Drivers: hv: Optimize the signaling on the write path
The host has already implemented the "read" side optimizations.
Leverage that to optimize "write" side signaling.
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 0010792..9bc55f0 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -53,6 +53,37 @@
return read;
}
+/*
+ * When we write to the ring buffer, check if the host needs to
+ * be signaled. Here is the details of this protocol:
+ *
+ * 1. The host guarantees that while it is draining the
+ * ring buffer, it will set the interrupt_mask to
+ * indicate it does not need to be interrupted when
+ * new data is placed.
+ *
+ * 2. The host guarantees that it will completely drain
+ * the ring buffer before exiting the read loop. Further,
+ * once the ring buffer is empty, it will clear the
+ * interrupt_mask and re-check to see if new data has
+ * arrived.
+ */
+
+static bool hv_need_to_signal(u32 old_write, struct hv_ring_buffer_info *rbi)
+{
+ if (rbi->ring_buffer->interrupt_mask)
+ return false;
+
+ /*
+ * This is the only case we need to signal when the
+ * ring transitions from being empty to non-empty.
+ */
+ if (old_write == rbi->ring_buffer->read_index)
+ return true;
+
+ return false;
+}
+
/*
* hv_get_next_write_location()
@@ -322,7 +353,7 @@
*
*/
int hv_ringbuffer_write(struct hv_ring_buffer_info *outring_info,
- struct scatterlist *sglist, u32 sgcount)
+ struct scatterlist *sglist, u32 sgcount, bool *signal)
{
int i = 0;
u32 bytes_avail_towrite;
@@ -331,6 +362,7 @@
struct scatterlist *sg;
u32 next_write_location;
+ u32 old_write;
u64 prev_indices = 0;
unsigned long flags;
@@ -359,6 +391,8 @@
/* Write to the ring buffer */
next_write_location = hv_get_next_write_location(outring_info);
+ old_write = next_write_location;
+
for_each_sg(sglist, sg, sgcount, i)
{
next_write_location = hv_copyto_ringbuffer(outring_info,
@@ -375,14 +409,16 @@
&prev_indices,
sizeof(u64));
- /* Make sure we flush all writes before updating the writeIndex */
- smp_wmb();
+ /* Issue a full memory barrier before updating the write index */
+ smp_mb();
/* Now, update the write location */
hv_set_next_write_location(outring_info, next_write_location);
spin_unlock_irqrestore(&outring_info->ring_lock, flags);
+
+ *signal = hv_need_to_signal(old_write, outring_info);
return 0;
}