hv_utils: drain the timesync packets on onchannelcallback

From: Vineeth Pillai <[email protected]>

commit b46b4a8a57c377b72a98c7930a9f6969d2d4784e

There could be instances where a system stall prevents the timesync packets to be consumed. And this might lead to more than one packet pending in the ring buffer. Current code empties one packet per callback and it might be a stale one. So drain all the packets from ring buffer on each callback.

The upstream commit uses HV_HYP_PAGE_SIZE, which is not defined in 4.19.y. Fixed this manually for 4.19.y by using PAGE_SIZE instead.

If there are multiple messages in the host-to-guest ringbuffer of the TimeSync device, 4.19.y only handles 1 message, and later the host puts new messages into the ringbuffer without signaling the guest because the ringbuffer is not empty, causing a "hung" ringbuffer. Backported the mainline fix for this issue.

---
 drivers/hv/hv_util.c | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

index 2003314dcfbe..4a131efe54ef 100644
@@ -294,10 +294,23 @@ static void timesync_onchannelcallback(void *context)
 	struct ictimesync_ref_data *refdata;
 	u8 *time_txf_buf = util_timesynch.recv_buffer;
 
-	vmbus_recvpacket(channel, time_txf_buf,
-			 PAGE_SIZE, &recvlen, &requestid);
+	/*
+	 * Drain the ring buffer and use the last packet to update
+	 * host_ts
+	 */
+	while (1) {
+		int ret = vmbus_recvpacket(channel, time_txf_buf,
+					   PAGE_SIZE, &recvlen,
+					   &requestid);
+		if (ret) {
+			pr_warn_once("TimeSync IC pkt recv failed (Err: %d)\n",
+				     ret);
+			break;
+		}
+
+		if (!recvlen)
+			break;
 
-	if (recvlen > 0) {
 		icmsghdrp = (struct icmsg_hdr *)&time_txf_buf[
 				sizeof(struct vmbuspipe_hdr)];
 

2.25.1