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SubjectRe: [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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