Re: [PATCHv2 08/10] vfio/mdev: Improve the create/remove sequence

[Pierre Morel]

On 09/05/2019 11:06, Cornelia Huck wrote:
> [vfio-ap folks: find a question regarding removal further down]
>
> On Wed, 8 May 2019 22:06:48 +0000
> Parav Pandit <parav@xxxxxxxxxxxx> wrote:
>
> > > -----Original Message-----
> > > From: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > Sent: Wednesday, May 8, 2019 12:10 PM
> > > To: Parav Pandit <parav@xxxxxxxxxxxx>
> > > Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > > kwankhede@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; cjia@xxxxxxxxxx
> > > Subject: Re: [PATCHv2 08/10] vfio/mdev: Improve the create/remove
> > > sequence
> > >
> > > On Tue, 30 Apr 2019 17:49:35 -0500
> > > Parav Pandit <parav@xxxxxxxxxxxx> wrote:
> > >    

...snip...

> > > > @@ -373,16 +330,15 @@ int mdev_device_remove(struct device *dev,
> > > bool force_remove)
> > > >   	mutex_unlock(&mdev_list_lock);
> > > >
> > > >   	type = to_mdev_type(mdev->type_kobj);
> > > > +	mdev_remove_sysfs_files(dev, type);
> > > > +	device_del(&mdev->dev);
> > > >   	parent = mdev->parent;
> > > > +	ret = parent->ops->remove(mdev);
> > > > +	if (ret)
> > > > +		dev_err(&mdev->dev, "Remove failed: err=%d\n", ret);
> > >
> > > I think carrying on with removal regardless of the return code of the
> > > ->remove callback makes sense, as it simply matches usual practice.
> > > However, are we sure that every vendor driver works well with that? I think
> > > it should, as removal from bus unregistration (vs. from the sysfs
> > > file) was always something it could not veto, but have you looked at the
> > > individual drivers?
> > >    
> > I looked at following drivers a little while back.
> > Looked again now.
> >
> > drivers/gpu/drm/i915/gvt/kvmgt.c which clears the handle valid in intel_vgpu_release(), which should finish first before remove() is invoked.
> >
> > s390 vfio_ccw_mdev_remove() driver drivers/s390/cio/vfio_ccw_ops.c remove() always returns 0.
> > s39 crypo fails the remove() once vfio_ap_mdev_release marks kvm null, which should finish before remove() is invoked.
>
> That one is giving me a bit of a headache (the ->kvm reference is
> supposed to keep us from detaching while a vm is running), so let's cc:
> the vfio-ap maintainers to see whether they have any concerns.

We are aware of this race and we did correct this in the IRQ patches for 
which it would have become a real issue.
We now increment/decrement the KVM reference counter inside open and 
release.
Should be right after this.

Thanks for the cc,
Pierre


--
Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany