linux-kernel - Re: [PATCH] shorten the x86_64 boot setup GDT to what the comment says

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Message-Id: <1163084072.31014.275411753@webmail.messagingengine.com>
Date:	Thu, 09 Nov 2006 15:54:32 +0100
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"Steven Rostedt" <rostedt@...dmis.org>,
	"LKML" <linux-kernel@...r.kernel.org>
Cc:	sct@...hat.com, ak@...e.de, herbert@...dor.apana.org.au,
	xen-devel@...ts.xensource.com, heukelum@...lshack.com
Subject: Re: [PATCH] shorten the x86_64 boot setup GDT to what the comment says

> 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@...dmis.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@...tmail.fm

-- 
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