linux-kernel - Re: [PATCH for 2.6.19] [3/9] x86_64: shorten the x86_64 boot setup GDT to what the comment says

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Date:	Wed, 15 Nov 2006 11:00:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <ak@...e.de>
Cc:	Steven Rostedt <rostedt@...dmis.org>, patches@...-64.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH for 2.6.19] [3/9] x86_64: shorten the x86_64 boot setup GDT to what the comment says


> From: Steven Rostedt <rostedt@...dmis.org>
> 
> 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.
> 
> AK: changed to compute true gdt size in assembler, fixed comment
> 
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Andi Kleen <ak@...e.de>

Acked-by: Ingo Molnar <mingo@...e.hu>

note, it seems to me that i386 had this fix years ago already.

	Ingo
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