linux-kernel - Re: [PATCH 0/8] x86 boot, pda and gdt cleanups

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Date:	Thu, 15 Mar 2007 10:55:41 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>,
	Jeremy Fitzhardinge <jeremy@...source.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [PATCH 0/8] x86 boot, pda and gdt cleanups

On Tue, 2007-03-13 at 13:48 -0700, Jeremy Fitzhardinge wrote:
>     * init_gdt should always use write_gdt_entry when touching the gdt;
>       if it doesn't and it ends up touching an already-installed gdt
>       under Xen, it will get a write fault.  This happens because
>       init_gdt ends up getting called twice in SMP (see below).

Hmm, this invalidated my assumption that write_gdt_entry is always a
write to this cpu's active gdt.  Better fix is not to call it twice
anyway...

>     * init_gdt should always be called before bringing up the cpu,
>       rather than by the cpu itself (and therefore, cpu_init() shouldn't
>       call it).  Obviously the the boot cpu is an exception.

Makes sense.

>     * secondary_cpu_init stops being necessary.

Indeed.

>     * On SMP, init_gdt can get called twice: first time in
>       smp_prepare_boot_cpu, and a second time in  trap_init.  On UP,
>       trap_init is the only caller.

Getting rid of the call in smp_prepare_boot_cpu currently works, but
it's fragile:  __get_cpu_var(x) && per_cpu(x, smp_processor_id()) will
differ, and changes made to __get_cpu_var(x) will vanish...

Fortunately, UP doesn't have to call init_gdt at all, so I think it's
better to place it in smp_prepare_boot_cpu only and then clean up the UP
code.  I'll try now...

Rusty.

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