[Xen-devel] [PATCH 3/3] x86/HPET: cache MSI message last written

["Jan Beulich" <JBeulich@xxxxxxxx>]

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);
 }
 
 /*

Attachment: x86-HPET-msg-cache.patch
Description: Text document

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