(+), 5 deletions(-)

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2.40.1