Re: [PATCH v2 1/1] Drivers: hv: vmbus: Skip VMBus module cleanup for non-nested root partition

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

On 8/5/2026 07:24, Michael Kelley wrote:
> The VMBus module initialization function, hv_acpi_init(), currently
> does nothing when running in the root partition and root is not nested
> in another VM. But the initialization function reports success, so the
> VMBus module is indeed loaded. VMBus functionality is not actually
> needed, but the VMBus module must be loaded so that hv_vmbus_exists()
> can answer correctly. Furthermore, the mshv_root dependency on the
> VMBus module is needed as described in the commit message for
> 840b740a35bf ("mshv: Add conditional VMBus dependency").
> 
> Loading the VMBus module without actually initializing it causes
> failures if the module should later be unloaded. The 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 exit function perform the same
> check for non-nested root partition, and do nothing in such a
> case, just like hv_acpi_init().
> 
> In the long run, the code that manages the Hyper-V provided SynIC
> should be refactored to better coordinate the requirements of
> root partition scenarios and normal VM scenarios, and to hopefully
> remove the hv_vmbus_exists() dependnecy between mshv_root and
> VMBus modules. Preventing the current unload failure scenario is
> an expediency until such a refactoring is done.
> 
> 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>
> ---
> v1: https://lore.kernel.org/linux-hyperv/20260804190517.101981-1-mhklinux@outlook.com/
> 
> Changes in v2:
> * Use a different solution: Allow the VMBus module to load but have the
>   unload function do nothing for non-nested root
> * Change the patch Subject and commit message to reflect the new approach
> 
>  drivers/hv/vmbus_drv.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index e19ec73b0187..f837153427f4 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -3024,6 +3024,9 @@ static void __exit vmbus_exit(void)
>  {
>  	int cpu;
>  
> +	if (hv_root_partition() && !hv_nested)
> +		return;
> +

Do you think it'd be useful to have a comment here either saying something to the effect of
"nothing was initialized, so let's skip the teardown", or alternatively pointing to the paired
check in hv_acpi_init()?

Either way,

Reviewed-by: Easwar Hariharan <easwar.hariharan@linux.microsoft.com>

>  	unregister_syscore(&hv_synic_syscore);
>  
>  	hv_remove_kexec_handler();