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Date: Thu, 7 Apr 2016 13:19:38 +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 12:39:21PM +0200, Florian Weimer wrote: > On 04/07/2016 12:31 PM, Peter Zijlstra wrote: > > On Thu, Apr 07, 2016 at 11:01:25AM +0200, Florian Weimer wrote: > >>> Because ideally this structure would be part of the initial (glibc) TCB > >>> with fixed offset etc. > >> > >> This is not possible because we have layering violations and code > >> assumes it knows the precise of the glibc TCB. I think Address > >> Sanitizer is in this category. This means we cannot adjust the TCB size > >> based on the kernel headers used to compile glibc, and there will have > >> to be some indirection. > > > > So with the proposed fixed sized object it would work, right? > > I didn't see a proposal for a fixed size buffer, in the sense that the > size of struct sockaddr_in is fixed. This thing proposed a single 64byte structure (with the possibility of eventually adding more 64byte structures). Basically: struct tlabi { union { __u8[64] __foo; struct { /* fields go here */ }; }; } __aligned__(64); 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.
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