hyperv: Enable sendbuf mechanism on the send path

We send packets using a copy-free mechanism (this is the Guest to Host transport
via VMBUS). While this is obviously optimal for large packets,
it may not be optimal for small packets. Hyper-V host supports
a second mechanism for sending packets that is "copy based". We implement that
mechanism in this patch.

In this version of the patch I have addressed a comment from David Miller.

With this patch (and all of the other offload and VRSS patches), we are now able
to almost saturate a 10G interface between Linux VMs on Hyper-V
on different hosts - close to  9 Gbps as measured via iperf.

Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/hyperv_net.h

@@ -140,6 +140,8 @@ struct hv_netvsc_packet {
 	void *send_completion_ctx;
 	void (*send_completion)(void *context);
 
+	u32 send_buf_index;
+
 	/* This points to the memory after page_buf */
 	struct rndis_message *rndis_msg;
 
@@ -582,6 +584,9 @@ struct nvsp_message {
 
 #define NETVSC_RECEIVE_BUFFER_SIZE		(1024*1024*16)	/* 16MB */
 #define NETVSC_RECEIVE_BUFFER_SIZE_LEGACY	(1024*1024*15)  /* 15MB */
+#define NETVSC_SEND_BUFFER_SIZE			(1024 * 1024)   /* 1MB */
+#define NETVSC_INVALID_INDEX			-1
+
 
 #define NETVSC_RECEIVE_BUFFER_ID		0xcafe
 
@@ -607,6 +612,15 @@ struct netvsc_device {
 	u32 recv_section_cnt;
 	struct nvsp_1_receive_buffer_section *recv_section;
 
+	/* Send buffer allocated by us */
+	void *send_buf;
+	u32 send_buf_size;
+	u32 send_buf_gpadl_handle;
+	u32 send_section_cnt;
+	u32 send_section_size;
+	unsigned long *send_section_map;
+	int map_words;
+
 	/* Used for NetVSP initialization protocol */
 	struct completion channel_init_wait;
 	struct nvsp_message channel_init_pkt;