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

[Wei Liu <wei.liu@kernel.org>]

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
>