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Subject[PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND
Date
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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