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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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