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SubjectRe: [PATCHv2 08/10] vfio/mdev: Improve the create/remove sequence
[vfio-ap folks: find a question regarding removal further down]

On Wed, 8 May 2019 22:06:48 +0000
Parav Pandit <parav@mellanox.com> wrote:

> > -----Original Message-----
> > From: Cornelia Huck <cohuck@redhat.com>
> > Sent: Wednesday, May 8, 2019 12:10 PM
> > To: Parav Pandit <parav@mellanox.com>
> > Cc: kvm@vger.kernel.org; linux-kernel@vger.kernel.org;
> > kwankhede@nvidia.com; alex.williamson@redhat.com; cjia@nvidia.com
> > Subject: Re: [PATCHv2 08/10] vfio/mdev: Improve the create/remove
> > sequence
> >
> > On Tue, 30 Apr 2019 17:49:35 -0500
> > Parav Pandit <parav@mellanox.com> wrote:
> >
> > > This patch addresses below two issues and prepares the code to address
> > > 3rd issue listed below.
> > >
> > > 1. mdev device is placed on the mdev bus before it is created in the
> > > vendor driver. Once a device is placed on the mdev bus without
> > > creating its supporting underlying vendor device, mdev driver's probe()
> > gets triggered.
> > > However there isn't a stable mdev available to work on.
> > >
> > > create_store()
> > > mdev_create_device()
> > > device_register()
> > > ...
> > > vfio_mdev_probe()
> > > [...]
> > > parent->ops->create()
> > > vfio_ap_mdev_create()
> > > mdev_set_drvdata(mdev, matrix_mdev);
> > > /* Valid pointer set above */
> > >
> > > Due to this way of initialization, mdev driver who want to use the

s/want/wants/

> > > mdev, doesn't have a valid mdev to work on.
> > >
> > > 2. Current creation sequence is,
> > > parent->ops_create()
> > > groups_register()
> > >
> > > Remove sequence is,
> > > parent->ops->remove()
> > > groups_unregister()
> > >
> > > However, remove sequence should be exact mirror of creation sequence.
> > > Once this is achieved, all users of the mdev will be terminated first
> > > before removing underlying vendor device.
> > > (Follow standard linux driver model).
> > > At that point vendor's remove() ops shouldn't failed because device is

s/failed/fail/

> > > taken off the bus that should terminate the users.

"because taking the device off the bus should terminate any usage" ?

> > >
> > > 3. When remove operation fails, mdev sysfs removal attempts to add the
> > > file back on already removed device. Following call trace [1] is observed.
> > >
> > > [1] call trace:
> > > kernel: WARNING: CPU: 2 PID: 9348 at fs/sysfs/file.c:327
> > > sysfs_create_file_ns+0x7f/0x90
> > > kernel: CPU: 2 PID: 9348 Comm: bash Kdump: loaded Not tainted
> > > 5.1.0-rc6-vdevbus+ #6
> > > kernel: Hardware name: Supermicro SYS-6028U-TR4+/X10DRU-i+, BIOS 2.0b
> > > 08/09/2016
> > > kernel: RIP: 0010:sysfs_create_file_ns+0x7f/0x90
> > > kernel: Call Trace:
> > > kernel: remove_store+0xdc/0x100 [mdev]
> > > kernel: kernfs_fop_write+0x113/0x1a0
> > > kernel: vfs_write+0xad/0x1b0
> > > kernel: ksys_write+0x5a/0xe0
> > > kernel: do_syscall_64+0x5a/0x210
> > > kernel: entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > >
> > > Therefore, mdev core is improved in following ways.
> > >
> > > 1. Before placing mdev devices on the bus, perform vendor drivers
> > > creation which supports the mdev creation.

"invoke the vendor driver ->create callback" ?

> > > This ensures that mdev specific all necessary fields are initialized

"that all necessary mdev specific fields are initialized" ?

> > > before a given mdev can be accessed by bus driver.
> > > This follows standard Linux kernel bus and device model similar to
> > > other widely used PCI bus.

