Messages in this thread | | | Subject | Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND | From | Yonghong Song <> | 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)) >
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