xen: use iret directly where possible

[ Expand, correct and clarify comments; minor code change. ]

Most of the time we can simply use the iret instruction to exit the kernel, rather than having to use the iret hypercall - the only exception is if we're returning into vm86 mode, or from delivering an NMI (which we don't support yet).

When running native, iret has the behaviour of testing for a pending interrupt atomically with re-enabling interrupts. Unfortunately there's no way to do this with Xen, so there's a window in which we could get a recursive exception after enabling events but before actually returning to userspace.

This causes a problem: if the nested interrupt causes one of the task's TIF_WORK_MASK flags to be set, they will not be checked again before returning to userspace. This means that pending work may be left pending indefinitely, until the process enters and leaves the kernel again. The net effect is that a pending signal or reschedule event could be delayed for an unbounded amount of time.

To deal with this, the xen event upcall handler checks to see if the EIP is within the critical section of the iret code, after events are (potentially) enabled up to the iret itself. If its within this range, it calls the iret critical section fixup, which adjusts the stack to deal with any unrestored registers, and then shifts the stack frame up to replace the previous invocation.

---
 arch/i386/kernel/asm-offsets.c |    1 
 arch/i386/kernel/entry.S       |   16 +++
 arch/i386/xen/enlighten.c      |    1 
 arch/i386/xen/xen-asm.S        |  185 +++++++++++++++++++++++++++++++++++++++-
 arch/i386/xen/xen-ops.h        |    1 
 5 files changed, 199 insertions(+), 5 deletions(-)

===================================================================

@@ -65,6 +65,7 @@ void foo(void)
 	OFFSET(TI_addr_limit, thread_info, addr_limit);
 	OFFSET(TI_restart_block, thread_info, restart_block);
 	OFFSET(TI_sysenter_return, thread_info, sysenter_return);
+	OFFSET(TI_cpu, thread_info, cpu);
 	BLANK();
 
 	OFFSET(GDS_size, Xgt_desc_struct, size);

===================================================================

@@ -1030,7 +1030,21 @@ ENTRY(xen_hypervisor_callback)
 	CFI_ADJUST_CFA_OFFSET 4
 	SAVE_ALL
 	TRACE_IRQS_OFF
-	mov %esp, %eax
+
+	/* Check to see if we got the event in the critical
+	   region in xen_iret_direct, after we've reenabled
+	   events and checked for pending events.  This simulates
+	   iret instruction's behaviour where it delivers a
+	   pending interrupt when enabling interrupts. */
+	movl PT_EIP(%esp),%eax
+	cmpl $xen_iret_start_crit,%eax
+	jb   1f
+	cmpl $xen_iret_end_crit,%eax
+	jae  1f
+
+	call xen_iret_crit_fixup
+
+1:	mov %esp, %eax
 	call xen_evtchn_do_upcall
 	jmp  ret_from_intr
 	CFI_ENDPROC

===================================================================

@@ -827,6 +827,7 @@ void __init xen_setup_vcpu_info_placemen
 		paravirt_ops.irq_disable = xen_irq_disable_direct;
 		paravirt_ops.irq_enable = xen_irq_enable_direct;
 		paravirt_ops.read_cr2 = xen_read_cr2_direct;
+		paravirt_ops.iret = xen_iret_direct;
 	}
 }
 

===================================================================

===================================================================

@@ -67,4 +67,5 @@ DECL_ASM(unsigned long, xen_save_fl_dire
 DECL_ASM(unsigned long, xen_save_fl_direct, void);
 DECL_ASM(void, xen_restore_fl_direct, unsigned long);
 
+void xen_iret_direct(void);
 #endif /* XEN_OPS_H */