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From:   Andrea Parri <parri.andrea@gmail.com>
To:     linux-kernel@vger.kernel.org, linux-hyperv@vger.kernel.org,
        netdev@vger.kernel.org
Cc:     "K . Y . Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        "David S . Miller" <davem@davemloft.net>,
        Michael Kelley <mikelley@microsoft.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Dexuan Cui <decui@microsoft.com>,
        Andrea Parri <parri.andrea@gmail.com>
Subject: [PATCH v2 3/3] Drivers: hv: vmbus: Add module parameter to cap the VMBus version
Date:   Thu, 10 Oct 2019 17:46:00 +0200
Message-Id: <20191010154600.23875-4-parri.andrea@gmail.com>
In-Reply-To: <20191010154600.23875-1-parri.andrea@gmail.com>
References: <20191010154600.23875-1-parri.andrea@gmail.com>
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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>
---
 drivers/hv/connection.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
index 2f6961ac8c996..f60d7330ff3fd 100644
--- a/drivers/hv/connection.c
+++ b/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;
@@ -237,6 +248,8 @@ int vmbus_connect(void)
 
 	for (i = 0; i < ARRAY_SIZE(vmbus_versions); i++) {
 		version = vmbus_versions[i];
+		if (version > max_version)
+			continue;
 
 		ret = vmbus_negotiate_version(msginfo, version);
 		if (ret == -ETIMEDOUT)
-- 
2.23.0