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

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Date:   Thu, 11 Nov 2021 12:32:00 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "hch@....de" <hch@....de>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "parav@...lanox.com" <parav@...lanox.com>,
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        "corbet@....net" <corbet@....net>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "yi.l.liu@...ux.intel.com" <yi.l.liu@...ux.intel.com>,
        "Tian, Jun J" <jun.j.tian@...el.com>, "Wu, Hao" <hao.wu@...el.com>,
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Subject: RE: [RFC 02/20] vfio: Add device class for /dev/vfio/devices

Hi Jason,

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, November 3, 2021 9:26 PM
> 
> 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;
> +}

you are right. After checking your ccw patch, the private free triggered
by vfio_ccw_free_private() should use kfree_rcu(). So it is not quite
same with other vfio_device users which only need kfree() to free the
vfio_device. So how can I address the difference when moving the vfio_device
alloc/free into vfio core? any suggestion?

@@ -164,14 +173,14 @@ static void vfio_ccw_free_private(struct vfio_ccw_private *private)
 	kmem_cache_free(vfio_ccw_io_region, private->io_region);
 	kfree(private->cp.guest_cp);
 	mutex_destroy(&private->io_mutex);
-	kfree(private);
+	vfio_uninit_group_dev(&private->vdev);
+	kfree_rcu(private, rcu);
 }

https://lore.kernel.org/kvm/10-v3-57c1502c62fd+2190-ccw_mdev_jgg@nvidia.com/

Regards,
Yi Liu