hv_utils: Add comment about max VMbus packet size in VSS driver

The VSS driver allocates a VMbus receive buffer significantly
larger than sizeof(hv_vss_msg), with no explanation. To help
prevent future mistakes, add a #define and comment about why
this is done.

No functional change.

Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/1644423070-75125-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>

Author: kelleymh

drivers/hv/hv_snapshot.c

@@ -31,6 +31,9 @@ static const int fw_versions[] = {
 	UTIL_FW_VERSION
 };
 
+/* See comment with struct hv_vss_msg regarding the max VMbus packet size */
+#define VSS_MAX_PKT_SIZE (HV_HYP_PAGE_SIZE * 2)
+
 /*
  * Timeout values are based on expecations from host
  */
@@ -298,7 +301,7 @@ void hv_vss_onchannelcallback(void *context)
 	if (vss_transaction.state > HVUTIL_READY)
 		return;
 
-	if (vmbus_recvpacket(channel, recv_buffer, HV_HYP_PAGE_SIZE * 2, &recvlen, &requestid)) {
+	if (vmbus_recvpacket(channel, recv_buffer, VSS_MAX_PKT_SIZE, &recvlen, &requestid)) {
 		pr_err_ratelimited("VSS request received. Could not read into recv buf\n");
 		return;
 	}
@@ -375,7 +378,7 @@ hv_vss_init(struct hv_util_service *srv)
 	}
 	recv_buffer = srv->recv_buffer;
 	vss_transaction.recv_channel = srv->channel;
-	vss_transaction.recv_channel->max_pkt_size = HV_HYP_PAGE_SIZE * 2;
+	vss_transaction.recv_channel->max_pkt_size = VSS_MAX_PKT_SIZE;
 
 	/*
 	 * When this driver loads, the user level daemon that

include/uapi/linux/hyperv.h

@@ -90,6 +90,17 @@ struct hv_vss_check_dm_info {
 	__u32 flags;
 } __attribute__((packed));
 
+/*
+ * struct hv_vss_msg encodes the fields that the Linux VSS
+ * driver accesses. However, FREEZE messages from Hyper-V contain
+ * additional LUN information that Linux doesn't use and are not
+ * represented in struct hv_vss_msg. A received FREEZE message may
+ * be as large as 6,260 bytes, so the driver must allocate at least
+ * that much space, not sizeof(struct hv_vss_msg). Other messages
+ * such as AUTO_RECOVER may be as large as 12,500 bytes. However,
+ * because the Linux VSS driver responds that it doesn't support
+ * auto-recovery, it should not receive such messages.
+ */
 struct hv_vss_msg {
 	union {
 		struct hv_vss_hdr vss_hdr;