xen/events: Handle VIRQ_TIMER before any other hardirq in event loop.

This avoids any other hardirq handler seeing a very stale jiffies
value immediately after wakeup from a long idle period. The one
observable symptom of this was a USB keyboard, with software keyboard
repeat, which would always repeat a key immediately that it was
pressed. This is due to the key press waking the guest, the key
handler immediately runs, sees an old jiffies value, and then that
jiffies value significantly updated, before the key is unpressed.
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
(cherry picked from commit bee980d9)
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/xen/events.c

@@ -1176,7 +1176,7 @@ static void __xen_evtchn_do_upcall(void)
 {
 	int start_word_idx, start_bit_idx;
 	int word_idx, bit_idx;
-	int i;
+	int i, irq;
 	int cpu = get_cpu();
 	struct shared_info *s = HYPERVISOR_shared_info;
 	struct vcpu_info *vcpu_info = __this_cpu_read(xen_vcpu);
@@ -1184,6 +1184,8 @@ static void __xen_evtchn_do_upcall(void)
 
 	do {
 		unsigned long pending_words;
+		unsigned long pending_bits;
+		struct irq_desc *desc;
 
 		vcpu_info->evtchn_upcall_pending = 0;
 
@@ -1194,6 +1196,17 @@ static void __xen_evtchn_do_upcall(void)
 		/* Clear master flag /before/ clearing selector flag. */
 		wmb();
 #endif
+		if ((irq = per_cpu(virq_to_irq, cpu)[VIRQ_TIMER]) != -1) {
+			int evtchn = evtchn_from_irq(irq);
+			word_idx = evtchn / BITS_PER_LONG;
+			pending_bits = evtchn % BITS_PER_LONG;
+			if (active_evtchns(cpu, s, word_idx) & (1ULL << pending_bits)) {
+				desc = irq_to_desc(irq);
+				if (desc)
+					generic_handle_irq_desc(irq, desc);
+			}
+		}
+
 		pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
 
 		start_word_idx = __this_cpu_read(current_word_idx);
@@ -1202,7 +1215,6 @@ static void __xen_evtchn_do_upcall(void)
 		word_idx = start_word_idx;
 
 		for (i = 0; pending_words != 0; i++) {
-			unsigned long pending_bits;
 			unsigned long words;
 
 			words = MASK_LSBS(pending_words, word_idx);
@@ -1231,8 +1243,7 @@ static void __xen_evtchn_do_upcall(void)
 
 			do {
 				unsigned long bits;
-				int port, irq;
-				struct irq_desc *desc;
+				int port;
 
 				bits = MASK_LSBS(pending_bits, bit_idx);