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mshv: Add conditional VMBus dependency

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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>
Reviewed-by: Hardik Garg <hargar@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
This commit is contained in:
Michael Kelley 2026-05-26 07:13:04 -07:00 • committed by Wei Liu
commit 840b740a35
2 changed files with 5 additions and 0 deletions
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1
drivers/hv/Kconfig
View file

@ -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

4
include/linux/hyperv.h
View file

@ -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;