linux-kernel - Re: [PATCH v4 1/2] dt-bindings: microsoft: Add vmbus message-connection-id property

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Message-Id: <1753240089-29558-1-git-send-email-hargar@linux.microsoft.com>
Date: Tue, 22 Jul 2025 20:08:09 -0700
From: Hardik Garg <hargar@...ux.microsoft.com>
To: krzk@...nel.org
Cc: apais@...rosoft.com,
	conor+dt@...nel.org,
	decui@...rosoft.com,
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Subject: Re: [PATCH v4 1/2] dt-bindings: microsoft: Add vmbus message-connection-id property

> Host is supposed to have multiple guests, so this feels like you are
> going to prepare for each guest different DTS with different connection
> ID. This feels like poor design. DTS is supposed to be relatively static
> configuration, not runtime choice vmguestid+1.

> The guest cannot access other configuration channels, can it? If it can,
> it would mean it can eavesdrop on other guests? So obviously it cannot.
> Therefore from guest point of view this is completely redundant. Guest
> cannot use any other value, thus guest should not configure it. The
> guest has only one channel and uses only this one which gets to right
> place to the host.

Thank you for your feedback. Let me explain the connection ID in more detail:

1. Message Port Architecture:
   - The connection ID specifies which Hyper-V hypervisor message port (mailbox slot) to use for communication between the host and guest
   - The hypervisor has multiple message ports, but historically VMBus only used one
   - With the introduction of VTL2 (Virtual Trust Level 2), the control plane can now be hosted in VTL2, requiring different message ports for communication

2. Control Plane Communication:
   - The VMBus control plane on the host needs to communicate with the VMBus driver in the guest
   - When the control plane is hosted in VTL2, it requires a different message port than the standard communication path
   - The connection ID tells the guest whether to use the standard or alternate message port for this control plane communication

3. Message Processing:
   - Each message is tagged with an ID
   - If the guest uses an incorrect ID, the host won't recognize the message and will drop it
   - This is not about choosing between multiple available channels - it's about using the correct mailbox slot for communication

4. Security and Isolation:
   - Each guest has its private hypervisor mailbox
   - Multiple guests using the same connection ID cannot interfere with each other
   - The connection ID is more like a "root ID" that helps enumerate devices on the bus, not a channel selector between guests

5. Dynamic Nature:
   - The connection ID is specified when the guest starts running
   - The host can change it during VM lifecycle events (reset/reboot)
   - This is why the VMBus driver needs to know the connection ID every time the kernel starts
   - The ID might be different from what it was during the last guest run




Thanks,
Hardik