Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpu area

[Eric W. Biederman]

Mike Travis <travis@xxxxxxx> writes:

> 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.

Weird.  Grub doesn't get involved in the decompression the kernel does it
all itself so we should be able to track where things go bad.

Last I looked the compressed code was formed by essentially.
objcopy vmlinux -O binary vmlinux.bin
gzip vmlinux.bin
And then we take on a magic header to the gzip compressed file.

Are things only bad with the change above?

Eric
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