Drivers: hv: vmbus: Implement a mechanism to tag the channel for low latency

[linux.git] / include / linux / hyperv.h

diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 50f493eedeb54226827793c5e694dc96d4a8fd44..755e8f566a47977c0638045d416a7877160afd34 100644 (file)
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -850,6 +850,31 @@ struct vmbus_channel {
         * ring lock to preserve the current behavior.
         */
        bool acquire_ring_lock;
+       /*
+        * For performance critical channels (storage, networking
+        * etc,), Hyper-V has a mechanism to enhance the throughput
+        * at the expense of latency:
+        * When the host is to be signaled, we just set a bit in a shared page
+        * and this bit will be inspected by the hypervisor within a certain
+        * window and if the bit is set, the host will be signaled. The window
+        * of time is the monitor latency - currently around 100 usecs. This
+        * mechanism improves throughput by:
+        *
+        * A) Making the host more efficient - each time it wakes up,
+        *    potentially it will process morev number of packets. The
+        *    monitor latency allows a batch to build up.
+        * B) By deferring the hypercall to signal, we will also minimize
+        *    the interrupts.
+        *
+        * Clearly, these optimizations improve throughput at the expense of
+        * latency. Furthermore, since the channel is shared for both
+        * control and data messages, control messages currently suffer
+        * unnecessary latency adversley impacting performance and boot
+        * time. To fix this issue, permit tagging the channel as being
+        * in "low latency" mode. In this mode, we will bypass the monitor
+        * mechanism.
+        */
+       bool low_latency;
 
 };
 
@@ -891,6 +916,16 @@ static inline void set_channel_pending_send_size(struct vmbus_channel *c,
        c->outbound.ring_buffer->pending_send_sz = size;
 }
 
+static inline void set_low_latency_mode(struct vmbus_channel *c)
+{
+       c->low_latency = true;
+}
+
+static inline void clear_low_latency_mode(struct vmbus_channel *c)
+{
+       c->low_latency = false;
+}
+
 void vmbus_onmessage(void *context);
 
 int vmbus_request_offers(void);