linux-kernel - Re: [PATCH 1/8] Remove cpu_gdt_table: use boot_gdt

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Date:	Wed, 07 Mar 2007 11:22:15 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, pazke@...pac.ru
Subject: Re: [PATCH 1/8] Remove cpu_gdt_table: use boot_gdt_table until
	migration to per-cpu

On Tue, 2007-03-06 at 14:20 +0100, Ingo Molnar wrote:
> * Rusty Russell <rusty@...tcorp.com.au> wrote:
> 
> > +/* The boot Global Descriptor Table: after boot we allocate a per-cpu copy */
> >  	.align L1_CACHE_BYTES
> >  ENTRY(boot_gdt_table)
> > -	.fill GDT_ENTRY_BOOT_CS,8,0
> > -	.quad 0x00cf9a000000ffff	/* kernel 4GB code at 0x00000000 */
> > -	.quad 0x00cf92000000ffff	/* kernel 4GB data at 0x00000000 */
> > -
> > -/*
> > - * The Global Descriptor Table contains 28 quadwords, per-CPU.
> > - */
> > -	.align L1_CACHE_BYTES
> > -ENTRY(cpu_gdt_table)
> >  	.quad 0x0000000000000000	/* NULL descriptor */
> >  	.quad 0x0000000000000000	/* 0x0b reserved */
> > -	.quad 0x0000000000000000	/* 0x13 reserved */
> > -	.quad 0x0000000000000000	/* 0x1b reserved */
> > +	.quad 0x00cf9a000000ffff	/* boot: 4GB code at 0x00000000 */
> > +	.quad 0x00cf92000000ffff	/* boot: 4GB data at 0x00000000 */
> >  	.quad 0x0000000000000000	/* 0x20 unused */
> >  	.quad 0x0000000000000000	/* 0x28 unused */
> >  	.quad 0x0000000000000000	/* 0x33 TLS entry 1 */
> 
> actually, the reason for the small boot GDT was that some systems 
> wouldnt even boot with a larger GDT. (there was some BIOS interaction, 
> forgot what it was - iirc it was mach-visws and also some other older 
> box)

Thanks for the explanation: I did wonder!  I left the descriptor
truncated to just the first few entries, so I'm *pretty sure* it won't
matter: the layout is identical.

I'd really like to try it and see, though; Andre?
Rusty.


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