RE: [PATCHv3 1/3] vfio/mdev: Improve the create/remove sequence

[Parav Pandit]

Hi Alex, Cornelia,

> -----Original Message-----
> From: Cornelia Huck <cohuck@xxxxxxxxxx>
> Sent: Wednesday, May 22, 2019 3:25 PM
> To: Parav Pandit <parav@xxxxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kwankhede@xxxxxxxxxx; alex.williamson@xxxxxxxxxx; cjia@xxxxxxxxxx
> Subject: Re: [PATCHv3 1/3] vfio/mdev: Improve the create/remove sequence
> 
> On Thu, 16 May 2019 18:30:32 -0500
> Parav Pandit <parav@xxxxxxxxxxxx> 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 wants to use the
> > 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 fail 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. Split the device registration/deregistration sequence so that some
> > things can be done between initialization of the device and hooking it
> > up to the bus respectively after deregistering it from the bus but
> > before giving up our final reference.
> > In particular, this means invoking the ->create and ->remove callbacks
> > in those new windows. This gives the vendor driver an initialized mdev
> > device to work with during creation.
> > At the same time, a bus driver who wish to bind to mdev driver also
> 
> s/who wish/that wishes/
> 
> > gets initialized mdev device.
> >
> > This follows standard Linux kernel bus and device model.
> >
> > 2. During remove flow, first remove the device from the bus. This
> > ensures that any bus specific devices are removed.
> > Once device is taken off the mdev bus, invoke remove() of mdev from
> > the vendor driver.
> >
> > 3. The driver core device model provides way to register and auto
> > unregister the device sysfs attribute groups at dev->groups.
> > Make use of dev->groups to let core create the groups and eliminate
> > code to avoid explicit groups creation and removal.
> >
> > To ensure, that new sequence is solid, a below stack dump of a process
> > is taken who attempts to remove the device while device is in use by
> > vfio driver and user application.
> > This stack dump validates that vfio driver guards against such device
> > removal when device is 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.
> >
> > Signed-off-by: Parav Pandit <parav@xxxxxxxxxxxx>
> > ---
> >  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(-)
> 
> Personally, I'd do a more compact patch description, but there's nothing
> really wrong with yours, either.
> 
> Patch also seems sane to me, although I'd probably have merged this and
> the next patch. But no reason to quibble further.
> 
> Reviewed-by: Cornelia Huck <cohuck@xxxxxxxxxx>

I missed to add your RB signature in v4.
If you send all 3 or just fist 2 of them in 5.2-rc, please include Cornelia's RB tag.