linux-kernel - Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

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Message-ID: <20211103132547.GM2744544@nvidia.com>
Date:   Wed, 3 Nov 2021 10:25:47 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
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Subject: Re: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

On Tue, Nov 02, 2021 at 09:53:29AM +0000, Liu, Yi L wrote:

> > 	vfio_uninit_group_dev(&mdev_state->vdev);
> > 	kfree(mdev_state->pages);
> > 	kfree(mdev_state->vconfig);
> > 	kfree(mdev_state);
> > 
> > pages/vconfig would logically be in a release function
> 
> I see. So the criteria is: the pointer fields pointing to a memory buffer
> allocated by the device driver should be logically be free in a release
> function. right? 

Often yes, that is usually a good idea

>I can see there are such fields in struct vfio_pci_core_device
> and mdev_state (both mbochs and mdpy). So we may go with your option #2.
> Is it? otherwise, needs to add release callback for all the related drivers.

Yes, that is the approx trade off

> > On the other hand ccw needs to rcu free the vfio_device, so that would
> > have to be global overhead with this api design.
> 
> not quite get. why ccw is special here? could you elaborate?

I added a rcu usage to it in order to fix a race

+static inline struct vfio_ccw_private *vfio_ccw_get_priv(struct subchannel *sch)
+{
+       struct vfio_ccw_private *private;
+
+       rcu_read_lock();
+       private = dev_get_drvdata(&sch->dev);
+       if (private && !vfio_device_try_get(&private->vdev))
+               private = NULL;
+       rcu_read_unlock();
+       return private;
+}
 

Jason