From: Peter Zijlstra <peterz@infradead.org>
To: Yonghong Song <yhs@fb.com>
Cc: mingo@kernel.org, torvalds@linux-foundation.org, ast@fb.com,
daniel@iogearbox.net, linux-kernel@vger.kernel.org,
x86@kernel.org, netdev@vger.kernel.org, kernel-team@fb.com,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH bpf v3] x86/cpufeature: bpf hack for clang not supporting asm goto
Date: Thu, 10 May 2018 12:06:34 +0200 [thread overview]
Message-ID: <20180510100634.GZ12217@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180504033119.2130788-1-yhs@fb.com>
On Thu, May 03, 2018 at 08:31:19PM -0700, Yonghong Song wrote:
> This approach is preferred since the already deployed bcc scripts, or
> any other bpf applicaitons utilizing LLVM JIT compilation functionality,
> will continue work with the new kernel without re-compilation and
> re-deployment.
So I really hate this and would much rather see the BPF build
environment changed. It not consistenyly having __BPF__ defined really
smells like a bug on your end.
Sometimes you just need to update tools... Is it really too hard to do
-D__BPF__ in the bpf build process that we need to mollest the kernel
for it?
> Note that this is a hack in the kernel to workaround bpf compilation issue.
> The hack will be removed once clang starts to support asm goto.
Note that that ^^ already mandates people re-deploy their bpf tools, so
why is llvm supporting asm-goto a better point to re-deploy than fixing
a consistent __BPF__ define for the bpf build environment?
> diff --git a/Makefile b/Makefile
> index 83b6c54..cfd8759 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 += -D__NO_CLANG_BPF_HACK
> KBUILD_AFLAGS += -DCC_HAVE_ASM_GOTO
> endif
I really think this is the wrong thing to do; but if the x86 maintainers
are willing to take this, I'll grudingly shut up.
Ingo, Thomas?
> diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
> index b27da96..42edd5d 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_CLANG_BPF_HACK
> /*
> * 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
>
next prev parent reply other threads:[~2018-05-10 10:06 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-04 3:31 [PATCH bpf v3] x86/cpufeature: bpf hack for clang not supporting asm goto Yonghong Song
2018-05-10 10:06 ` Peter Zijlstra [this message]
2018-05-10 15:52 ` Alexei Starovoitov
2018-05-10 16:20 ` Borislav Petkov
2018-05-10 17:57 ` Gianluca Borello
2018-05-10 17:58 ` Alexei Starovoitov
2018-05-10 18:16 ` Borislav Petkov
2018-05-10 10:49 ` Borislav Petkov
2018-05-12 16:03 Alexei Starovoitov
2018-05-12 20:30 ` Thomas Gleixner
2018-05-13 17:43 ` Alexei Starovoitov
2018-05-13 18:02 ` Thomas Gleixner
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