Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Easwar Hariharan <easwar.hariharan@linux.microsoft.com>]

On 8/4/2026 15:17, Michael Kelley wrote:
> From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 12:40 PM
>>
>> On 8/4/2026 12:05, Michael Kelley wrote:
>>> The VMBus module should not be loaded when Linux is running directly
>>> in the root partition and root is not nested in another VM. Current
>>> code checks this condition and skips VMBus module initialization, which
>>> works. But it returns 0 as the result, so Linux thinks the module has
>>> successfully loaded. Later, if the module were to be unloaded, the
>>> VMBus module unload code tries to clean up things that were never
>>> initialized, resulting in memory faults and a panic.
>>>
>>> Fix this by having VMBus module initialization return -ENODEV for this
>>> case. The module is then not loaded, and the unload path can never run.
>>>
>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
>>> Closes: https://lore.kernel.org/linux-
>> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
>>> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
>>> ---
>>>  drivers/hv/vmbus_drv.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
>>> index e19ec73b0187..849d7e1a7320 100644
>>> --- a/drivers/hv/vmbus_drv.c
>>> +++ b/drivers/hv/vmbus_drv.c
>>> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
>>>  		return -ENODEV;
>>>
>>>  	if (hv_root_partition() && !hv_nested)
>>> -		return 0;
>>> +		return -ENODEV;
>>>
>>>  	/*
>>>  	 * Get ACPI resources first.
>>
>> This seems straightforward:
> 
> Alas, it's not so straightforward, as Sashiko pointed out.  I knew that
> the mshv module has a dependency on the vmbus module, but had
> forgotten. There's a reason for the dependency as described in the
> commit message for 840b740a35bf.
> 
> There's another easy way to fix the VMBus module unload problem.
> I'll send a v2. :-)
> 
> Michael
> 
I may be missing something, but mshv_root is used in 3 cases:

1) Baremetal root partition, which requires no VMBus

2) L1VH aka Direct Virtualization, which is conditioned on hv_l1vh_partition(), which is not the check here in the vmbus driver

3) For the OpenHCL paravisor, where I honestly don't know what the dependency chain looks like, but based on a quick glance
at https://github.com/microsoft/OHCL-Linux-Kernel/ and a cursory grep, doesn't seem to rely on hv_root_partition() but does rely
on VMbus.

I feel like Sashiko's review falls into item 1, but then again, there may just be a mismatch
between reality and my mental model, or my mental model may becorrect, but the code doesn't match it.

Thanks,
Easwar (he/him)