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Re: TLS register for NPTL

To: Thiemo Seufer <ths@networkno.de>
Subject: Re: TLS register for NPTL
From: Ralf Baechle <ralf@linux-mips.org>
Date: Mon, 20 Aug 2007 15:54:49 +0100
Cc: Carl van Schaik <carl@ok-labs.com>, linux-mips@linux-mips.org
In-reply-to: <20070820080627.GF4479@networkno.de>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <46C93BB5.9050809@ok-labs.com> <20070820080627.GF4479@networkno.de>
Sender: linux-mips-bounce@linux-mips.org
User-agent: Mutt/1.5.14 (2007-02-12)
On Mon, Aug 20, 2007 at 09:06:27AM +0100, Thiemo Seufer wrote:

> > It seems the rdhwr emulation is used/proposed for accessing the thread
> > word in NPTL.
> > I've been reading some of the posts from 2005 about this choice of this
> > and what I have missed is anyone talking about using the "k0" register
> > for TLS. It seems logical that the kernel could always restore k0 on
> > returning to user-land and having k1 only for the last part of returning
> > to user is sufficient. Any reason why this was not looked at?
> 
> The TLB handlers need k0/k1 as well and have no good place to save/restore
> a register.

It can be done but would require several extra instructions in the most
performance sensitive parts of the OS.

Aside, latest MIPS processors support a hardware implementation of rdhwr $29,
so there is no more emulation overhead for this instruction at full binary
compatibility.

  Ralf

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