Re: [PATCH v2 1/1] mshv: Add conditional VMBus dependency

[Wei Liu]

On Tue, May 26, 2026 at 07:13:04AM -0700, Michael Kelley wrote:
> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> 
> When the VMBus driver is not part of the kernel (CONFIG_HYPERV_VMBUS=n),
> the MSHV root driver fails to link:
> 
> ERROR: modpost: "hv_vmbus_exists" [drivers/hv/mshv_root.ko] undefined!
> 
> Fix this while meeting these requirements:
> * It must be possible to include the MSHV root driver without the
>   VMBus driver. In such case, the MSHV root driver can be built-in
>   to the kernel image, or it can be built as a separate module.
> * If both the MSHV root driver and the VMBus driver are present, the
>   MSHV root driver and VMBus driver can both be built-in, or they can
>   both be separate modules. Or the MSHV root driver can be a module
>   while the VMBus driver can be built-in, but the reverse is
>   disallowed. Regardless of the build choices, the VMBus driver must
>   be loaded before the MSHV driver in order for the SynIC to be
>   managed properly (see comments in the MSHV SynIC code).
> 
> The fix has two parts:
> * Add a Kconfig entry for MSHV_ROOT to depend on HYPERV_VMBUS if
>   HYPERV_VMBUS is present. The entry disallows MSHV_ROOT being
>   built-in when HYPERV_VMBUS is a module, but without requiring that
>   HYPERV_VMBUS be built.
> * Add a stub implementation of hv_vmbus_exists() for when the
>   VMBus driver is not present so that the MSHV root driver has
>   no module dependency on VMBus. When the VMBus driver *is*
>   present, the module dependency ensures that the VMBus driver
>   loads first when both are built as modules.
> 
> Existing code ensures that the VMBus driver loads first if it is
> built-in. The VMBus driver uses subsys_initcall(), which is
> initcall level 4. The MSHV root driver uses module_init(), which
> becomes device_init() when built-in, and device_init() is
> initcall level 6.
> 
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Closes: https://lore.kernel.org/all/20260520074044.923728-1-arnd@xxxxxxxxxx/
> Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
> Reviewed-by: Jork Loeser <jloeser@xxxxxxxxxxxxxxxxxxx>

Applied. Thanks everyone.