Re: [PATCH 4.9 84/96] x86/asm: Use register variable to get stack pointer value

[Andrey Ryabinin <aryabinin@virtuozzo.com>]

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 */