Why kernel reloads these segment registers, such as ds, es, .etc in startup_32 ?

[Jacky]

And another confusing:

In uncompressed code,

================
arch/x86/boot/compressed/head_32.S:

ENTRY(startup_32)
...
    testb   $(1<<6), BP_loadflags(%esi)
    jnz 1f

    cli
    movl    $__BOOT_DS, %eax
    movl    %eax, %ds
    movl    %eax, %es
   ...
===============

Kernel already set these segment resisters.


At 2013-05-02 16:14:23,Jacky <jackyclivia at 163.com> wrote:

Hi, All,

Why kernel reloads these segment registers in startup_32 again ? Just liking the following:

=======================
ENTRY(startup_32)
    movl pa(stack_start),%ecx

    /* test KEEP_SEGMENTS flag to see if the bootloader is asking
        us to not reload segments */
    testb $(1<<6), BP_loadflags(%esi)
    jnz 2f

/*
 * Set segments to known values.
 */
    lgdt pa(boot_gdt_descr)
    movl $(__BOOT_DS),%eax
    movl %eax,%ds
    ...
======================

Considering bootloader, such as GRUB has already load these segments, so why kernel reloads them again while wasting CPU ?

Thanks,
Jacky





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