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

[Michael Kelley]

From: Easwar Hariharan <easwar.hariharan@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, August 5, 2026 10:28 AM
> 
> 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@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/linux-hyperv/20260721154943.A09BD1F00A3D@xxxxxxxxxxxxxxx/
> > Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
> > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > ---
> > v1: https://lore.kernel.org/linux-hyperv/20260804190517.101981-1-mhklinux@xxxxxxxxxxx/
> >
> > 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()?

I generally lean toward adding comments for anything that is non-obvious,
so yes. Actually, a comment is really needed in hv_acpi_init() to explain why
the code intentionally allows the module to be loaded even though no initialization
is done. Then a comment here can refer back to that.

I'll spin a v3.

Michael