linux-kernel - RE: [PATCH 1/6] vfio: Introduce .migration_reset

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Message-ID: <BN9PR11MB52765AD421658A2C867AD77E8CD5A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 21 Nov 2025 07:13:38 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Winiarski, Michal" <michal.winiarski@...el.com>, Jason Gunthorpe
	<jgg@...pe.ca>, Alex Williamson <alex@...zbot.org>, Yishai Hadas
	<yishaih@...dia.com>, Longfang Liu <liulongfang@...wei.com>, Shameer Kolothum
	<skolothumtho@...dia.com>, Brett Creeley <brett.creeley@....com>, "Cabiddu,
 Giovanni" <giovanni.cabiddu@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, qat-linux <qat-linux@...el.com>,
	"virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/6] vfio: Introduce .migration_reset_state() callback

> From: Winiarski, Michal <michal.winiarski@...el.com>
> Sent: Thursday, November 20, 2025 8:37 PM
> 
> Resetting the migration device state is typically delegated to PCI
> .reset_done() callback.
> With VFIO, reset is usually called under vdev->memory_lock, which causes
> lockdep to report a following circular locking dependency scenario:
> 
> 0: set_device_state
> driver->state_mutex -> migf->lock
> 1: data_read
> migf->lock -> mm->mmap_lock
> 2: vfio_pin_dma
> mm->mmap_lock -> vdev->memory_lock
> 3: vfio_pci_ioctl_reset
> vdev->memory_lock -> driver->state_mutex
> 
> Introduce a .migration_reset_state() callback called outside of
> vdev->memory_lock to break the dependency chain.

so it kind of unifies the deferred_reset logic cross all drivers.

sounds reasonable as nobody should expect a concrete sequence of
a reset done vs. a racing set_device_state.

> 
> +static void vfio_pci_dev_migration_reset_state(struct vfio_pci_core_device
> *vdev)
> +{
> +	lockdep_assert_not_held(&vdev->memory_lock);
> +
> +	if (!vdev->vdev.mig_ops->migration_reset_state)
> +		return;

mig_ops could be NULL.

> @@ -1230,6 +1242,8 @@ static int vfio_pci_ioctl_reset(struct
> vfio_pci_core_device *vdev,
>  	ret = pci_try_reset_function(vdev->pdev);
>  	up_write(&vdev->memory_lock);
> 
> +	vfio_pci_dev_migration_reset_state(vdev);
> +

only if the previous reset succeeds.

> @@ -2486,8 +2501,10 @@ static int vfio_pci_dev_set_hot_reset(struct
> vfio_device_set *dev_set,
> 
>  err_undo:
>  	list_for_each_entry_from_reverse(vdev, &dev_set->device_list,
> -					 vdev.dev_set_list)
> +					 vdev.dev_set_list) {
>  		up_write(&vdev->memory_lock);
> +		vfio_pci_dev_migration_reset_state(vdev);
> +	}

ditto

btw some reset paths are missed in drivers/vfio/pci/vfio_pci_config.c,
e.g. for vFLR emulation.