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    SubjectRe: BOOT_CS
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    Date
    hpa@zytor.com (H. Peter Anvin) writes:

    > Anyone happen to know of any legitimate reason not to reload %cs in
    > head.S?

    Other than the fact it is strongly rude and error prone to depend on
    the contents of a global descriptor table you did not setup?

    If we did the lgdt boot_gdt before hand I don't see any problems
    though.

    But at the point we could as easily do lgdt cpu_gdt_descr, and use
    __KERNEL_CS which is better anyway.

    > I think the following would be a lot cleaner, as well as a
    > lot safer (the jump and indirect branch aren't guaranteed to have the
    > proper effects, although technically neither should be required due to
    > the %cr0 write):
    >
    > @@ -117,10 +147,7 @@
    > movl %cr0,%eax
    > orl $0x80000000,%eax
    > movl %eax,%cr0 /* ..and set paging (PG) bit */
    > - jmp 1f /* flush the prefetch-queue */
    > -1:
    > - movl $1f,%eax
    > - jmp *%eax /* make sure eip is relocated */
    > + ljmp $__BOOT_CS,$1f /* Clear prefetch and normalize %eip
    > */
    > 1:
    > /* Set up the stack pointer */
    > lss stack_start,%esp
    >
    >
    > I've been doing some cleanups in head.S after making the early page
    > tables dynamic.

    That is almost nice. Care to export where the bottom of the page
    tables or even better where the bottom of the kernel is for those
    folks who want to place their ramdisk as low in memory as possible?

    Eric
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