Messages in this thread Patch in this message |  | | From | Yonghong Song <> | Subject | [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND | Date | Sat, 14 Apr 2018 21:27:38 -0700 |
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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)) -- 2.9.5
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