Messages in this thread | | | Subject | Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND | From | Yonghong Song <> | Date | Wed, 2 May 2018 07:01:18 -0700 |
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Hi, Peter,
Ping again. Did you get chances to think about this issue again?
Thanks!
Yonghong
On 4/27/18 9:34 AM, Yonghong Song wrote: > Hi, Peter, > > Just wanted to ping again so that you did not miss the email below. > Please let me know your opinion. > > Thanks! > > Yonghong > > > On 4/23/18 9:50 AM, Yonghong Song wrote: >> 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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