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Message-ID: <41651b53-7e99-4394-7edc-13c17b3bc9dd@fb.com>
Date: Thu, 19 Apr 2018 09:36:00 -0700
From: Yonghong Song <yhs@...com>
To: <peterz@...radead.org>, <mingo@...nel.org>, <ast@...com>,
<daniel@...earbox.net>, <linux-kernel@...r.kernel.org>,
<x86@...nel.org>
CC: <kernel-team@...com>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with
NO_BPF_WORKAROUND
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@...nel.org>
> Signed-off-by: Yonghong Song <yhs@...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))
>
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