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 <[email protected]> > > 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 <[email protected]> > --- > 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 >
