Date: Tue, 5 Apr 2016 18:47:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Florian Weimer <fweimer@redhat.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, linux-api <linux-api@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Andrew Hunter <ahh@google.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Andi Kleen <andi@firstfloor.org>, Dave Watson <davejwatson@fb.com>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread
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On Tue, Apr 05, 2016 at 06:02:25PM +0200, Florian Weimer wrote:
> On 04/04/2016 10:48 PM, Mathieu Desnoyers wrote:
> 
> > Moreover, the feature set that the application knows about, glibc
> > knows about, and the kernel knows about are three different things.
> > My intent here is to have glibc stay out of the way as much as possible,
> > since this is really an interface between various applications/libraries
> > and the kernel.
> 
> Surely glibc can allocate the space based on what is advertised as
> needed by the kernel?  Why would it limit itself to what is supported by
> the kernel headers it is compiled against if the actual size can be
> queried from the kernel?

I guess the question is; can we do thread local variable arrays like:

  __thread uint32_t[x]; /* with x being a runtime constant */

Because then we can do:

  __thread struct thread_local_abi tla;

where sizeof(struct thread_local_abi) is a runtime variable.

Without that we cannot have this thread-local-abi structure be part of
the immediately addressable TLS space. That is, we then need a pointer
like:

  __thread struct thread_local_abi *tla;

and every usage will need the extra pointer deref.

Because ideally this structure would be part of the initial (glibc) TCB
with fixed offset etc.