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

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Date:	Tue, 13 Mar 2007 17:23:52 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	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, 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.

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

%gs is best set to the offset of the local cpu's area from the "master"
per-cpu area, not set to the local cpu area's address.  In the former
case, booting with %gs at offset 0 works naturally, in the latter case,
hoops need to be jumped through to make it work.  See how much nicer the
x86 code is post pda->percpu conversion.

So, even if we leave the PDA and place the per-cpu area immediately
after it, we still can't use "%gs:var" to access a per-cpu variable: we
need to do a subtract, so why bother using the segment reg?

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
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...

In summary, containing the PDA infection to x86-64 is possible, but
curing that patient is non-trivial 8)

Rusty.

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