On Tue, 26 May 2026, Michael Kelley wrote:
From: Michael Kelley <[email protected]>
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 <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Michael Kelley <[email protected]>
Acked-by: Arnd Bergmann <[email protected]>
Reviewed-by: Jork Loeser <[email protected]>
---
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 <[email protected]>
