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

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

[Michael Kelley]

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;
+
 	unregister_syscore(&hv_synic_syscore);
 
 	hv_remove_kexec_handler();
-- 
2.25.1

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

[Easwar Hariharan]

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();