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Date:   Mon, 15 Apr 2019 09:37:21 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     peter maydell <peter.maydell@...aro.org>
Cc:     Will Deacon <will.deacon@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        carlos <carlos@...hat.com>,
        richard earnshaw <richard.earnshaw@....com>
Subject: Re: rseq/arm32: choosing rseq code signature

----- On Apr 15, 2019, at 9:30 AM, peter maydell peter.maydell@...aro.org wrote:

> On Mon, 15 Apr 2019 at 14:11, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> ----- On Apr 11, 2019, at 3:55 PM, peter maydell peter.maydell@...aro.org wrote:
>>
>> > On Thu, 11 Apr 2019 at 18:51, Mathieu Desnoyers
>> > <mathieu.desnoyers@...icios.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".

I'm actually very much concerned about environments with big endian
data and little endian code. Which gcc compiler flags do I need to
use to test it ?

I'm concerned about a signature mismatch between what is passed to
the rseq system call ("data-endian signature") and what is generated
in the code ("instruction-endian signature").

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

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