The connection-id determines which hypervisor communication channel the
guest should use to talk to the VMBus host. This patch adds support to
read this value from the DeviceTree where it exists as a property under
the vmbus node with the compatible ID "microsoft,message-connection-id".
The property name follows the format <vendor>,<field> where
"vendor": "microsoft" and "field": "message-connection-id"

Reading from DeviceTree allows platforms to specify their preferred
communication channel, making it more flexible. If the property is
not found in the DeviceTree, use the default connection ID
(VMBUS_MESSAGE_CONNECTION_ID or VMBUS_MESSAGE_CONNECTION_ID_4
based on protocol version).

Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
---
v3: https://lore.kernel.org/all/6a92ca86-ad6b-4d49-af6e-1ed7651b8ab8@linux.microsoft.com
v2: https://lore.kernel.org/all/096edaf7-cc90-42b6-aff4-c5f088574e1e@linux.microsoft.com
v1: https://lore.kernel.org/all/6acee4bf-cb04-43b9-9476-e8d811d26dfd@linux.microsoft.com
---
 drivers/hv/connection.c |  6 ++++--
 drivers/hv/vmbus_drv.c  | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
index be490c598785..15d2b652783d 100644
--- a/drivers/hv/connection.c
+++ b/drivers/hv/connection.c
@@ -99,11 +99,13 @@ int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo, u32 version)
 	if (version >= VERSION_WIN10_V5) {
 		msg->msg_sint = VMBUS_MESSAGE_SINT;
 		msg->msg_vtl = ms_hyperv.vtl;
-		vmbus_connection.msg_conn_id = VMBUS_MESSAGE_CONNECTION_ID_4;
 	} else {
 		msg->interrupt_page = virt_to_phys(vmbus_connection.int_page);
-		vmbus_connection.msg_conn_id = VMBUS_MESSAGE_CONNECTION_ID;
 	}
+	/* Set default connection ID if not provided via DeviceTree */
+	if (!vmbus_connection.msg_conn_id)
+		vmbus_connection.msg_conn_id = (version >= VERSION_WIN10_V5) ?
+			VMBUS_MESSAGE_CONNECTION_ID_4 : VMBUS_MESSAGE_CONNECTION_ID;
 
 	/*
 	 * shared_gpa_boundary is zero in non-SNP VMs, so it's safe to always
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index c236081d0a87..b78d5499e4bc 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -2541,10 +2541,23 @@ static int vmbus_device_add(struct platform_device *pdev)
 	struct of_range range;
 	struct of_range_parser parser;
 	struct device_node *np = pdev->dev.of_node;
+	unsigned int conn_id;
 	int ret;
 
 	vmbus_root_device = &pdev->dev;
 
+	/*
+	 * Read connection ID from DeviceTree. The property name follows the
+	 * format <vendor>,<field> where:
+	 * - vendor: "microsoft"
+	 * - field: "message-connection-id"
+	 */
+	ret = of_property_read_u32(np, "microsoft,message-connection-id", &conn_id);
+	if (!ret) {
+		pr_info("VMBus message connection ID: %u\n", conn_id);
+	    vmbus_connection.msg_conn_id = conn_id;
+	}
+
 	ret = of_range_parser_init(&parser, np);
 	if (ret)
 		return ret;
-- 
2.40.4

On Thu, Jun 19, 2025 at 04:06:35PM GMT, Hardik Garg wrote:
> The connection-id determines which hypervisor communication channel the
> guest should use to talk to the VMBus host. This patch adds support to
> read this value from the DeviceTree where it exists as a property under
> the vmbus node with the compatible ID "microsoft,message-connection-id".
> The property name follows the format <vendor>,<field> where
> "vendor": "microsoft" and "field": "message-connection-id"
> 
> Reading from DeviceTree allows platforms to specify their preferred
> communication channel, making it more flexible. If the property is
> not found in the DeviceTree, use the default connection ID
> (VMBUS_MESSAGE_CONNECTION_ID or VMBUS_MESSAGE_CONNECTION_ID_4
> based on protocol version).
> 
> Signed-off-by: Hardik Garg <hargar@linux.microsoft.com>
> ---
> v3: https://lore.kernel.org/all/6a92ca86-ad6b-4d49-af6e-1ed7651b8ab8@linux.microsoft.com
> v2: https://lore.kernel.org/all/096edaf7-cc90-42b6-aff4-c5f088574e1e@linux.microsoft.com
> v1: https://lore.kernel.org/all/6acee4bf-cb04-43b9-9476-e8d811d26dfd@linux.microsoft.com
> ---
>  drivers/hv/connection.c |  6 ++++--
>  drivers/hv/vmbus_drv.c  | 13 +++++++++++++
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/connection.c b/drivers/hv/connection.c
> index be490c598785..15d2b652783d 100644
> --- a/drivers/hv/connection.c
> +++ b/drivers/hv/connection.c
> @@ -99,11 +99,13 @@ int vmbus_negotiate_version(struct vmbus_channel_msginfo *msginfo, u32 version)
>  	if (version >= VERSION_WIN10_V5) {
>  		msg->msg_sint = VMBUS_MESSAGE_SINT;
>  		msg->msg_vtl = ms_hyperv.vtl;
> -		vmbus_connection.msg_conn_id = VMBUS_MESSAGE_CONNECTION_ID_4;
>  	} else {
>  		msg->interrupt_page = virt_to_phys(vmbus_connection.int_page);
> -		vmbus_connection.msg_conn_id = VMBUS_MESSAGE_CONNECTION_ID;
>  	}
> +	/* Set default connection ID if not provided via DeviceTree */
> +	if (!vmbus_connection.msg_conn_id)

Your binding said connection ID 0 is a valid one, so this is wrong. Or
binding is wrong.

Best regards,
Krzysztof