x86_64: Early segment setup for VT

VT is very picky about when it can enter execution.
Get all segments setup and get LDT and TR into valid state to allow
VT execution under VMware and KVM (untested).

This makes the boot decompression run under VT, which makes it several
orders of magnitude faster on 64-bit Intel hardware.

Before, I was seeing times up to a minute or more to decompress a 1.3MB kernel
on a very fast box.
Signed-off-by: NZachary Amsden <zach@vmware.com>
Signed-off-by: NAndi Kleen <ak@suse.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/x86_64/boot/compressed/head.S

@@ -195,6 +195,11 @@ ENTRY(startup_64)
 	movl	%eax, %ds
 	movl	%eax, %es
 	movl	%eax, %ss
+	movl	%eax, %fs
+	movl	%eax, %gs
+	lldt	%ax
+	movl    $0x20, %eax
+	ltr	%ax
 
 	/* Compute the decompressed kernel start address.  It is where
 	 * we were loaded at aligned to a 2M boundary. %rbp contains the
@@ -295,6 +300,8 @@ gdt:
 	.quad	0x0000000000000000	/* NULL descriptor */
 	.quad	0x00af9a000000ffff	/* __KERNEL_CS */
 	.quad	0x00cf92000000ffff	/* __KERNEL_DS */
+	.quad	0x0080890000000000	/* TS descriptor */
+	.quad   0x0000000000000000	/* TS continued */
 gdt_end:
 	.bss
 /* Stack for uncompression */