Drivers: hv: vmbus: Add module parameter to cap the VMBus version

Currently, Linux guests negotiate the VMBus version with Hyper-V
and use the highest available VMBus version they can connect to.
This has some drawbacks: by using the highest available version,
certain code paths are never executed and can not be tested when
the guest runs on the newest host.

Add the module parameter "max_version", to upper-bound the VMBus
versions guests can negotiate.
Suggested-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

drivers/hv/connection.c

@@ -14,6 +14,7 @@
 #include <linux/wait.h>
 #include <linux/delay.h>
 #include <linux/mm.h>
+#include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/hyperv.h>
@@ -55,6 +56,16 @@ static __u32 vmbus_versions[] = {
 	VERSION_WS2008
 };
 
+/*
+ * Maximal VMBus protocol version guests can negotiate.  Useful to cap the
+ * VMBus version for testing and debugging purpose.
+ */
+static uint max_version = VERSION_WIN10_V5_2;
+
+module_param(max_version, uint, S_IRUGO);
+MODULE_PARM_DESC(max_version,
+		 "Maximal VMBus protocol version which can be negotiated");
+
 int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo, u32 version)
 {
 	int ret = 0;
@@ -240,6 +251,8 @@ int vmbus_connect(void)
 			goto cleanup;
 
 		version = vmbus_versions[i];
+		if (version > max_version)
+			continue;
 
 		ret = vmbus_negotiate_version(msginfo, version);
 		if (ret == -ETIMEDOUT)