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Date:   Wed, 30 Sep 2020 18:58:48 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
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        Sean Christopherson <sean.j.christopherson@intel.com>,
        Yu Wang <yu1.wang@intel.com>,
        Reinette Chatre <reinette.chatre@intel.com>,
        Yakui Zhao <yakui.zhao@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Fengwei Yin <fengwei.yin@intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>,
        Zhenyu Wang <zhenyuw@linux.intel.com>
Subject: Re: [PATCH v4 04/17] x86/acrn: Introduce hypercall interfaces
Message-ID: <20200930235848.GD28786@gate.crashing.org>
References: <20200922114311.38804-1-shuo.a.liu@intel.com> <20200922114311.38804-5-shuo.a.liu@intel.com> <20200927105152.GG88650@kroah.com> <6f9a2b83-6904-2290-6c4f-526672390beb@intel.com> <20200930111612.GZ2628@hirez.programming.kicks-ass.net> <20200930161036.GY28786@gate.crashing.org> <20200930171346.GC2628@hirez.programming.kicks-ass.net> <CAKwvOdnpU=w4uStcP+UUr9wfoE5U-hW0cMt1bizcX4zQ4=-gOg@mail.gmail.com> <20200930194240.GH2628@hirez.programming.kicks-ass.net>
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On Wed, Sep 30, 2020 at 09:42:40PM +0200, Peter Zijlstra wrote:
> > Looks like yes. You can even check different GCC versions via the
> > dropdown in the top right.
> 
> That only tells me it compiles it, not if that (IMO) weird construct is
> actually guaranteed to work as expected.
> 
> I'd almost dive into the GCC archives to read the back-story to this
> 'feature', it just seems to weird to me. A well, for another day that.

It was documented in 1996 (<https://gcc.gnu.org/g:c1f7febfcb10>), the
feature is older than that though.

In 2004 (in <https://gcc.gnu.org/g:805c33df1366>) the documentation for
this was made more explicit (it has been rewritten since, but it still
says the same thing).


Segher