Drivers: hv: balloon: account for vmbus packet header in max_pkt_size

Commit adae1e93 ("Drivers: hv: vmbus: Copy packets sent by Hyper-V
out of the ring buffer") introduced a notion of maximum packet size in
vmbus channel and used that size to initialize a buffer holding all
incoming packet along with their vmbus packet header. hv_balloon uses
the default maximum packet size VMBUS_DEFAULT_MAX_PKT_SIZE which matches
its maximum message size, however vmbus_open expects this size to also
include vmbus packet header. This leads to 4096 bytes
dm_unballoon_request messages being truncated to 4080 bytes. When the
driver tries to read next packet it starts from a wrong read_index,
receives garbage and prints a lot of "Unhandled message: type:
<garbage>" in dmesg.

Allocate the buffer with HV_HYP_PAGE_SIZE more bytes to make room for
the header.

Fixes: adae1e93 ("Drivers: hv: vmbus: Copy packets sent by Hyper-V out of the ring buffer")
Suggested-by: Michael Kelley (LINUX) <mikelley@microsoft.com>
Suggested-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Signed-off-by: Yanming Liu <yanminglr@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20220119202052.3006981-1-yanminglr@gmail.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/hv_balloon.c

@@ -1660,6 +1660,13 @@ static int balloon_connect_vsp(struct hv_device *dev)
 	unsigned long t;
 	int ret;
 
+	/*
+	 * max_pkt_size should be large enough for one vmbus packet header plus
+	 * our receive buffer size. Hyper-V sends messages up to
+	 * HV_HYP_PAGE_SIZE bytes long on balloon channel.
+	 */
+	dev->channel->max_pkt_size = HV_HYP_PAGE_SIZE * 2;
+
 	ret = vmbus_open(dev->channel, dm_ring_size, dm_ring_size, NULL, 0,
 			 balloon_onchannelcallback, dev);
 	if (ret)