From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: peter maydell <peter.maydell@linaro.org>
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>
Subject: Re: rseq/arm32: choosing rseq code signature
Date: Mon, 15 Apr 2019 09:37:21 -0400 (EDT) [thread overview]
Message-ID: <71495082.295.1555335441325.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CAFEAcA8YP5vvZpa_5dvv00koupsry9_3x2EB0cjvE6vtoi8sog@mail.gmail.com>
----- On Apr 15, 2019, at 9:30 AM, peter maydell peter.maydell@linaro.org wrote:
> 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".
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
next prev parent reply other threads:[~2019-04-15 13:37 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-09 19:32 rseq/arm32: choosing rseq code signature Mathieu Desnoyers
2019-04-10 20:29 ` Mathieu Desnoyers
2019-04-11 16:42 ` Will Deacon
2019-04-11 17:51 ` Mathieu Desnoyers
2019-04-11 19:55 ` Peter Maydell
2019-04-15 13:11 ` Mathieu Desnoyers
2019-04-15 13:30 ` Peter Maydell
2019-04-15 13:37 ` Mathieu Desnoyers [this message]
2019-04-16 13:39 ` Mathieu Desnoyers
2019-04-17 10:37 ` Richard Earnshaw (lists)
2019-04-17 14:43 ` Mathieu Desnoyers
2019-04-17 15:30 ` Mathieu Desnoyers
2019-04-18 16:18 ` Richard Earnshaw (lists)
2019-04-11 12:24 ` Florian Weimer
2019-04-15 13:22 ` Mathieu Desnoyers
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