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

[Jork Loeser <jloeser@linux.microsoft.com>]

On Tue, 26 May 2026, Michael Kelley wrote:

> From: Michael Kelley <mhklinux@outlook.com>
>
> 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@arndb.de>
> Closes: https://lore.kernel.org/all/20260520074044.923728-1-arnd@kernel.org/
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> Reviewed-by: Jork Loeser <jloeser@linux.microsoft.com>
> ---
> Changes in v2:
> * Instead of putting IS_ENABLED(CONFIG_HYPERV_VMBUS) around each of
>  the two calls to hv_vmbus_exists() in mshv_synic.c, provide a stub
>  for hv_vmbus_exists() when CONFIG_HYPERV_VMBUS is not set. The
>  effect is the same as in v1, but the code is cleaner. [Jork Loeser]
>
> Arnd: I've kept your Ack even though I changed how hv_vmbus_exists()
> is stubbed out since the effect is the same. Let me know if
> you have any concerns.
>
> drivers/hv/Kconfig     | 1 +
> include/linux/hyperv.h | 4 ++++
> 2 files changed, 5 insertions(+)
>
> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> index 2d0b3fcb0ff8..aa11bcefddf2 100644
> --- a/drivers/hv/Kconfig
> +++ b/drivers/hv/Kconfig
> @@ -74,6 +74,7 @@ config MSHV_ROOT
> 	# e.g. When withdrawing memory, the hypervisor gives back 4k pages in
> 	# no particular order, making it impossible to reassemble larger pages
> 	depends on PAGE_SIZE_4KB
> +	depends on HYPERV_VMBUS if HYPERV_VMBUS
> 	select EVENTFD
> 	select VIRT_XFER_TO_GUEST_WORK
> 	select HMM_MIRROR
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index 41a3d82f0722..734b7ef98f4d 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1304,7 +1304,11 @@ static inline void *hv_get_drvdata(struct hv_device *dev)
>
> struct device *hv_get_vmbus_root_device(void);
>
> +#if IS_ENABLED(CONFIG_HYPERV_VMBUS)
> bool hv_vmbus_exists(void);
> +#else
> +static inline bool hv_vmbus_exists(void) { return false; }
> +#endif
>
> struct hv_ring_buffer_debug_info {
> 	u32 current_interrupt_mask;
> -- 
> 2.25.1
>

Thank you!

Reviewed-by: Jork Loeser <jloeser@linux.microsoft.com>