Re: [PATCH v2] asm goto: eradicate CC_HAS_ASM_GOTO

Alexandre Belloni
2022-08-19T20:04:50Z

On 19/08/2022 12:06:40-0700, Nick Desaulniers wrote:
> GCC has supported asm goto since 4.5, and Clang has since version 9.0.0.
> The minimum supported versions of these tools for the build according to
> Documentation/process/changes.rst are 5.1 and 11.0.0 respectively.
>
> Remove the feature detection script, Kconfig option, and clean up some
> fallback code that is no longer supported.
>
> The removed script was also testing for a GCC specific bug that was
> fixed in the 4.7 release.
>
> Also remove workarounds for bpftrace using clang older than 9.0.0, since
> other BPF backend fixes are required at this point.
>
> Link: https://lore.kernel.org/lkml/CAK7LNATSr=BXKfkdW8f-H5VT_w=xBpT2ZQcZ7rm6JfkdE+QnmA@mail.gmail.com/
> Link: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48637
> Acked-by: Borislav Petkov <bp@suse.de>
> Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

> ---
>
> Changes v1 -> v2:
> https://lore.kernel.org/linux-kbuild/20220819170053.2686006-1-ndesaulniers@google.com/
> * Pick up Boris' ack.
> * Drop line about Dash compat as per Alexandre.
> * Drop Alexandre's reported by as per Masahiro.
> * s/Kbuild/asm goto/ in oneline as per Masahiro.
> * Remove entirety of bpftrace workarounds as per Alexei.
> * Fix mistake in arch/x86/include/asm/rmwcc.h in v1 where I removed too
> much; we still need guards for __GCC_ASM_FLAG_OUTPUTS__.
>
> Documentation/kbuild/kconfig-language.rst | 4 ++--
> arch/Kconfig | 3 +--
> arch/um/include/asm/cpufeature.h | 15 ---------------
> arch/x86/Makefile | 4 ----
> arch/x86/include/asm/cpufeature.h | 15 ---------------
> arch/x86/include/asm/rmwcc.h | 6 +++---
> arch/x86/kvm/emulate.c | 2 +-
> init/Kconfig | 4 ----
> scripts/gcc-goto.sh | 22 ----------------------
> tools/arch/x86/include/asm/rmwcc.h | 21 ---------------------
> 10 files changed, 7 insertions(+), 89 deletions(-)
> delete mode 100755 scripts/gcc-goto.sh
>
> diff --git a/Documentation/kbuild/kconfig-language.rst b/Documentation/kbuild/kconfig-language.rst
> index 7fb398649f51..858ed5d80def 100644
> --- a/Documentation/kbuild/kconfig-language.rst
> +++ b/Documentation/kbuild/kconfig-language.rst
> @@ -525,8 +525,8 @@ followed by a test macro::
> If you need to expose a compiler capability to makefiles and/or C source files,
> `CC_HAS_` is the recommended prefix for the config option::
>
> - config CC_HAS_ASM_GOTO
> - def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
> + config CC_HAS_FOO
> + def_bool $(success,$(srctree)/scripts/cc-check-foo.sh $(CC))
>
> Build as module only
> ~~~~~~~~~~~~~~~~~~~~
> diff --git a/arch/Kconfig b/arch/Kconfig
> index f330410da63a..5dbf11a5ba4e 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -53,7 +53,6 @@ config KPROBES
> config JUMP_LABEL
> bool "Optimize very unlikely/likely branches"
> depends on HAVE_ARCH_JUMP_LABEL
> - depends on CC_HAS_ASM_GOTO
> select OBJTOOL if HAVE_JUMP_LABEL_HACK
> help
> This option enables a transparent branch optimization that
> @@ -1361,7 +1360,7 @@ config HAVE_PREEMPT_DYNAMIC_CALL
>
> config HAVE_PREEMPT_DYNAMIC_KEY
> bool
> - depends on HAVE_ARCH_JUMP_LABEL && CC_HAS_ASM_GOTO
> + depends on HAVE_ARCH_JUMP_LABEL
> select HAVE_PREEMPT_DYNAMIC
> help
> An architecture should select this if it can handle the preemption
> diff --git a/arch/um/include/asm/cpufeature.h b/arch/um/include/asm/cpufeature.h
> index 19cd7ed6ec3c..4b6d1b526bc1 100644
> --- a/arch/um/include/asm/cpufeature.h
> +++ b/arch/um/include/asm/cpufeature.h
> @@ -65,20 +65,6 @@ extern void setup_clear_cpu_cap(unsigned int bit);
>
> #define setup_force_cpu_bug(bit) setup_force_cpu_cap(bit)
>
> -#if defined(__clang__) && !defined(CONFIG_CC_HAS_ASM_GOTO)
> -
> -/*
> - * Workaround for the sake of BPF compilation which utilizes kernel
> - * headers, but clang does not support ASM GOTO and fails the build.
