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port-amd64/42645: Instant reboot with big modules (e.g. miniroot.kmod) on amd64

>Number:         42645
>Category:       port-amd64
>Synopsis:       Instant reboot with big modules (e.g. miniroot.kmod) on amd64
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    port-amd64-maintainer
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jan 19 22:40:00 +0000 2010
>Originator:     Vladimir 'phcoder' Serbinenko
>Release:        5.0.1 and -current
sys/arch/amd64/amd64/locore.S seems to map only pages in the range 
0x100000-0x1800000 When total amount of loaded modules is higher than that 
kernel instantly reboots (triple fault). It happes in following chunk of code:
         * 6.
         * Finally, we're in long mode. However, we're still
         * in the identity mapped area (could not jump out
         * of that earlier because it would have been a > 32bit
         * jump). We can do that now, so here we go.
        movabsq $longmode_hi,%rax
        jmp     *%rax
         * We have arrived.
         * There's no need anymore for the identity mapping in low
         * memory, remove it.
        movq    $KERNBASE,%r8

        movq    $(NKL2_KIMG_ENTRIES+1),%rcx
        leaq    (PROC0_PTP2_OFF)(%rsi),%rbx
        addq    %r8, %rbx
1:      movq    $0,(%rbx)
        addq    $8,%rbx
        loop    1b

More exactly on 1:      movq    $0,(%rbx)
It tries to remove some mappings but since page table is after modules if big 
modules were loaded the memory holding pagetable isn't mapped. Triple fault.
Make a miniroot.kmod with 32MiB image. Load it in bootloader, type boot and see 
how you get triple-fault reboot instead.
add necessary entries to initial pagetable. LAzy solution would be just to map 
low 4GiB. Since boot protocol has 32-bit pointers this value can't be exceeded 
and mapping with 2M pages should be quite easy

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