"This follows standard practice on other Linux device model buses." ?

> > >
> > > 2. During remove flow, first remove the device from the bus. This
> > > ensures that any bus specific devices and data is cleared.
> > > Once device is taken of the mdev bus, perform remove() of mdev from

s/of/off/

> > > the vendor driver.
> > >
> > > 3. Linux core device model provides way to register and auto
> > > unregister the device sysfs attribute groups at dev->groups.

"The driver core provides a way to automatically register and
unregister sysfs attributes via dev->groups." ?

> > > Make use of this groups to let core create the groups and simplify
> > > code to avoid explicit groups creation and removal.
> > >
> > > A below stack dump of a mdev device remove process also ensures that
> > > vfio driver guards against device removal already in use.
> > >
> > > cat /proc/21962/stack
> > > [<0>] vfio_del_group_dev+0x216/0x3c0 [vfio] [<0>]
> > > mdev_remove+0x21/0x40 [mdev] [<0>]
> > > device_release_driver_internal+0xe8/0x1b0
> > > [<0>] bus_remove_device+0xf9/0x170
> > > [<0>] device_del+0x168/0x350
> > > [<0>] mdev_device_remove_common+0x1d/0x50 [mdev] [<0>]
> > > mdev_device_remove+0x8c/0xd0 [mdev] [<0>] remove_store+0x71/0x90
> > > [mdev] [<0>] kernfs_fop_write+0x113/0x1a0 [<0>] vfs_write+0xad/0x1b0
> > > [<0>] ksys_write+0x5a/0xe0 [<0>] do_syscall_64+0x5a/0x210 [<0>]
> > > entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > [<0>] 0xffffffffffffffff
> > >
> > > This prepares the code to eliminate calling device_create_file() in
> > > subsquent patch.

I find this stack dump and explanation more confusing than
enlightening. Maybe just drop it?

> >
> > I'm afraid I have a bit of a problem following this explanation, so let me try
> > to summarize what the patch does to make sure that I understand it
> > correctly:
> >
> > - Add the sysfs groups to device->groups so that the driver core deals
> > with proper registration/deregistration.
> > - Split the device registration/deregistration sequence so that some
> > things can be done between initialization of the device and hooking
> > it up to the infrastructure respectively after deregistering it from
> > the infrastructure but before giving up our final reference. In
> > particular, this means invoking the ->create and ->remove callback in
> > those new windows. This gives the vendor driver an initialized mdev
> > device to work with during creation.
> > - Don't allow ->remove to fail, as the device is already removed from
> > the infrastructure at that point in time.
> >
> You got all the points pretty accurate.

Ok, good.

>
> > >
> > > Signed-off-by: Parav Pandit <parav@mellanox.com>
> > > ---
> > > drivers/vfio/mdev/mdev_core.c | 94 +++++++++-----------------------
> > > drivers/vfio/mdev/mdev_private.h | 2 +-
> > > drivers/vfio/mdev/mdev_sysfs.c | 2 +-
> > > 3 files changed, 27 insertions(+), 71 deletions(-)
> >
> > (...)

> > > @@ -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.

> samples/vfio-mdev/mbochs.c mbochs_remove() always returns 0.
>
> > >
> > > - ret = mdev_device_remove_ops(mdev, force_remove);
> > > - if (ret) {
> > > - mdev->active = true;
> > > - return ret;
> > > - }
> > > -
> > > - mdev_remove_sysfs_files(dev, type);
> > > - device_unregister(dev);
> > > + /* Balances with device_initialize() */
> > > + put_device(&mdev->dev);
> > > mdev_put_parent(parent);
> > >
> > > return 0;
> >
> > I think that looks sane in general, but the commit message might benefit
> > from tweaking.
> Part of your description is more crisp than my commit message, I can probably take snippet from it to improve?
> Or any specific entries in commit message that I should address?

I have added some comments inline (mostly some wording tweaks).

Feel free to take anything from my summary as well.

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