Re: [PATCH] mshv: add vmbus dependency

[Jork Loeser <jloeser@linux.microsoft.com>]

On Wed, 20 May 2026, Arnd Bergmann wrote:

> On Wed, May 20, 2026, at 15:46, Michael Kelley wrote:
>> From: Arnd Bergmann <arnd@kernel.org> Sent: Wednesday, May 20, 2026 12:40 AM
>>>
>>> When the vmbus driver is not part of the kernel, the mvhv_root
>>> driver now fails to link:
>>>
>>> ERROR: modpost: "hv_vmbus_exists" [drivers/hv/mshv_root.ko] undefined!
>>>
>>> Avoid this by adding an explicit Kconfig dependency. Note that
>>> stubbing out the hv_vmbus_exists() based on configuration would
>>> also work for some cases, but not with MSHV_ROOT=y and HYPERV_VMBUS=m.
>>
>> Conceptually, the MSHV root code should not have a dependency on
>> VMBus. The "does VMBus exist?" question should handled differently
>> by setting up a boolean in the core Hyper-V code that defaults to "false".
>> If the VMBus driver loads, it would set the boolean to "true". MSHV
>> root code would query the boolean.
>
> That makes sense to me, but it's outside of my area for a drive-by
> build fix. Please treat my patch as a 'Reported-by' then, I expect
> this can easily be addressed by Jork or someone else in the
> hyperv team.

Apologies for letting this linger - we were looking for the best fix 
internally. While Michael is correct that conceptually the dependency is 
not ideal, I think it's the right fix *for now*.

Yes, this would not allow MSHV root without VMBUS, which is relevant for 
MSHV root only, which misses a few other components. For L1VH it is the 
right fix that also enforces module load order.

Ultimately (and this is planned), we need a SYNIC driver, and the 
hv_vmbus_exists() will vanish for this usecase. It is a hack even now.

Best,
Jork