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    SubjectRe: [PATCH] shorten the x86_64 boot setup GDT to what the comment says
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    > Andi,
    >
    > Stephen Tweedie, Herbert Xu, and myself have been struggling with a very
    > nasty bug in Xen. But it also pointed out a small bug in the x86_64
    > kernel boot setup.
    >
    > The GDT limit being setup by the initial bzImage code when entering into
    > protected mode is way too big. The comment by the code states that the
    > size of the GDT is 2048, but the actual size being set up is much bigger
    > (32768). This happens simply because of one extra '0'.
    >
    > Instead of setting up a 0x800 size, 0x8000 is set up. On bare metal this
    > is fine because the CPU wont load any segments unless they are
    > explicitly used. But unfortunately, this breaks Xen on vmx FV, since it
    > (for now) blindly loads all the segments into the VMCS if they are less
    > than the gdt limit. Since the real mode segments are around 0x3000, we
    > are
    > getting junk into the VMCS and that later causes an exception.
    >
    > Stephen Tweedie has written up a patch to fix the Xen side and will be
    > submitting that to those folks. But that doesn't excuse the GDT limit
    > being a magnitude too big.
    >
    > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
    >
    > Index: linux-2.6.19-rc2/arch/x86_64/boot/setup.S
    > ===================================================================
    > --- linux-2.6.19-rc2.orig/arch/x86_64/boot/setup.S 2006-11-08
    > 21:37:58.000000000 -0500
    > +++ linux-2.6.19-rc2/arch/x86_64/boot/setup.S 2006-11-08
    > 21:38:16.000000000 -0500
    > @@ -840,7 +840,7 @@ idt_48:
    > .word 0 # idt limit = 0
    > .word 0, 0 # idt base = 0L
    > gdt_48:
    > - .word 0x8000 # gdt limit=2048,
    > + .word 0x800 # gdt limit=2048,
    > # 256 GDT entries
    >
    > .word 0, 0 # gdt base (filled in later)

    The limit should be the offset of the last byte of the gdt table. So
    I think what was meant was really 0x7ff. Comparing this code with the
    i386-version, why does x86_64 need 256 entries here, while i386 is happy
    with just the code-segment and data-segment descriptors?

    Greetings,
    Alexander
    --
    Alexander van Heukelum
    heukelum@fastmail.fm

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