Messages in this thread | | | From | "Alexander van Heukelum" <> | Subject | Re: [PATCH] shorten the x86_64 boot setup GDT to what the comment says | Date | Thu, 09 Nov 2006 15:54:32 +0100 |
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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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