Messages in this thread Patches in this message | | | Subject | Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND | From | Yonghong Song <> | Date | Mon, 23 Apr 2018 09:50:59 -0700 |
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Hi, Peter,
Please see comments below.
On 4/23/18 3:52 AM, Peter Zijlstra wrote: > On Fri, Apr 20, 2018 at 11:06:03AM -0700, Yonghong Song wrote: >> On 4/20/18 1:19 AM, Peter Zijlstra wrote: > >>> Hurm, so adding __BPF__ for BPF compiles isn't an option? It seems to me >>> having a CPP flag to identify BPF compile context might be useful in >>> general. >> >> With "clang -target bpf", we already have __BPF__ defined. >> For tracing, esp. ptrace.h is included, "clang -target <native_arch>" where >> "-target <native_arch>" can be omitted, is typically used. > >> The reason is the native architecture header files typically >> include a lot of various asm related stuff where "-target bpf" cannot >> really handle. We relay on native clang to flush out all these >> asm constructs and only bpf program needed stuff survives >> reach to backend compiler. > > So because 'clang -target bpf' is 'broken', you do a work-around using
'clang -target bpf' is 'broken' in this case because the x86 arch has a lot of inline asm's in the header file where bpf target cannot handle. For most networking related bpf programs where `asm` is rarely involved, `clang -target bpf` works fine most of time.
> 'clang -target <native_arch>'. But because that doesn't set __BPF__ you
`clang -target <native_arch>` should work, regardless of whether __BPF__ macro is setup or not. The reason it doesn't work now is due to its lacking asm-goto support. So to use `clang -target <native_arch>` is not really a workaround for `target bpf`. It by itself should work.
> want to add NO_BPF_WORKAROUND to the kernel instead of adding __BPF__ to > your build rules to better mimick -target bpf, which you should be > using. > > How is that sane? Why not use 'clang -target <native_arch> -D__BPF__'
To workaround the asm-goto issue, the suggested macro __BPF__ can be added to user space and kernel. But note that `clang -target <native_arch>` will not define the macro __BPF__, so this requires user space change.
Also, to make sure people understand that this is a WORKAROUND for asm-goto issue and is not a lasting thing we want to support. I have the following change for cpufeature.h:
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index b27da9602a6d..c832118defa1 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -140,6 +140,7 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
#define setup_force_cpu_bug(bit) setup_force_cpu_cap(bit)
+#ifndef __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 +196,9 @@ 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)) diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile index 4d6a6edd4bf6..b229e5090e4a 100644 As mentioned above, user space needs to add this new macro definition. Specifically for kernel/samples/bpf: diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile index 4d6a6edd4bf6..b229e5090e4a 100644 --- a/samples/bpf/Makefile +++ b/samples/bpf/Makefile @@ -255,7 +255,7 @@ $(obj)/tracex5_kern.o: $(obj)/syscall_nrs.h $(obj)/%.o: $(src)/%.c $(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \ -I$(srctree)/tools/testing/selftests/bpf/ \ - -D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \ + -D__KERNEL__ -D__BPF_WORKAROUND__ -Wno-unused-value -Wno-pointer-sign \ -D__TARGET_ARCH_$(ARCH) -Wno-compare-distinct-pointer-types \ -Wno-gnu-variable-sized-type-not-at-end \ -Wno-address-of-packed-member -Wno-tautological-compare \ Please let me know whether this approach is okay to you or not, whether the name __BPF_WORKAROUND__ is better than __BPF__ or not, or we could use the earlier approach which does not require user space change.
Thanks!
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