[PATCH v2 1/2] Drivers: hv: vmbus: Provide option to skip VMBus unload on panic

[PATCH v2 1/2] Drivers: hv: vmbus: Provide option to skip VMBus unload on panic

[Michael Kelley]

From: Michael Kelley <mhklinux@outlook.com>

Currently, VMBus code initiates a VMBus unload in the panic path so
that if a kdump kernel is loaded, it can start fresh in setting up its
own VMBus connection. However, a driver for the VMBus virtual frame
buffer may need to flush dirty portions of the frame buffer back to
the Hyper-V host so that panic information is visible in the graphics
console. To support such flushing, provide exported functions for the
frame buffer driver to specify that the VMBus unload should not be
done by the VMBus driver, and to initiate the VMBus unload itself.
Together these allow a frame buffer driver to delay the VMBus unload
until after it has completed the flush.

Ideally, the VMBus driver could use its own panic-path callback to do
the unload after all frame buffer drivers have finished. But DRM frame
buffer drivers use the kmsg dump callback, and there are no callbacks
after that in the panic path. Hence this somewhat messy approach to
properly sequencing the frame buffer flush and the VMBus unload.

Fixes: 3671f3777758 ("drm/hyperv: Add support for drm_panic")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
Changes in v2: None

 drivers/hv/channel_mgmt.c |  1 +
 drivers/hv/hyperv_vmbus.h |  1 -
 drivers/hv/vmbus_drv.c    | 25 ++++++++++++++++++-------
 include/linux/hyperv.h    |  3 +++
 4 files changed, 22 insertions(+), 8 deletions(-)

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 74fed2c073d4..5de83676dbad 100644
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -944,6 +944,7 @@ void vmbus_initiate_unload(bool crash)
 	else
 		vmbus_wait_for_unload();
 }
+EXPORT_SYMBOL_GPL(vmbus_initiate_unload);
 
 static void vmbus_setup_channel_state(struct vmbus_channel *channel,
 				      struct vmbus_channel_offer_channel *offer)
diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
index cdbc5f5c3215..5d3944fc93ae 100644
--- a/drivers/hv/hyperv_vmbus.h
+++ b/drivers/hv/hyperv_vmbus.h
@@ -440,7 +440,6 @@ void hv_vss_deinit(void);
 int hv_vss_pre_suspend(void);
 int hv_vss_pre_resume(void);
 void hv_vss_onchannelcallback(void *context);
-void vmbus_initiate_unload(bool crash);
 
 static inline void hv_poll_channel(struct vmbus_channel *channel,
 				   void (*cb)(void *))
diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 6785ad63a9cb..97dfa529d250 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -69,19 +69,29 @@ bool vmbus_is_confidential(void)
 }
 EXPORT_SYMBOL_GPL(vmbus_is_confidential);
 
+static bool skip_vmbus_unload;
+
+/*
+ * Allow a VMBus framebuffer driver to specify that in the case of a panic,
+ * it will do the VMbus unload operation once it has flushed any dirty
+ * portions of the framebuffer to the Hyper-V host.
+ */
+void vmbus_set_skip_unload(bool skip)
+{
+	skip_vmbus_unload = skip;
+}
+EXPORT_SYMBOL_GPL(vmbus_set_skip_unload);
+
 /*
  * The panic notifier below is responsible solely for unloading the
  * vmbus connection, which is necessary in a panic event.
- *
- * Notice an intrincate relation of this notifier with Hyper-V
- * framebuffer panic notifier exists - we need vmbus connection alive
- * there in order to succeed, so we need to order both with each other
- * [see hvfb_on_panic()] - this is done using notifiers' priorities.
  */
 static int hv_panic_vmbus_unload(struct notifier_block *nb, unsigned long val,
 			      void *args)
 {
-	vmbus_initiate_unload(true);
+	if (!skip_vmbus_unload)
+		vmbus_initiate_unload(true);
+
 	return NOTIFY_DONE;
 }
 static struct notifier_block hyperv_panic_vmbus_unload_block = {
@@ -2848,7 +2858,8 @@ static void hv_crash_handler(struct pt_regs *regs)
 {
 	int cpu;
 
-	vmbus_initiate_unload(true);
+	if (!skip_vmbus_unload)
+		vmbus_initiate_unload(true);
 	/*
 	 * In crash handler we can't schedule synic cleanup for all CPUs,
 	 * doing the cleanup for current CPU only. This should be sufficient
diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
index dfc516c1c719..b0502a336eb3 100644
--- a/include/linux/hyperv.h
+++ b/include/linux/hyperv.h
@@ -1334,6 +1334,9 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
 			bool fb_overlap_ok);
 void vmbus_free_mmio(resource_size_t start, resource_size_t size);
 
+void vmbus_initiate_unload(bool crash);
+void vmbus_set_skip_unload(bool skip);
+
 /*
  * GUID definitions of various offer types - services offered to the guest.
  */
-- 
2.25.1

[PATCH v2 2/2] drm/hyperv: During panic do VMBus unload after frame buffer is flushed

[Michael Kelley]

From: Michael Kelley <mhklinux@outlook.com>

In a VM, Linux panic information (reason for the panic, stack trace,
etc.) may be written to a serial console and/or a virtual frame buffer
for a graphics console. The latter may need to be flushed back to the
host hypervisor for display.

