linux-kernel - Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU number of running thread

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Message-ID: <20160407122528.GS3430@twins.programming.kicks-ass.net>
Date:	Thu, 7 Apr 2016 14:25:28 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Florian Weimer <fweimer@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	linux-api <linux-api@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Andrew Hunter <ahh@...gle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Watson <davejwatson@...com>, Chris Lameter <cl@...ux.com>,
	Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH v6 1/5] Thread-local ABI system call: cache CPU
 number of running thread

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.