xen: fold xen_sysexit into xen_iret

xen_sysexit and xen_iret were doing essentially the same thing.  Rather
than having a separate implementation for xen_sysexit, we can just strip
the stack back to an iret frame and jump into xen_iret.  This removes
a lot of code and complexity - specifically, another critical region.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/entry_32.S

@@ -1044,15 +1044,8 @@ ENTRY(xen_hypervisor_callback)
 
 	jmp  xen_iret_crit_fixup
 
-1:	cmpl $xen_sysexit_start_crit,%eax
-	jb   2f
-	cmpl $xen_sysexit_end_crit,%eax
-	jae  2f
-
-	jmp xen_sysexit_crit_fixup
-
 ENTRY(xen_do_upcall)
-2:	mov %esp, %eax
+1:	mov %esp, %eax
 	call xen_evtchn_do_upcall
 	jmp  ret_from_intr
 	CFI_ENDPROC

arch/x86/xen/xen-asm.S

@@ -107,6 +107,20 @@ ENDPATCH(xen_restore_fl_direct)
 	ENDPROC(xen_restore_fl_direct)
 	RELOC(xen_restore_fl_direct, 2b+1)
 
+/*
+	We can't use sysexit directly, because we're not running in ring0.
+	But we can easily fake it up using iret.  Assuming xen_sysexit
+	is jumped to with a standard stack frame, we can just strip it
+	back to a standard iret frame and use iret.
+ */
+ENTRY(xen_sysexit)
+	movl PT_EAX(%esp), %eax			/* Shouldn't be necessary? */
+	orl $X86_EFLAGS_IF, PT_EFLAGS(%esp)
+	lea PT_EIP(%esp), %esp
+
+	jmp xen_iret
+ENDPROC(xen_sysexit)
+
 /*
 	This is run where a normal iret would be run, with the same stack setup:
 	      8: eflags
@@ -276,62 +290,6 @@ ENTRY(xen_iret_crit_fixup)
 2:	jmp xen_do_upcall
 
 
-ENTRY(xen_sysexit)
-	/* Store vcpu_info pointer for easy access.  Do it this
-	   way to avoid having to reload %fs */
-#ifdef CONFIG_SMP
-	GET_THREAD_INFO(%eax)
-	movl TI_cpu(%eax),%eax
-	movl __per_cpu_offset(,%eax,4),%eax
-	mov per_cpu__xen_vcpu(%eax),%eax
-#else
-	movl per_cpu__xen_vcpu, %eax
-#endif
-
-	/* We can't actually use sysexit in a pv guest,
-	   so fake it up with iret */
-	pushl $__USER_DS		/* user stack segment */
-	pushl %ecx			/* user esp */
-	pushl PT_EFLAGS+2*4(%esp)	/* user eflags */
-	pushl $__USER_CS		/* user code segment */
-	pushl %edx			/* user eip */
-
-xen_sysexit_start_crit:
-	/* Unmask events... */
-	movb $0, XEN_vcpu_info_mask(%eax)
-	/* ...and test for pending.
-	   There's a preempt window here, but it doesn't
-	   matter because we're within the critical section. */
-	testb $0xff, XEN_vcpu_info_pending(%eax)
-
-	/* If there's something pending, mask events again so we
-	   can directly inject it back into the kernel. */
-	jnz   1f
-
-	movl PT_EAX+5*4(%esp),%eax
-2:	iret
-1:	movb $1, XEN_vcpu_info_mask(%eax)
-xen_sysexit_end_crit:
-	addl $5*4, %esp		/* remove iret frame */
-	/* no need to re-save regs, but need to restore kernel %fs */
-	mov $__KERNEL_PERCPU, %eax
-	mov %eax, %fs
-	jmp xen_do_upcall
-.section __ex_table,"a"
-	.align 4
-	.long 2b,iret_exc
-.previous
-
-	.globl xen_sysexit_start_crit, xen_sysexit_end_crit
-/*
-	sysexit fixup is easy, since the old frame is still sitting there
-	on the stack.  We just need to remove the new recursive
-	interrupt and return.
- */
-ENTRY(xen_sysexit_crit_fixup)
-	addl $PT_OLDESP+5*4, %esp		/* remove frame+iret */
-	jmp xen_do_upcall
-
 /*
 	Force an event check by making a hypercall,
 	but preserve regs before making the call.