> - */
> -#ifndef __BPF_TRACING__
> -#warning "Compiler lacks ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
> -#endif
> -
> -#define static_cpu_has(bit) boot_cpu_has(bit)
> -
> -#else
> -
> /*
> * Static testing of CPU features. Used the same as boot_cpu_has(). It
> * statically patches the target code for additional performance. Use
> @@ -137,7 +123,6 @@ static __always_inline bool _static_cpu_has(u16 bit)
> boot_cpu_has(bit) : \
> _static_cpu_has(bit) \
> )
> -#endif
>
> #define cpu_has_bug(c, bit) cpu_has(c, (bit))
> #define set_cpu_bug(c, bit) set_cpu_cap(c, (bit))
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 7854685c5f25..bafbd905e6e7 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -286,10 +286,6 @@ vdso_install:
>
> archprepare: checkbin
> checkbin:
> -ifndef CONFIG_CC_HAS_ASM_GOTO
> - @echo Compiler lacks asm-goto support.
> - @exit 1
> -endif
> ifdef CONFIG_RETPOLINE
> ifeq ($(RETPOLINE_CFLAGS),)
> @echo "You are building kernel with non-retpoline compiler." >&2
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index ea34cc31b047..1a85e1fb0922 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -155,20 +155,6 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
>
> #define setup_force_cpu_bug(bit) setup_force_cpu_cap(bit)
>
> -#if defined(__clang__) && !defined(CONFIG_CC_HAS_ASM_GOTO)
> -
> -/*
> - * Workaround for the sake of BPF compilation which utilizes kernel
> - * headers, but clang does not support ASM GOTO and fails the build.
> - */
> -#ifndef __BPF_TRACING__
> -#warning "Compiler lacks ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
> -#endif
> -
> -#define static_cpu_has(bit) boot_cpu_has(bit)
> -
> -#else
> -
> /*
> * Static testing of CPU features. Used the same as boot_cpu_has(). It
> * statically patches the target code for additional performance. Use
> @@ -208,7 +194,6 @@ static __always_inline bool _static_cpu_has(u16 bit)
> boot_cpu_has(bit) : \
> _static_cpu_has(bit) \
> )
> -#endif
>
> #define cpu_has_bug(c, bit) cpu_has(c, (bit))
> #define set_cpu_bug(c, bit) set_cpu_cap(c, (bit))
> diff --git a/arch/x86/include/asm/rmwcc.h b/arch/x86/include/asm/rmwcc.h
> index 8a9eba191516..7fa611216417 100644
> --- a/arch/x86/include/asm/rmwcc.h
> +++ b/arch/x86/include/asm/rmwcc.h
> @@ -11,7 +11,7 @@
>
> #define __CLOBBERS_MEM(clb...) "memory", ## clb
>
> -#if !defined(__GCC_ASM_FLAG_OUTPUTS__) && defined(CONFIG_CC_HAS_ASM_GOTO)
> +#ifndef __GCC_ASM_FLAG_OUTPUTS__
>
> /* Use asm goto */
>
> @@ -27,7 +27,7 @@ cc_label: c = true; \
> c; \
> })
>
> -#else /* defined(__GCC_ASM_FLAG_OUTPUTS__) || !defined(CONFIG_CC_HAS_ASM_GOTO) */
> +#else /* defined(__GCC_ASM_FLAG_OUTPUTS__) */
>
> /* Use flags output or a set instruction */
>
> @@ -40,7 +40,7 @@ cc_label: c = true; \
> c; \
> })
>
> -#endif /* defined(__GCC_ASM_FLAG_OUTPUTS__) || !defined(CONFIG_CC_HAS_ASM_GOTO) */
> +#endif /* defined(__GCC_ASM_FLAG_OUTPUTS__) */
>
> #define GEN_UNARY_RMWcc_4(op, var, cc, arg0) \
> __GEN_RMWcc(op " " arg0, var, cc, __CLOBBERS_MEM())
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index b4eeb7c75dfa..08613c65138d 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -493,7 +493,7 @@ FOP_END;
>
> /*
> * XXX: inoutclob user must know where the argument is being expanded.
