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Date:   Tue, 29 Jan 2019 21:56:18 +0000
From:   Joseph Myers <joseph@codesourcery.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC:     carlos <carlos@redhat.com>, Florian Weimer <fweimer@redhat.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Rich Felker <dalias@libc.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at C
 startup and thread creation (v6)
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On Tue, 29 Jan 2019, Mathieu Desnoyers wrote:

> I recalled that aarch64 defines RSEQ_SIG to a different value which maps to
> a valid trap instruction. So I plan to move the RSEQ_SIG define to per-arch
> headers like this:
> 
>  sysdeps/unix/sysv/linux/aarch64/bits/rseq.h                  |   24 ++
>  sysdeps/unix/sysv/linux/arm/bits/rseq.h                      |   24 ++
>  sysdeps/unix/sysv/linux/bits/rseq.h                          |   23 ++
>  sysdeps/unix/sysv/linux/mips/bits/rseq.h                     |   24 ++
>  sysdeps/unix/sysv/linux/powerpc/bits/rseq.h                  |   24 ++
>  sysdeps/unix/sysv/linux/s390/bits/rseq.h                     |   24 ++
>  sysdeps/unix/sysv/linux/x86/bits/rseq.h                      |   24 ++
> 
> where "bits/rseq.h" contains a #error:
> 
> # error "Architecture does not define RSEQ_SIG.
> 
> sys/rseq.h will now include <bits/rseq.h>.

We're trying to reduce the number of cases where most or all new glibc 
architecture ports need to provide a bits/ header, by making the generic 
headers handle the common case.  So a generic header with a #error, and 
lots of architecture-specific headers mostly with the same value for 
RSEQ_SIG, seems unfortunate.  I'd hope the generic header could use a 
generic value, with architecture-specific variants only for architectures 
with some reason for a different value.

-- 
Joseph S. Myers
joseph@codesourcery.com