* Andy Lutomirski <[email protected]> wrote:
> As per https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383, GCC only
> allows -mpreferred-stack-boundary=3 on x86_64 if -mno-sse is set.
> That means that cc-option will not detect
> -mpreferred-stack-boundary=3 support, because we test for it before
> setting -mno-sse.
>
> Fix it by reordering the Makefile bits.
>
> Compile-tested only. This should help avoid code generation issues
> such as the one that was worked around in b96fecbfa8c8 ("x86/fpu:
> Fix boot crash in the early FPU code").
>
> I'm a bit concerned that we could still have problems on older GCC
> versions given that our asm code does not respect GCC's idea of the
> ABI-required stack alignment.
>
> Cc: Linus Torvalds <[email protected]>
> Cc: Jan Kara <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Signed-off-by: Andy Lutomirski <[email protected]>
> ---
> arch/x86/Makefile | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 118e6debc483..344dd2110b2a 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -39,6 +39,12 @@ ifdef CONFIG_X86_NEED_RELOCS
> LDFLAGS_vmlinux := --emit-relocs
> endif
>
> +# prevent gcc from generating any FP code by mistake
> +# This must be before we try -mpreferred-stack-boundary -- see
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
> +KBUILD_CFLAGS += -mno-sse -mno-mmx -mno-sse2 -mno-3dnow
> +KBUILD_CFLAGS += $(call cc-option,-mno-avx,)
> +
So the 'stack boundary' is the RSP that GCC generates before it calls another
function from within an existing function, right?
So looking at this I question the choice of -mpreferred-stack-boundary=3. Why
not
do -mpreferred-stack-boundary=2?
My reasoning: on modern uarchs there's no penalty for 32-bit misalignment of
64-bit variables, only if they cross 64-byte cache lines, which should be rare
with a chance of 1:16. This small penalty (of at most +1 cycle in some
circumstances IIRC) should be more than counterbalanced by the compression of
the
stack by 5% on average.
... using stack-boundary=1 or stack-boundary=0 would probably be
counterproductive, as these more exotic misalignments get treated progressively
worse by x86 CPUs.
... but I have not measured any of this and even the 5% is just a possibly
overoptimistic guess.
Thanks,
Ingo
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