[PATCH] hyperv: move vmbus_root_device outside of the VMBus driver

[Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>]

Conceptually, we shouldn't have to build the VMBus driver to answer the
question "does VMBus exist?" So, move vmbus_root_device and the
functions that access it directly to hyperv.h. Also, introduce
hv_set_vmbus_root_device() to allow vmbus_drv to set the root device.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/r/20260520074044.923728-1-arnd@kernel.org/
Fixes: f1a9e67c1138 ("mshv: limit SynIC management to MSHV-owned resources")
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
---
 drivers/hv/vmbus_drv.c | 27 ++++++---------------------
 include/linux/hyperv.h | 18 ++++++++++++++++--
 2 files changed, 22 insertions(+), 23 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 71f1b4d52f7f..b26e6b42fa49 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -47,9 +47,6 @@ struct vmbus_dynid {
 	struct hv_vmbus_device_id id;
 };
 
-/* VMBus Root Device */
-static struct device  *vmbus_root_device;
-
 static int hyperv_cpuhp_online;
 
 static DEFINE_PER_CPU(long, vmbus_evt);
@@ -95,18 +92,6 @@ static struct resource *fb_mmio;
 static struct resource *hyperv_mmio;
 static DEFINE_MUTEX(hyperv_mmio_lock);
 
-struct device *hv_get_vmbus_root_device(void)
-{
-	return vmbus_root_device;
-}
-EXPORT_SYMBOL_GPL(hv_get_vmbus_root_device);
-
-bool hv_vmbus_exists(void)
-{
-	return vmbus_root_device != NULL;
-}
-EXPORT_SYMBOL_GPL(hv_vmbus_exists);
-
 static u8 channel_monitor_group(const struct vmbus_channel *channel)
 {
 	return (u8)channel->offermsg.monitorid / 32;
@@ -903,7 +888,7 @@ static int vmbus_dma_configure(struct device *child_device)
 	 * On x86/x64 coherence is assumed and these calls have no effect.
 	 */
 	hv_setup_dma_ops(child_device,
-		device_get_dma_attr(vmbus_root_device) == DEV_DMA_COHERENT);
+		device_get_dma_attr(hv_get_vmbus_root_device()) == DEV_DMA_COHERENT);
 	return 0;
 }
 
@@ -2172,7 +2157,7 @@ int vmbus_device_register(struct hv_device *child_device_obj)
 		     &child_device_obj->channel->offermsg.offer.if_instance);
 
 	child_device_obj->device.bus = &hv_bus;
-	child_device_obj->device.parent = vmbus_root_device;
+	child_device_obj->device.parent = hv_get_vmbus_root_device();
 	child_device_obj->device.release = vmbus_device_release;
 
 	child_device_obj->device.dma_parms = &child_device_obj->dma_parms;
@@ -2561,7 +2546,7 @@ static int vmbus_acpi_add(struct platform_device *pdev)
 	struct acpi_device *ancestor;
 	struct acpi_device *device = ACPI_COMPANION(&pdev->dev);
 
-	vmbus_root_device = &device->dev;
+	hv_set_vmbus_root_device(&device->dev);
 
 	/*
 	 * Older versions of Hyper-V for ARM64 fail to include the _CCA
@@ -2648,7 +2633,7 @@ static int vmbus_device_add(struct platform_device *pdev)
 	struct device_node *np = pdev->dev.of_node;
 	int ret;
 
-	vmbus_root_device = &pdev->dev;
+	hv_set_vmbus_root_device(&pdev->dev);
 
 	ret = of_range_parser_init(&parser, np);
 	if (ret)
@@ -2969,7 +2954,7 @@ static int __init hv_acpi_init(void)
 	if (ret)
 		return ret;
 
-	if (!vmbus_root_device) {
+	if (!hv_vmbus_exists()) {
 		ret = -ENODEV;
 		goto cleanup;
 	}
@@ -3000,7 +2985,7 @@ static int __init hv_acpi_init(void)
 
 cleanup:
 	platform_driver_unregister(&vmbus_platform_driver);
-	vmbus_root_device = NULL;
+	hv_set_vmbus_root_device(NULL);
 	return ret;
 }
 
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index 5459e776ec17..3e8ea7d9653c 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1302,9 +1302,23 @@ static inline void *hv_get_drvdata(struct hv_device *dev)
 	return dev_get_drvdata(&dev->device);
 }
 
-struct device *hv_get_vmbus_root_device(void);
+/* Do *not* use this outside of hyperv.h */
+static struct device *__vmbus_root_device __read_mostly;
 
-bool hv_vmbus_exists(void);
+static inline void hv_set_vmbus_root_device(struct device *dev)
+{
+	__vmbus_root_device = dev;
+}
+
+static inline struct device *hv_get_vmbus_root_device(void)
+{
+	return __vmbus_root_device;
+}
+
+static inline bool hv_vmbus_exists(void)
+{
+	return hv_get_vmbus_root_device();
+}
 
 struct hv_ring_buffer_debug_info {
 	u32 current_interrupt_mask;
-- 
2.54.0