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SubjectRe: [PATCH 4.9 84/96] x86/asm: Use register variable to get stack pointer value
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On 01/15/2018 03:35 PM, Greg Kroah-Hartman wrote:
> 4.9-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andrey Ryabinin <aryabinin@virtuozzo.com>
>
> commit 196bd485ee4f03ce4c690bfcf38138abfcd0a4bc upstream.
>
> Currently we use current_stack_pointer() function to get the value
> of the stack pointer register. Since commit:
>
> f5caf621ee35 ("x86/asm: Fix inline asm call constraints for Clang")
>
> ... we have a stack register variable declared. It can be used instead of
> current_stack_pointer() function which allows to optimize away some
> excessive "mov %rsp, %<dst>" instructions:
>
> -mov %rsp,%rdx
> -sub %rdx,%rax
> -cmp $0x3fff,%rax
> -ja ffffffff810722fd <ist_begin_non_atomic+0x2d>
>
> +sub %rsp,%rax
> +cmp $0x3fff,%rax
> +ja ffffffff810722fa <ist_begin_non_atomic+0x2a>
>
> Remove current_stack_pointer(), rename __asm_call_sp to current_stack_pointer
> and use it instead of the removed function.
>
> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Link: http://lkml.kernel.org/r/20170929141537.29167-1-aryabinin@virtuozzo.com
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> [dwmw2: We want ASM_CALL_CONSTRAINT for retpoline]

If we want ASM_CALL_CONSTRAINT it would be more correct to backport
f5caf621ee35 ("x86/asm: Fix inline asm call constraints for Clang")
and some fixes for it:
520a13c530ae ("x86/asm: Fix inline asm call constraints for GCC 4.4")
ca26cffa4e4a ("x86/asm: Allow again using asm.h when building for the 'bpf' clang target")

Because ASM_CALL_CONSTRAINT added in f5caf621ee35, not in this patch.

The end result looks fine though. So it's ok to keep it that way.


> Signed-off-by: David Woodhouse <dwmw@amazon.co.ku>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> arch/x86/include/asm/asm.h | 11 +++++++++++
> arch/x86/include/asm/thread_info.h | 11 -----------
> arch/x86/kernel/irq_32.c | 6 +++---
> arch/x86/kernel/traps.c | 2 +-
> arch/x86/mm/tlb.c | 2 +-
> 5 files changed, 16 insertions(+), 16 deletions(-)
>
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -125,4 +125,15 @@
> /* For C file, we already have NOKPROBE_SYMBOL macro */
> #endif
>
> +#ifndef __ASSEMBLY__
> +/*
> + * This output constraint should be used for any inline asm which has a "call"
> + * instruction. Otherwise the asm may be inserted before the frame pointer
> + * gets set up by the containing function. If you forget to do this, objtool
> + * may print a "call without frame pointer save/setup" warning.
> + */
> +register unsigned long current_stack_pointer asm(_ASM_SP);
> +#define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer)
> +#endif
> +
> #endif /* _ASM_X86_ASM_H */

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