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From:   Michael Kelley <mikelley@microsoft.com>
To:     sthemmin@microsoft.com, kys@microsoft.com, haiyangz@microsoft.com,
        wei.liu@kernel.org, decui@microsoft.com,
        linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        vkuznets@redhat.com
Cc:     mikelley@microsoft.com
Subject: [PATCH 1/1] hv_utils: Add comment about max VMbus packet size in VSS driver
Date:   Wed,  9 Feb 2022 08:11:10 -0800
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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>
---
 drivers/hv/hv_snapshot.c    |  7 +++++--
 include/uapi/linux/hyperv.h | 11 +++++++++++
 2 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv_snapshot.c b/drivers/hv/hv_snapshot.c
index 6018b9d..0d2184b 100644
--- a/drivers/hv/hv_snapshot.c
+++ b/drivers/hv/hv_snapshot.c
@@ -31,6 +31,9 @@
 	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 @@ static void vss_on_reset(void)
 	}
 	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
diff --git a/include/uapi/linux/hyperv.h b/include/uapi/linux/hyperv.h
index daf82a2..aaa502a 100644
--- a/include/uapi/linux/hyperv.h
+++ b/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;
-- 
1.8.3.1