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Date:   Thu, 20 Sep 2018 20:20:28 +0000
From:   Joseph Myers <joseph@codesourcery.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC:     carlos <carlos@redhat.com>, Florian Weimer <fweimer@redhat.com>,
        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>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init
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On Thu, 20 Sep 2018, Mathieu Desnoyers wrote:

> Are you saying glibc has an explicit check for the kernel version visible
> from /proc before using specific features ? If so, how can this work with
> the variety of feature backports we find in the distribution kernels out
> there ?

See sysdeps/unix/sysv/linux/dl-sysdep.c and 
sysdeps/unix/sysv/linux/dl-osinfo.h.  As I said, Carlos has proposed 
removing that check.

> For too-old headers at compile time, one possibility is that we don't event
> expose the __rseq_abi TLS symbol. OTOH, if we need to keep exposing it anyway
> for ABI consistency purposes, then we'd leave its cpu_id field at the initial
> value (-1). But that would require that we copy linux/rseq.h into the glibc
> source tree.

The ABI needs to be independent of the kernel headers used.  I don't think 
you need to copy linux/rseq.h; all you should need is to e.g. define an 
array of suitable size and alignment with the relevant member initialized 
and a suitable explanatory comment.

-- 
Joseph S. Myers
joseph@codesourcery.com