linux-kernel - Re: [PATCH] x86, build: remove -m16 workaround for unsupported versions of GCC

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Date:   Tue, 01 Dec 2020 21:49:15 -0800
From:   hpa@...or.com
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
CC:     David Woodhouse <dwmw2@...radead.org>, x86@...nel.org,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] x86, build: remove -m16 workaround for unsupported versions of GCC

On November 30, 2020 5:13:06 PM PST, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>A revert of the following two commits.
>commit de3accdaec88 ("x86, build: Build 16-bit code with -m16 where
>possible")
>commit a9cfccee6604 ("x86, build: Change code16gcc.h from a C header to
>an assembly header")
>
>Since commit 0bddd227f3dc ("Documentation: update for gcc 4.9
>requirement") the minimum supported version of GCC is gcc-4.9.  It's
>now
>safe to remove this code.
>
>Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>---
> arch/x86/Makefile         |  9 +--------
> arch/x86/boot/code16gcc.h | 12 ------------
> 2 files changed, 1 insertion(+), 20 deletions(-)
> delete mode 100644 arch/x86/boot/code16gcc.h
>
>diff --git a/arch/x86/Makefile b/arch/x86/Makefile
>index 1bf21746f4ce..7116da3980be 100644
>--- a/arch/x86/Makefile
>+++ b/arch/x86/Makefile
>@@ -24,14 +24,7 @@ endif
> 
># How to compile the 16-bit code.  Note we always compile for
>-march=i386;
> # that way we can complain to the user if the CPU is insufficient.
>-#
>-# The -m16 option is supported by GCC >= 4.9 and clang >= 3.5. For
>-# older versions of GCC, include an *assembly* header to make sure
>that
>-# gcc doesn't play any games behind our back.
>-CODE16GCC_CFLAGS := -m32 -Wa,$(srctree)/arch/x86/boot/code16gcc.h
>-M16_CFLAGS	 := $(call cc-option, -m16, $(CODE16GCC_CFLAGS))
>-
>-REALMODE_CFLAGS	:= $(M16_CFLAGS) -g -Os -DDISABLE_BRANCH_PROFILING \
>+REALMODE_CFLAGS	:= -m16 -g -Os -DDISABLE_BRANCH_PROFILING \
> 		   -Wall -Wstrict-prototypes -march=i386 -mregparm=3 \
> 		   -fno-strict-aliasing -fomit-frame-pointer -fno-pic \
> 		   -mno-mmx -mno-sse
>diff --git a/arch/x86/boot/code16gcc.h b/arch/x86/boot/code16gcc.h
>deleted file mode 100644
>index e19fd7536307..000000000000
>--- a/arch/x86/boot/code16gcc.h
>+++ /dev/null
>@@ -1,12 +0,0 @@
>-/* SPDX-License-Identifier: GPL-2.0 */
>-#
>-# code16gcc.h
>-#
>-# This file is added to the assembler via -Wa when compiling 16-bit C
>code.
>-# This is done this way instead via asm() to make sure gcc does not
>reorder
>-# things around us.
>-#
>-# gcc 4.9+ has a real -m16 option so we can drop this hack long term.
>-#
>-
>-	.code16gcc

With enthusiasm:

Acked-by: H. Peter Anvin <hpa@...or.com>
-- 
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