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

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Message-ID: <ff23k7d3uhdskwnx42ogdcno72qwkcuxjcbgoocesvjf3saybq@drqmfu6k3bwy>
Date: Mon, 24 Nov 2025 22:29:25 +0100
From: "Winiarski, Michal" <michal.winiarski@...el.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: 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

On Fri, Nov 21, 2025 at 08:13:38AM +0100, Tian, Kevin wrote:
> > 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.

I'll replace it with pure mig_ops check, as the migration_reset_state()
is not optional.

> 
> > @@ -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.

Ok.

> 
> > @@ -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.
> 

I'll add the missing paths in v2.

Thanks,
-Michał