Re: failed 'ljmp' in linear addressing mode

[linux-os \(Dick Johnson\)]

On Wed, 22 Nov 2006, Jun Sun wrote:

>
> I am plowing along as I am learning about the in'n'outs about i386.  I am
> totally stuck on this one.  I would appreciate any help.
>
> As you can see, the function turns off paging mode (of course it
> runs from identically mapped page) and tries to jump to an absolute
> address at 0x10000000.  It appears the machine would reboot when running
> "ljmp" instruction.
>
> Any pointers?
>
> I was not too certain about the %cs, %ds business and did quite
> a few experiments with different values but with no luck.
>
> Thanks in advance.
>
> Jun
>
> --------------------------
>
> /* [JSUN] we will map this page into identity mapping before execution */
>        .align  4096
> ENTRY(do_os_switching)
> 	/* interrupt is disabled! */
>
>        /* the black magic, some copied form relocate_new_kernel, JSUN */
>        /* Set cr0 to a known state:
>         * 31 0 == Paging disabled
>         * 18 0 == Alignment check disabled
>         * 16 0 == Write protect disabled
>         * 3  0 == No task switch
>         * 2  0 == Don't do FP software emulation.
>         * 0  1 == Proctected mode enabled
>         */
>
>        movl    %cr0, %eax
>        andl    $~((1<<31)|(1<<18)|(1<<16)|(1<<3)|(1<<2)), %eax
>        orl     $(1<<0), %eax
>        movl    %eax, %cr0
>
>
>        /* JSUN, 0x11 was the boot up value for cr0.
>        movl    $0x11, %eax
>        movl    %eax, %cr0
>        */
>
>        /* clear cr4 */
>        movl    $0, %eax
>        movl    %eax, %cr4
>
> 	/* why this? */
>        jmp     1f
> 1:
>
>        /* clear cr3, flush TLB */
>        movl    $0, %eax
>        movl    %eax, %cr3
>
>        /*
>        movl    $(__KERNEL_DS),%eax
>        movl    %eax,%ds
>        movl    %eax,%es
>        movl    %eax,%fs
>        movl    %eax,%gs
>        movl    %eax,%ss
>        */
>
>        ljmp    $(__KERNEL_CS), $0x10000000
>

I think it probably resets the instant that you turn off paging. To
turn off paging, you need to copy some code (properly linked) to an
area where there is a 1:1 mapping between virtual and physical addresses.
A safe place is somewhere below 1 megabyte. Then you need to set up a
call descriptor so you can call that code (you can ljump if you never
plan to get back). You then need to clear interrupts on all CPUs (use a 
spin-lock). Once you are executing from the new area, you reset your
segments to the new area. The call descriptor would have already set
CS, as would have the long-jump. At this time you can turn off paging
and flush the TLB. You are now in linear-address protected mode.


Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_


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