The current Hyper-V DRM driver for the frame buffer does the flushing
*after* the VMBus connection has been unloaded, such that panic messages
are not displayed on the graphics console. A user with a Hyper-V graphics
console is left with just a hung empty screen after a panic. The enhanced
control that DRM provides over the panic display in the graphics console
is similarly non-functional.

Commit 3671f3777758 ("drm/hyperv: Add support for drm_panic") added
the Hyper-V DRM driver support to flush the virtual frame buffer. It
provided necessary functionality but did not handle the sequencing
problem with VMBus unload.

Fix the full problem by using VMBus functions to suppress the VMBus
unload that is normally done by the VMBus driver in the panic path. Then
after the frame buffer has been flushed, do the VMBus unload so that a
kdump kernel can start cleanly. As expected, CONFIG_DRM_PANIC must be
selected for these changes to have effect. As a side benefit, the
enhanced features of the DRM panic path are also functional.

Fixes: 3671f3777758 ("drm/hyperv: Add support for drm_panic")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
Changes in v2: Removed test of CONFIG_PRINTK in deciding whether
   to have VMBus skip the unload. A separate patch by Jocelyn Falempe
   incorporates the CONFIG_PRINTK dependency into CONFIG_DRM_PANIC.

 drivers/gpu/drm/hyperv/hyperv_drm_drv.c     |  5 +++++
 drivers/gpu/drm/hyperv/hyperv_drm_modeset.c | 15 ++++++++-------
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_drv.c b/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
index 06b5d96e6eaf..b6bf6412ae34 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_drv.c
@@ -150,6 +150,10 @@ static int hyperv_vmbus_probe(struct hv_device *hdev,
 		goto err_free_mmio;
 	}
 
+	/* If DRM panic path is stubbed out VMBus code must do the unload */
+	if (IS_ENABLED(CONFIG_DRM_PANIC))
+		vmbus_set_skip_unload(true);
+
 	drm_client_setup(dev, NULL);
 
 	return 0;
@@ -169,6 +173,7 @@ static void hyperv_vmbus_remove(struct hv_device *hdev)
 	struct drm_device *dev = hv_get_drvdata(hdev);
 	struct hyperv_drm_device *hv = to_hv(dev);
 
+	vmbus_set_skip_unload(false);
 	drm_dev_unplug(dev);
 	drm_atomic_helper_shutdown(dev);
 	vmbus_close(hdev->channel);
diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
index 7978f8c8108c..d48ca6c23b7c 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_modeset.c
@@ -212,15 +212,16 @@ static void hyperv_plane_panic_flush(struct drm_plane *plane)
 	struct hyperv_drm_device *hv = to_hv(plane->dev);
 	struct drm_rect rect;
 
-	if (!plane->state || !plane->state->fb)
-		return;
+	if (plane->state && plane->state->fb) {
+		rect.x1 = 0;
+		rect.y1 = 0;
+		rect.x2 = plane->state->fb->width;
+		rect.y2 = plane->state->fb->height;
 
-	rect.x1 = 0;
-	rect.y1 = 0;
-	rect.x2 = plane->state->fb->width;
-	rect.y2 = plane->state->fb->height;
+		hyperv_update_dirt(hv->hdev, &rect);
+	}
 
-	hyperv_update_dirt(hv->hdev, &rect);
+	vmbus_initiate_unload(true);
 }
 
 static const struct drm_plane_helper_funcs hyperv_plane_helper_funcs = {
-- 
2.25.1

Re: [PATCH v2 1/2] Drivers: hv: vmbus: Provide option to skip VMBus unload on panic

[Wei Liu]

Dexuan and Long, can you share your thoughts on this patch?

