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From:   Peter Maydell <peter.maydell@linaro.org>
Date:   Mon, 15 Apr 2019 14:30:09 +0100
Message-ID: <CAFEAcA8YP5vvZpa_5dvv00koupsry9_3x2EB0cjvE6vtoi8sog@mail.gmail.com>
Subject: Re: rseq/arm32: choosing rseq code signature
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Will Deacon <will.deacon@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        carlos <carlos@redhat.com>,
        richard earnshaw <richard.earnshaw@arm.com>
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On Mon, 15 Apr 2019 at 14:11, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> ----- On Apr 11, 2019, at 3:55 PM, peter maydell peter.maydell@linaro.org wrote:
>
> > On Thu, 11 Apr 2019 at 18:51, Mathieu Desnoyers
> > <mathieu.desnoyers@efficios.com> wrote:
> >>  * This translates to the following instruction pattern in the T16 instruction
> >>  * set:
> >>  *
> >>  * little endian:
> >>  * def3        udf    #243      ; 0xf3
> >>  * e7f5        b.n    <7f5>
> >>  *
> >>  * big endian:
> >>  * e7f5        b.n    <7f5>
> >>  * def3        udf    #243      ; 0xf3
> >
> > Do we really care about big-endian instruction-ordering for Thumb?
> > It requires (AIUI) either an ARMv7R CPU which implements and sets
> > SCTLR.IE to 1, or a v6-or-earlier CPU using BE32, and it's going to
> > be even rarer than normal BE8 big-endian...
>
> I don't think we care enough about it to look for a trick to
> turn the branch into something else (which would not branch away from the
> udf instruction), but considering this signature will be ABI, it's good to
> be thorough documentation-wise and cover all existing cases.

I think if you want to document it it would be helpful to
readers to make it clear that this is the ultra-rare
big-endian-instruction-order "big endian Thumb", not the only
moderately-rare little-endian-instructions-big-endian-data
"big endian Thumb".

thanks
-- PMM