> - * Relying on CONFIG_CC_HAS_ASM_GOTO would allow us to remove _fault.
> + * Using asm goto would allow us to remove _fault.
> */
> #define asm_safe(insn, inoutclob...) \
> ({ \
> diff --git a/init/Kconfig b/init/Kconfig
> index 80fe60fa77fb..532362fcfe31 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -70,11 +70,7 @@ config CC_CAN_LINK_STATIC
> default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m64-flag) -static) if 64BIT
> default $(success,$(srctree)/scripts/cc-can-link.sh $(CC) $(CLANG_FLAGS) $(USERCFLAGS) $(USERLDFLAGS) $(m32-flag) -static)
>
> -config CC_HAS_ASM_GOTO
> - def_bool $(success,$(srctree)/scripts/gcc-goto.sh $(CC))
> -
> config CC_HAS_ASM_GOTO_OUTPUT
> - depends on CC_HAS_ASM_GOTO
> def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
>
> config CC_HAS_ASM_GOTO_TIED_OUTPUT
> diff --git a/scripts/gcc-goto.sh b/scripts/gcc-goto.sh
> deleted file mode 100755
> index 8b980fb2270a..000000000000
> --- a/scripts/gcc-goto.sh
> +++ /dev/null
> @@ -1,22 +0,0 @@
> -#!/bin/sh
> -# SPDX-License-Identifier: GPL-2.0
> -# Test for gcc 'asm goto' support
> -# Copyright (C) 2010, Jason Baron <jbaron@redhat.com>
> -
> -cat << "END" | $@ -x c - -fno-PIE -c -o /dev/null
> -int main(void)
> -{
> -#if defined(__arm__) || defined(__aarch64__)
> - /*
> - * Not related to asm goto, but used by jump label
> - * and broken on some ARM GCC versions (see GCC Bug 48637).
> - */
> - static struct { int dummy; int state; } tp;
> - asm (".long %c0" :: "i" (&tp.state));
> -#endif
> -
> -entry:
> - asm goto ("" :::: entry);
> - return 0;
> -}
> -END
> diff --git a/tools/arch/x86/include/asm/rmwcc.h b/tools/arch/x86/include/asm/rmwcc.h
> index fee7983a90b4..11ff975242ca 100644
> --- a/tools/arch/x86/include/asm/rmwcc.h
> +++ b/tools/arch/x86/include/asm/rmwcc.h
> @@ -2,8 +2,6 @@
> #ifndef _TOOLS_LINUX_ASM_X86_RMWcc
> #define _TOOLS_LINUX_ASM_X86_RMWcc
>
> -#ifdef CONFIG_CC_HAS_ASM_GOTO
> -
> #define __GEN_RMWcc(fullop, var, cc, ...) \
> do { \
> asm_volatile_goto (fullop "; j" cc " %l[cc_label]" \
> @@ -20,23 +18,4 @@ cc_label: \
> #define GEN_BINARY_RMWcc(op, var, vcon, val, arg0, cc) \
> __GEN_RMWcc(op " %1, " arg0, var, cc, vcon (val))
>
> -#else /* !CONFIG_CC_HAS_ASM_GOTO */
> -
> -#define __GEN_RMWcc(fullop, var, cc, ...) \
> -do { \
> - char c; \
> - asm volatile (fullop "; set" cc " %1" \
> - : "+m" (var), "=qm" (c) \
> - : __VA_ARGS__ : "memory"); \
> - return c != 0; \
> -} while (0)
> -
> -#define GEN_UNARY_RMWcc(op, var, arg0, cc) \
> - __GEN_RMWcc(op " " arg0, var, cc)
> -
> -#define GEN_BINARY_RMWcc(op, var, vcon, val, arg0, cc) \
> - __GEN_RMWcc(op " %2, " arg0, var, cc, vcon (val))
> -
> -#endif /* CONFIG_CC_HAS_ASM_GOTO */
> -
> #endif /* _TOOLS_LINUX_ASM_X86_RMWcc */
> --
> 2.37.1.595.g718a3a8f04-goog
>

--
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering