x86/vpic: force int output to low when in init mode

When the PIC is on the init sequence prevent interrupt delivery. The
state of the registers is in the process of being set during the init
phase, so it makes sense to prevent any int line changes during that
process.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/vpic.c

@@ -101,11 +101,14 @@ static void vpic_update_int_output(struct hvm_hw_vpic *vpic)
     irq = vpic_get_highest_priority_irq(vpic);
     TRACE_3D(TRC_HVM_EMUL_PIC_INT_OUTPUT, vpic->int_output, vpic->is_master,
              irq);
-    if ( vpic->int_output == (irq >= 0) )
+    if ( vpic->int_output == (!vpic->init_state && irq >= 0) )
         return;
 
-    /* INT line transition L->H or H->L. */
-    vpic->int_output = !vpic->int_output;
+    /*
+     * INT line transition L->H or H->L.
+     * Force line status to L when in init mode.
+     */
+    vpic->int_output = !vpic->init_state && !vpic->int_output;
 
     if ( vpic->int_output )
     {