x86/HPET: cache MSI message last written

Rather than spending measurable amounts of time reading back the most
recently written message, cache it in space previously unused, and thus
accelerate the CPU's entering of the intended C-state.

hpet_msi_read() ends up being unused after this change, but rather than
removing the function, it's being marked "unused" in order - that way
it can easily get used again should a new need for it arise.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -253,17 +253,19 @@ static void hpet_msi_mask(struct irq_des
 
 static void hpet_msi_write(struct hpet_event_channel *ch, struct msi_msg *msg)
 {
+    ch->msi.msg = *msg;
     if ( iommu_intremap )
         iommu_update_ire_from_msi(&ch->msi, msg);
     hpet_write32(msg->data, HPET_Tn_ROUTE(ch->idx));
     hpet_write32(msg->address_lo, HPET_Tn_ROUTE(ch->idx) + 4);
 }
 
-static void hpet_msi_read(struct hpet_event_channel *ch, struct msi_msg *msg)
+static void __attribute__((__unused__))
+hpet_msi_read(struct hpet_event_channel *ch, struct msi_msg *msg)
 {
     msg->data = hpet_read32(HPET_Tn_ROUTE(ch->idx));
     msg->address_lo = hpet_read32(HPET_Tn_ROUTE(ch->idx) + 4);
-    msg->address_hi = 0;
+    msg->address_hi = MSI_ADDR_BASE_HI;
     if ( iommu_intremap )
         iommu_read_msi_from_ire(&ch->msi, msg);
 }
@@ -285,20 +287,19 @@ static void hpet_msi_ack(struct irq_desc
 
 static void hpet_msi_set_affinity(struct irq_desc *desc, const cpumask_t *mask)
 {
-    struct msi_msg msg;
-    unsigned int dest;
+    struct hpet_event_channel *ch = desc->action->dev_id;
+    struct msi_msg msg = ch->msi.msg;
 
-    dest = set_desc_affinity(desc, mask);
-    if (dest == BAD_APICID)
+    msg.dest32 = set_desc_affinity(desc, mask);
+    if ( msg.dest32 == BAD_APICID )
         return;
 
-    hpet_msi_read(desc->action->dev_id, &msg);
     msg.data &= ~MSI_DATA_VECTOR_MASK;
     msg.data |= MSI_DATA_VECTOR(desc->arch.vector);
     msg.address_lo &= ~MSI_ADDR_DEST_ID_MASK;
-    msg.address_lo |= MSI_ADDR_DEST_ID(dest);
-    msg.dest32 = dest;
-    hpet_msi_write(desc->action->dev_id, &msg);
+    msg.address_lo |= MSI_ADDR_DEST_ID(msg.dest32);
+    if ( msg.data != ch->msi.msg.data || msg.dest32 != ch->msi.msg.dest32 )
+        hpet_msi_write(ch, &msg);
 }
 
 /*