From: Rik van Riel <riel@redhat.com>
Subject: [PATCH] Fix FS/GS registers for VT bootup
Date: Thu, 14 Jun 2007 14:24:52 -0400
Bugzilla: 224671
Message-Id: <467187F4.3090306@redhat.com>
Changelog: [xen] x86_64: Fix FS/GS registers for VT bootup


The attached upstream patch reorders some of the early boot code
and makes sure FS and GS are cleared.  This speeds up booting of
the kernel under VT, since otherwise VT could trap on every
instruction executed because FS and GS have invalid (real mode)
values in them.

Fixes bug 224671

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.

Initialize FS and GS to __KERNEL_DS as well.  The actual value of them is not
important, but it is important to reload them in protected mode.  At this time,
they still retain the real mode values from initial boot.  VT disallows
execution of code under such conditions, which means hardware virtualization
can not be used to boot the kernel on Intel platforms, making the boot time
painfully slow.

This requires moving the GS load before the load of GS_BASE, so just move
all the segments loads there to keep them together in the code.

Signed-off-by: Zachary Amsden <zach@vmware.com>

Index: linux-2.6.19/arch/x86_64/kernel/head.S
===================================================================
--- linux-2.6.19.orig/arch/x86_64/kernel/head.S	2006-11-29 13:57:37.000000000 -0800
+++ linux-2.6.19/arch/x86_64/kernel/head.S	2007-01-11 16:57:24.000000000 -0800
@@ -163,6 +163,20 @@ startup_64:
 	 */
 	lgdt	cpu_gdt_descr
 
+	/* set up data segments. actually 0 would do too */
+	movl $__KERNEL_DS,%eax
+	movl %eax,%ds	
+	movl %eax,%ss
+	movl %eax,%es
+
+	/*
+	 * We don't really need to load %fs or %gs, but load them anyway
+	 * to kill any stale realmode selectors.  This allows execution
+	 * under VT hardware.
+	 */
+	movl %eax,%fs
+	movl %eax,%gs
+			
 	/* 
 	 * Setup up a dummy PDA. this is just for some early bootup code
 	 * that does in_interrupt() 
@@ -173,12 +187,6 @@ startup_64:
 	shrq	$32,%rdx
 	wrmsr	
 
-	/* set up data segments. actually 0 would do too */
-	movl $__KERNEL_DS,%eax
-	movl %eax,%ds	
-	movl %eax,%ss
-	movl %eax,%es
-			
 	/* esi is pointer to real mode structure with interesting info.
 	   pass it to C */
 	movl	%esi, %edi