otherwise Xen is _completely_ unusable with 5 or more VCPUs.
( when !CONFIG_HAVE_SPARSE_IRQ)

based on Alex's patch

also add +1 offset after redir_entries

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Alex Nixon <alex.nixon@citrix.com>

Index: linux-2.6/arch/x86/kernel/io_apic.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/io_apic.c
+++ linux-2.6/arch/x86/kernel/io_apic.c
@@ -3625,16 +3625,21 @@ int __init probe_nr_irqs(void)
 {
 	int idx;
 	int nr = 0;
+#ifndef CONFIG_XEN
+	int nr_min = 32;
+#else
+	int nr_min = NR_IRQS;
+#endif
 
 	for (idx = 0; idx < nr_ioapics; idx++)
-		nr += io_apic_get_redir_entries(idx);
+		nr += io_apic_get_redir_entries(idx) + 1;
 
 	/* double it for hotplug and msi and nmi */
 	nr <<= 1;
 
 	/* something wrong ? */
-	if (nr < 32)
-		nr = 32;
+	if (nr < nr_min)
+		nr = nr_min;
 
 	return nr;
 }
--
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