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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 26, 2018 at 11:35:04AM +0000, Andrew Cooper 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>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> 
> RFC until support is actually committed to binutils mainline.

Since RFC has been dropped from the subject, is this now committed?

> ---
>  xen/arch/x86/Rules.mk                 |  4 ++++
>  xen/include/asm-x86/alternative-asm.h | 14 ++++++++++++--
>  xen/include/asm-x86/alternative.h     | 13 ++++++++++---
>  3 files changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/xen/arch/x86/Rules.mk b/xen/arch/x86/Rules.mk
> index e169d67..bf5047f 100644
> --- 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)
> +
>  CFLAGS += -mno-red-zone -fpic -fno-asynchronous-unwind-tables
>  
>  # Xen doesn't use SSE interally.  If the compiler supports it, also skip the
> diff --git a/xen/include/asm-x86/alternative-asm.h 
> b/xen/include/asm-x86/alternative-asm.h
> index 25f79fe..9e46bed 100644
> --- 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

I'm not able to find any online document about the .nop directive, and
I cannot really figure out the purpose of the second parameter.

Also, after this patch is applied it seems like the padding is not
going to be 0x90, because as will already emit optimized nops. Are
those nops more optimized than the ones added by the alternatives
framework?

I would expect the nops added at runtime would be more optimized than
the ones added at build time, because the runtime ones could take into
account the specific CPU model Xen is running on.

Thanks, Roger.

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