linux-kernel - Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with NO_BPF

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Message-ID: <20180423105209.GX4064@hirez.programming.kicks-ass.net>
Date:   Mon, 23 Apr 2018 12:52:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yonghong Song <yhs@...com>
Cc:     mingo@...nel.org, ast@...com, daniel@...earbox.net,
        linux-kernel@...r.kernel.org, x86@...nel.org, kernel-team@...com
Subject: Re: [PATCH v2] x86/cpufeature: guard asm_volatile_goto usage with
 NO_BPF_WORKAROUND

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__'