Date: Thu, 7 Apr 2016 14:25:28 +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 Thu, Apr 07, 2016 at 02:03:53PM +0200, Florian Weimer wrote:
> > struct tlabi {
> > 	union {
> > 		__u8[64] __foo;
> > 		struct {
> > 			/* fields go here */
> > 		};
> > 	};
> > } __aligned__(64);
> 
> That's not really “fixed size” as far as an ABI is concerned, due to the
> possibility of future extensions.

sizeof(struct tlabi) is always the same, right? How is that not fixed?

> > People objected against the fixed size scheme, but it being possible to
> > get a fixed TCB offset and reduce indirections is a big win IMO.
> 
> It's a difficult trade-off.  It's not an indirection as such, it's avoid
> loading the dynamic TLS offset.

What we _want_ is being able to use %[gf]s:offset and have it work (I
forever forget which segment register userspace TLS uses).

> Let me repeat that the ELF TLS GNU ABI has very limited support for
> static offsets at present, and it is difficult to make them available
> more widely without code generation at run time (in the form of text
> relocations, but still).

Do you have a pointer to something I can read? Because I'm clearly not
understanding the full issue here.