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

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Date:	Mon, 30 Jun 2008 10:57:31 -0700
From:	Mike Travis <travis@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Christoph Lameter <clameter@....com>,
	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

H. Peter Anvin wrote:
> Mike Travis wrote:
>>
>> FYI, I did try this out and it caused the bootloader to scramble the
>> loaded data.  The first corruption I found was the .x86cpuvendor.init
>> section contained all zeroes.
>>
> 
> Explain what you mean with "the bootloader" in this context.
> 
>     -hpa


After the code was loaded (the compressed code, it seems that my GRUB
doesn't support uncompressed loading), the above section contained
zeroes.  I snapped it fairly early, around secondary_startup_64, and
then printed it in x86_64_start_kernel.

The object file had the correct data (as displayed by objdump) so I'm
assuming that the bootloading process didn't load the section correctly.

Below was the linker script I used:

--- linux-2.6.tip.orig/include/asm-generic/vmlinux.lds.h
+++ linux-2.6.tip/include/asm-generic/vmlinux.lds.h
@@ -373,9 +373,13 @@

 #ifdef CONFIG_HAVE_ZERO_BASED_PER_CPU
 #define PERCPU(align)                                                  \
-       . = ALIGN(align);                                               \
+       .data.percpu.abs = .;                                           \
        percpu : { } :percpu                                            \
-       __per_cpu_load = .;                                             \
+       .data.percpu.rel : AT(.data.percpu.abs - LOAD_OFFSET) {         \
+               BYTE(0)                                                 \
+               . = ALIGN(align);                                       \
+               __per_cpu_load = .;                                     \
+       }                                                               \
        .data.percpu 0 : AT(__per_cpu_load - LOAD_OFFSET) {             \
                *(.data.percpu.first)                                   \
                *(.data.percpu.shared_aligned)                          \
@@ -383,8 +387,8 @@
                *(.data.percpu.page_aligned)                            \
                ____per_cpu_size = .;                                   \
        }                                                               \
-       . = __per_cpu_load + ____per_cpu_size;                          \
-       data : { } :data
+       . = __per_cpu_load + ____per_cpu_size;
+
 #else
 #define PERCPU(align)                                                  \
        . = ALIGN(align);                                               \

It showed all the correct address in the map and __per_cpu_load was a
relative symbol (which was the objective.)

Btw, our simulator, which only loads uncompressed code, had the data correct,
so it *may* only be a result of the code being compressed.

Thanks,
Mike
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