linux-kernel - Re: _proxy_pda still makes linking modules fail

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Date:	Tue, 13 Mar 2007 16:57:47 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcin 'Qrczak' Kowalczyk <qrczak@....org.pl>,
	linux-kernel@...r.kernel.org
Subject: Re: _proxy_pda still makes linking modules fail

On Tue, Mar 13, 2007 at 05:23:52PM +1100, Rusty Russell wrote:
> On Tue, 2007-03-13 at 08:59 +1100, Rusty Russell wrote:
> > On Mon, 2007-03-12 at 10:48 +0100, Andi Kleen wrote:
> > > > Rusty's pda->per_cpu patch will deal with this once and for all; have
> > > 
> > > Not on x86-64.
> > 
> > Indeed.  Perhaps it's time I join the modern world and compile a 64-bit
> > kernel...
> > 
> > Will prepare patches,
> 
> No, I don't think I will.  The PDA concept has gone too far in x86-64 to
> be undone.  In particular, it's been put in GCC 4.1 for
> CONFIG_CC_STACKPROTECTOR, which assumes %gs:40 will give the stack
> canary.

Yes that was always ugly, but I don't know a better way.

> For the record: the PDA should never have existed, that's what percpu
> vars were supposed to be for.  Something went wrong here 8(

PDA predates per cpu.

> The ideal solution has always been to use __thread, but no architecture
> has yet managed it (I tried for i386, and it quickly caused unbearable

I tried it too, but __thread is hopeless for kernel code

> pain).  On x86-64 that uses "%fs" on x86-64, not "%gs" as the kernel
> does, but I might try that if I feel particularly masochistic soon...

Then swapgs wouldn't work anymore (there is no swapfs)

-Andi
> 
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