Use common IRET macro for all exits from XenLinux. Currently

uses slow iret path every time (but most returns will do that
anyway, since they'll be to userspace). Fast return (to kernel)
can be implemented inside the macro and respect the 
supervisor_mode_kernel feature flag.

Signed-off-by: Keir Fraser <keir@xensource.com>

linux-2.6-xen-sparse/arch/x86_64/kernel/entry-xen.S

@@ -325,7 +325,7 @@ ENTRY(int_ret_from_sys_call)
 	testb $3,CS-ARGOFFSET(%rsp)
         jnz 1f
         /* Need to set the proper %ss (not NULL) for ring 3 iretq */
-        movl $__KERNEL_DS|3,SS-ARGOFFSET(%rsp)
+        movl $__KERNEL_DS,SS-ARGOFFSET(%rsp)
         jmp retint_restore_args   # retrun from ring3 kernel
 1:              
 	movl $_TIF_ALLWORK_MASK,%edi
@@ -510,13 +510,7 @@ retint_restore_args:
 	jnz restore_all_enable_events	# != 0 => reenable event delivery      
         XEN_PUT_VCPU_INFO(%rsi)
 		
-	RESTORE_ARGS 0,8,0						
-	testb $3,8(%rsp)                # check CS
-	jnz  user_mode
-kernel_mode:
-        orb   $3,1*8(%rsp)
-	iretq
-user_mode:
+	RESTORE_ARGS 0,8,0
 	HYPERVISOR_IRET 0
 	
 	/* edi: workmask, edx: work */
@@ -826,11 +820,6 @@ scrit:	/**** START OF CRITICAL REGION ****/
 	jnz  14f			# process more events if necessary...
 	XEN_PUT_VCPU_INFO(%rsi)
         RESTORE_ARGS 0,8,0
-        testb $3,8(%rsp)                # check CS
-        jnz  crit_user_mode
-        orb   $3,1*8(%rsp)
-        iretq
-crit_user_mode:
         HYPERVISOR_IRET 0
         
 14:	XEN_LOCKED_BLOCK_EVENTS(%rsi)