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From: Peter Zijlstra <peterz@infradead.org>
To: Yonghong Song <yhs@fb.com>
Cc: mingo@kernel.org, ast@fb.com, daniel@iogearbox.net,
	linux-kernel@vger.kernel.org, x86@kernel.org, kernel-team@fb.com
Subject: Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF_WORKAROUND
Date: Mon, 23 Apr 2018 12:52:09 +0200	[thread overview]
Message-ID: <20180423105209.GX4064@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <fd14097e-31c5-0c0e-cbe5-d399299ca489@fb.com>

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 <native_arch>'. But because that doesn't set __BPF__ you
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__'

  reply	other threads:[~2018-04-23 10:52 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-15  4:27 Yonghong Song
2018-04-19 16:36 ` Yonghong Song
2018-04-20  8:19 ` Peter Zijlstra
2018-04-20 18:06   ` Yonghong Song
2018-04-23 10:52     ` Peter Zijlstra [this message]
2018-04-23 16:50       ` Yonghong Song
2018-04-27 16:34         ` Yonghong Song
2018-05-02 14:01           ` Yonghong Song

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