x86: xen: 64-bit kernel RPL should be 0

Under Xen 64 bit guests actually run their kernel in ring 3,
however the hypervisor takes care of squashing descriptor the
RPLs transparently (in order to allow them to continue to
differentiate between user and kernel space CS using the RPL).
Therefore the Xen paravirt backend should use RPL==0 instead of
1 (or 3). Using RPL==1 causes generic arch code to take
incorrect code paths because it uses "testl $3, <CS>, je foo"
type tests for a userspace CS and this considers 1==userspace.

This issue was previously masked because get_kernel_rpl() was
omitted when setting CS in kernel_thread(). This was fixed when
kernel_thread() was unified with 32 bit in
f443ff4.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Christian Kujau <lists@nerdbynature.de>
Cc: Jeremy Fitzhardinge <Jeremy.Fitzhardinge@citrix.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <1263377768-19600-2-git-send-email-ian.campbell@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1151,9 +1151,13 @@ asmlinkage void __init xen_start_kernel(void)
 
 	/* keep using Xen gdt for now; no urgent need to change it */
 
+#ifdef CONFIG_X86_32
 	pv_info.kernel_rpl = 1;
 	if (xen_feature(XENFEAT_supervisor_mode_kernel))
 		pv_info.kernel_rpl = 0;
+#else
+	pv_info.kernel_rpl = 0;
+#endif
 
 	/* set the limit of our address space */
 	xen_reserve_top();