[PATCH] Drivers: hv: balloon: account for vmbus packet header in max_pkt_size

[Yanming Liu <yanminglr@gmail.com>]

Commit adae1e931acd ("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: adae1e931acd ("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>
---
The patch was "[PATCH v2] hv: account for packet descriptor in maximum
packet size". As pointed out by Michael Kelley [1], other hv drivers
already overallocate a lot, and hv_balloon is hopefully the only
remaining affected driver. It's better to just fix hv_balloon. Patch
summary is changed to reflect this new (much smaller) scope.

[1] https://lore.kernel.org/linux-hyperv/CY4PR21MB1586D30C6CEC81EFC37A9848D7599@CY4PR21MB1586.namprd21.prod.outlook.com/

 drivers/hv/hv_balloon.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
index ca873a3b98db..f2d05bff4245 100644
--- a/drivers/hv/hv_balloon.c
+++ b/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)
-- 
2.34.1