On Tue, Feb 17, 2026 at 10:23:34AM -0800, Michael Kelley wrote:
> From: Michael Kelley <mhklinux@outlook.com>
> 
> Currently, VMBus code initiates a VMBus unload in the panic path so
> that if a kdump kernel is loaded, it can start fresh in setting up its
> own VMBus connection. However, a driver for the VMBus virtual frame
> buffer may need to flush dirty portions of the frame buffer back to
> the Hyper-V host so that panic information is visible in the graphics
> console. To support such flushing, provide exported functions for the
> frame buffer driver to specify that the VMBus unload should not be
> done by the VMBus driver, and to initiate the VMBus unload itself.
> Together these allow a frame buffer driver to delay the VMBus unload
> until after it has completed the flush.
> 
> Ideally, the VMBus driver could use its own panic-path callback to do
> the unload after all frame buffer drivers have finished. But DRM frame
> buffer drivers use the kmsg dump callback, and there are no callbacks
> after that in the panic path. Hence this somewhat messy approach to
> properly sequencing the frame buffer flush and the VMBus unload.
> 
> Fixes: 3671f3777758 ("drm/hyperv: Add support for drm_panic")
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
> Changes in v2: None
> 
>  drivers/hv/channel_mgmt.c |  1 +
>  drivers/hv/hyperv_vmbus.h |  1 -
>  drivers/hv/vmbus_drv.c    | 25 ++++++++++++++++++-------
>  include/linux/hyperv.h    |  3 +++
>  4 files changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
> index 74fed2c073d4..5de83676dbad 100644
> --- a/drivers/hv/channel_mgmt.c
> +++ b/drivers/hv/channel_mgmt.c
> @@ -944,6 +944,7 @@ void vmbus_initiate_unload(bool crash)
>  	else
>  		vmbus_wait_for_unload();
>  }
> +EXPORT_SYMBOL_GPL(vmbus_initiate_unload);
>  
>  static void vmbus_setup_channel_state(struct vmbus_channel *channel,
>  				      struct vmbus_channel_offer_channel *offer)
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index cdbc5f5c3215..5d3944fc93ae 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -440,7 +440,6 @@ void hv_vss_deinit(void);
>  int hv_vss_pre_suspend(void);
>  int hv_vss_pre_resume(void);
>  void hv_vss_onchannelcallback(void *context);
> -void vmbus_initiate_unload(bool crash);
>  
>  static inline void hv_poll_channel(struct vmbus_channel *channel,
>  				   void (*cb)(void *))
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 6785ad63a9cb..97dfa529d250 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -69,19 +69,29 @@ bool vmbus_is_confidential(void)
>  }
>  EXPORT_SYMBOL_GPL(vmbus_is_confidential);
>  
> +static bool skip_vmbus_unload;
> +
> +/*
> + * Allow a VMBus framebuffer driver to specify that in the case of a panic,
> + * it will do the VMbus unload operation once it has flushed any dirty
> + * portions of the framebuffer to the Hyper-V host.
> + */
> +void vmbus_set_skip_unload(bool skip)
> +{
> +	skip_vmbus_unload = skip;
> +}
> +EXPORT_SYMBOL_GPL(vmbus_set_skip_unload);
> +
>  /*
>   * The panic notifier below is responsible solely for unloading the
>   * vmbus connection, which is necessary in a panic event.
> - *
> - * Notice an intrincate relation of this notifier with Hyper-V
> - * framebuffer panic notifier exists - we need vmbus connection alive
> - * there in order to succeed, so we need to order both with each other
> - * [see hvfb_on_panic()] - this is done using notifiers' priorities.
>   */
>  static int hv_panic_vmbus_unload(struct notifier_block *nb, unsigned long val,
>  			      void *args)
>  {
> -	vmbus_initiate_unload(true);
> +	if (!skip_vmbus_unload)
> +		vmbus_initiate_unload(true);
> +
>  	return NOTIFY_DONE;
>  }
>  static struct notifier_block hyperv_panic_vmbus_unload_block = {
> @@ -2848,7 +2858,8 @@ static void hv_crash_handler(struct pt_regs *regs)
>  {
>  	int cpu;
>  
> -	vmbus_initiate_unload(true);
> +	if (!skip_vmbus_unload)
> +		vmbus_initiate_unload(true);
>  	/*
>  	 * In crash handler we can't schedule synic cleanup for all CPUs,
>  	 * doing the cleanup for current CPU only. This should be sufficient
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index dfc516c1c719..b0502a336eb3 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1334,6 +1334,9 @@ int vmbus_allocate_mmio(struct resource **new, struct hv_device *device_obj,
>  			bool fb_overlap_ok);
>  void vmbus_free_mmio(resource_size_t start, resource_size_t size);
>  
> +void vmbus_initiate_unload(bool crash);
> +void vmbus_set_skip_unload(bool skip);
> +
>  /*
>   * GUID definitions of various offer types - services offered to the guest.
>   */
> -- 
> 2.25.1
>