Re: [Xen-devel] [PATCH v2 7/7] x86/build: Use new .nop directive when available

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.02.18 at 12:35, <andrew.cooper3@xxxxxxxxxx> wrote:
> Newer versions of binutils are capable of emitting an exact number bytes worth
> of optimised nops.  Use this in preference to .skip when available.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

In principle
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
However, ...

> RFC until support is actually committed to binutils mainline.

... upstream looks to have switched to .nops now, so the prereq
to the R-b is that you consistently switch over.

> --- a/xen/arch/x86/Rules.mk
> +++ b/xen/arch/x86/Rules.mk
> @@ -28,6 +28,10 @@ $(call as-option-add,CFLAGS,CC,".equ \"x\"$$(comma)1", \
>  $(call as-option-add,CFLAGS,CC,\
>      ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
>  
> +# Check to see whether the assmbler supports the .nop directive.
> +$(call as-option-add,CFLAGS,CC,\
> +    ".L1: .L2: .nop (.L2 - .L1)$$(comma)9",-DHAVE_AS_NOP_DIRECTIVE)

Do you really need the (arbitrary) second argument here?

> --- a/xen/include/asm-x86/alternative-asm.h
> +++ b/xen/include/asm-x86/alternative-asm.h
> @@ -1,6 +1,8 @@
>  #ifndef _ASM_X86_ALTERNATIVE_ASM_H_
>  #define _ASM_X86_ALTERNATIVE_ASM_H_
>  
> +#include <asm/nops.h>
> +
>  #ifdef __ASSEMBLY__
>  
>  /*
> @@ -18,6 +20,14 @@
>      .byte \pad_len
>  .endm
>  
> +.macro mknops nr_bytes
> +#ifdef HAVE_AS_NOP_DIRECTIVE
> +    .nop \nr_bytes, ASM_NOP_MAX

And do you really need to specify ASM_NOP_MAX here? What's
wrong with letting the assembler pick what it wants as the longest
NOP?

Jan


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