lguest: suppress interrupts for single insn, not range.

The last patch reduced our interrupt-suppression region to one address,
so simplify the code somewhat.

Also, remove the obsolete undefined instruction ranges and the comment
which refers to lguest_guest.S instead of head_32.S.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/lguest/hypercalls.c

@@ -211,10 +211,9 @@ static void initialize(struct lg_cpu *cpu)
 
 	/*
 	 * The Guest tells us where we're not to deliver interrupts by putting
-	 * the range of addresses into "struct lguest_data".
+	 * the instruction address into "struct lguest_data".
 	 */
-	if (get_user(cpu->lg->noirq_start, &cpu->lg->lguest_data->noirq_start)
-	    || get_user(cpu->lg->noirq_end, &cpu->lg->lguest_data->noirq_end))
+	if (get_user(cpu->lg->noirq_iret, &cpu->lg->lguest_data->noirq_iret))
 		kill_guest(cpu, "bad guest page %p", cpu->lg->lguest_data);
 
 	/*

drivers/lguest/interrupts_and_traps.c

@@ -204,8 +204,7 @@ void try_deliver_interrupt(struct lg_cpu *cpu, unsigned int irq, bool more)
 	 * They may be in the middle of an iret, where they asked us never to
 	 * deliver interrupts.
 	 */
-	if (cpu->regs->eip >= cpu->lg->noirq_start &&
-	   (cpu->regs->eip < cpu->lg->noirq_end))
+	if (cpu->regs->eip == cpu->lg->noirq_iret)
 		return;
 
 	/* If they're halted, interrupts restart them. */
@@ -395,8 +394,9 @@ static bool direct_trap(unsigned int num)
  * The Guest has the ability to turn its interrupt gates into trap gates,
  * if it is careful.  The Host will let trap gates can go directly to the
  * Guest, but the Guest needs the interrupts atomically disabled for an
- * interrupt gate.  It can do this by pointing the trap gate at instructions
- * within noirq_start and noirq_end, where it can safely disable interrupts.
+ * interrupt gate.  The Host could provide a mechanism to register more
+ * "no-interrupt" regions, and the Guest could point the trap gate at
+ * instructions within that region, where it can safely disable interrupts.
  */
 
 /*M:006

drivers/lguest/lg.h

@@ -102,7 +102,7 @@ struct lguest {
 
 	struct pgdir pgdirs[4];
 
-	unsigned long noirq_start, noirq_end;
+	unsigned long noirq_iret;
 
 	unsigned int stack_pages;
 	u32 tsc_khz;