linux-kernel - Re: [crash, bisected] Re: [PATCH 3/4] x86

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Message-ID: <485BF73C.7010802@sgi.com>
Date:	Fri, 20 Jun 2008 11:30:20 -0700
From:	Mike Travis <travis@....com>
To:	Christoph Lameter <clameter@....com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu
 area

Christoph Lameter wrote:
> On Fri, 20 Jun 2008, Jeremy Fitzhardinge wrote:
> 
>> So it seems the problem is that the pre-initialized gdt_page is being lost and
>> replaced with zero.  Linker script bug?
> 
> Is the pre initialized gdt page in the per cpu area? Does not look like 
> it. The loader setup for the percpu section changes with zero basing. 
> Maybe that has bad side effects?

Yes, it is... The fixup logic is this:

0000000000004000 D per_cpu__gdt_page
ffffffff81911000 A __per_cpu_load

arch/x86/kernel/cpu/common.c:

DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
        [GDT_ENTRY_KERNEL_CS] = { { { 0x0000ffff, 0x00cf9a00 } } },
...


arch/x86/kernel/head_64.S:

startup_64:
...
       /*
         * Fix up per_cpu__gdt_page offset when basing percpu
         * variables at zero.  This is only needed for the boot cpu.
         */
        addq    $__per_cpu_load, early_gdt_descr_base(%rip)

ENTRY(secondary_startup_64)
...
        /*
         * We must switch to a new descriptor in kernel space for the GDT
         * because soon the kernel won't have access anymore to the userspace
         * addresses where we're currently running on. We have to do that here
         * because in 32bit we couldn't load a 64bit linear address.
         */
        lgdt    early_gdt_descr(%rip)
...
        .globl early_gdt_descr
early_gdt_descr:
        .word   GDT_ENTRIES*8-1
early_gdt_descr_base:
        .quad   per_cpu__gdt_page

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