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Subject: Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with
 NO_BPF_WORKAROUND
From:   Yonghong Song <yhs@fb.com>
To:     <peterz@infradead.org>, <mingo@kernel.org>, <ast@fb.com>,
        <daniel@iogearbox.net>, <linux-kernel@vger.kernel.org>,
        <x86@kernel.org>
CC:     <kernel-team@fb.com>, netdev <netdev@vger.kernel.org>
References: <20180415042738.1788215-1-yhs@fb.com>
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Date:   Thu, 19 Apr 2018 09:36:00 -0700
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Hi, Peter,

Just pinging. Did you get chances to look at this?

[ cc netdev as well so folks are aware of the issue. ]

Thanks!

Yonghong

On 4/14/18 9:27 PM, Yonghong Song wrote:
> Commit d0266046ad54 ("x86: Remove FAST_FEATURE_TESTS")
> removed X86_FAST_FEATURE_TESTS and make macro static_cpu_has() always
> use __always_inline function _static_cpu_has() funciton.
> The static_cpu_has() uses gcc feature asm_volatile_goto construct,
> which is not supported by clang.
> 
> Currently, for BPF programs written in C, the only widely-supported
> compiler is clang. Because of clang lacking support
> of asm_volatile_goto construct, if you try to compile
> bpf programs under samples/bpf/,
>     $ make -j20 && make headers_install && make samples/bpf/
> you will see a lot of failures like below:
> 
>    =========================
>    clang  -nostdinc -isystem /usr/lib/gcc/x86_64-redhat-linux/7/include \
>           -I/home/yhs/work/bpf-next/arch/x86/include \
>           -I./arch/x86/include/generated  -I/home/yhs/work/bpf-next/include \
>           -I./include -I/home/yhs/work/bpf-next/arch/x86/include/uapi \
>           -I./arch/x86/include/generated/uapi \
>           -I/home/yhs/work/bpf-next/include/uapi -I./include/generated/uapi \
>           -include /home/yhs/work/bpf-next/include/linux/kconfig.h
>           -Isamples/bpf \
> 	 -I/home/yhs/work/bpf-next/tools/testing/selftests/bpf/ \
> 	 -D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \
> 	 -D__TARGET_ARCH_x86 -Wno-compare-distinct-pointer-types \
> 	 -Wno-gnu-variable-sized-type-not-at-end \
> 	 -Wno-address-of-packed-member -Wno-tautological-compare \
> 	 -Wno-unknown-warning-option  \
> 	 -O2 -emit-llvm -c /home/yhs/work/bpf-next/samples/bpf/xdp_redirect_cpu_kern.c \
>           -o -| llc -march=bpf -filetype=obj -o samples/bpf/xdp_redirect_cpu_kern.o
>    In file included from /home/yhs/work/bpf-next/samples/bpf/xdp_redirect_cpu_kern.c:10:
>    In file included from /home/yhs/work/bpf-next/include/uapi/linux/in.h:24:
>    In file included from /home/yhs/work/bpf-next/include/linux/socket.h:8:
>    In file included from /home/yhs/work/bpf-next/include/linux/uio.h:13:
>    In file included from /home/yhs/work/bpf-next/include/linux/thread_info.h:38:
>    In file included from /home/yhs/work/bpf-next/arch/x86/include/asm/thread_info.h:53:
>    /home/yhs/work/bpf-next/arch/x86/include/asm/cpufeature.h:150:2: error: 'asm goto' constructs are not supported yet
>          asm_volatile_goto("1: jmp 6f\n"
>          ^
>    /home/yhs/work/bpf-next/include/linux/compiler-gcc.h:296:42: note: expanded from macro 'asm_volatile_goto'
>    #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)
>                                             ^
>    1 error generated.
>    =========================
>    ...
>    In file included from /home/yhs/work/bpf-next/samples/bpf/tracex4_kern.c:7:
>    In file included from /home/yhs/work/bpf-next/include/linux/ptrace.h:6:
>    In file included from /home/yhs/work/bpf-next/include/linux/sched.h:14:
>    In file included from /home/yhs/work/bpf-next/include/linux/pid.h:5:
>    In file included from /home/yhs/work/bpf-next/include/linux/rculist.h:11:
>    In file included from /home/yhs/work/bpf-next/include/linux/rcupdate.h:40:
>    In file included from /home/yhs/work/bpf-next/include/linux/preempt.h:81:
>    In file included from /home/yhs/work/bpf-next/arch/x86/include/asm/preempt.h:7:
>    In file included from /home/yhs/work/bpf-next/include/linux/thread_info.h:38:
>    In file included from /home/yhs/work/bpf-next/arch/x86/include/asm/thread_info.h:53:
>    /home/yhs/work/bpf-next/arch/x86/include/asm/cpufeature.h:150:2: error: 'asm goto' constructs are not supported yet
>    ...
>    =========================
> 
> This patch adds a preprocessor guard NO_BPF_WORKAROUND around the
> asm_volatile_goto based static_cpu_has(). NO_BPF_WORKAROUND is set
> at toplevel Makefile when compiler supports asm-goto. That is,
> if the compiler supports asm-goto, the kernel build will use
> asm-goto version of static_cpu_has().
> 
> For clang compilation for bpf programs where only header files
> are accessed, NO_BPF_WORKAROUND is not defined and asm-goto version
> of static_cpu_has() will not be accessed, hence the above compilation
> error will disappear.
> 
> If later clang compiler starts to support asm_volatile_goto,
> the change in this patch can be reverted.
> 
> Fixes: d0266046ad54 ("x86: Remove FAST_FEATURE_TESTS")
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>   Makefile                          | 1 +
>   arch/x86/include/asm/cpufeature.h | 5 +++++
>   2 files changed, 6 insertions(+)
> 
> Changelog:
>   v1 -> v2:
>     Use NO_BPF_WORKAROUND macro instead of CC_HAVE_ASM_GOTO
>     to make it explicit that this is a workaround.
> 
> diff --git a/Makefile b/Makefile
> index c1a608a..7b81f1f 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -504,6 +504,7 @@ export RETPOLINE_CFLAGS
>   ifeq ($(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-goto.sh $(CC) $(KBUILD_CFLAGS)), y)
>     CC_HAVE_ASM_GOTO := 1
>     KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO
> +  KBUILD_CFLAGS += -DNO_BPF_WORKAROUND
>     KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO
>   endif
>   
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index b27da96..638417b 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -140,6 +140,8 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
>   
>   #define setup_force_cpu_bug(bit) setup_force_cpu_cap(bit)
>   
> +/* this macro is a temporary hack for bpf until clang gains asm-goto support */
> +#ifdef NO_BPF_WORKAROUND
>   /*
>    * Static testing of CPU features.  Used the same as boot_cpu_has().
>    * These will statically patch the target code for additional
> @@ -195,6 +197,9 @@ static __always_inline __pure bool _static_cpu_has(u16 bit)
>   		boot_cpu_has(bit) :				\
>   		_static_cpu_has(bit)				\
>   )
> +#else
> +#define static_cpu_has(bit)		boot_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))
>