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SubjectRe: [PATCH RFC] vfio/mdev: delay uevent after initialization complete
On Wed, Feb 14, 2018 at 06:20:35PM +0100, Cornelia Huck wrote:
> [cc:ing Greg for his opinion on this; retaining quoting for context]

Ick, just found this in my inbox, sorry for the delay...

>
> On Tue, 13 Feb 2018 17:15:02 -0700
> Alex Williamson <alex.williamson@redhat.com> wrote:
>
> > On Tue, 13 Feb 2018 14:09:01 +0100
> > Cornelia Huck <cohuck@redhat.com> wrote:
> >
> > > On Mon, 12 Feb 2018 14:20:57 -0700
> > > Alex Williamson <alex.williamson@redhat.com> wrote:
> > >
> > > > On Fri, 9 Feb 2018 11:27:16 +0100
> > > > Cornelia Huck <cohuck@redhat.com> wrote:
> > > >
> > > > > The registration code first registers the mdev device, and then
> > > > > proceeds to populate sysfs. An userspace application that listens
> > > > > for the ADD uevent is therefore likely to look for sysfs entries
> > > > > that have not yet been created.
> > > > >
> > > > > The canonical way to fix this is to use attribute groups that are
> > > > > registered by the driver core before it sends the ADD uevent; I
> > > > > unfortunately did not find a way to make this work in this case,
> > > > > though.
> > > > >
> > > > > An alternative approach is to suppress uevents before we register
> > > > > with the core and generate the ADD uevent ourselves after the
> > > > > sysfs infrastructure is in place.
> > > > >
> > > > > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > > > > ---
> > > > >
> > > > > This feels like a band-aid, but I can't figure out how to handle creating
> > > > > attribute groups when there's a callback in the parent involved.
> > > > >
> > > > > This should address the issue with libvirt's processing of mdevs raised in
> > > > > https://www.redhat.com/archives/libvir-list/2018-February/msg00023.html
> > > > > - although libvirt will still need to deal with older kernels, of course.
> > > > >
> > > > > Best to consider this an untested patch :)
> > > >
> > > > I agree, this feels like a band-aide. If every device in the kernel
> > > > needs to suppress udev events until until some key component is added,
> > > > that suggests that either udev is broken in general or not being used
> > > > as intended.
> > >
> > > I think udev is working exactly as designed - it's more a problem of
> > > when the kernel is sending what kind of notification to userspace, and
> > > the particular issue here is how the code sending the event (driver
> > > core) and the code assembling part of the user interface (mdev)
> > > interact.
> > >
> > > > Zongyong submitted a different proposal to fix this
> > > > here[1]. That proposal seems a bit more sound and has precedence
> > > > elsewhere in the kernel. What do you think of that approach? We
> > > > don't need both afaict. Thanks,
> > > >
> > > > Alex
> > > >
> > > > [1]https://patchwork.kernel.org/patch/10196197/
> > >
> > > Zongyong's patch is sending an additional CHANGE uevent, and I agree
> > > that doing both does not make sense. However, I think the CHANGE uevent
> > > is not quite suitable in this case, and delaying the ADD uevent is
> > > better.
> > >
> > > [Warning, the following may be a bit rambling.]
> > >
> > > The Linux driver model works under the assumption that any device is
> > > represented as an in-kernel object that exposes information and knobs
> > > through sysfs. As long as the device exists, userspace can poke at the
> > > sysfs entries and retrieve information or configure things.
> > >
> > > The idea of the 'ADD' uevent is basically to let userspace know that
> > > there is now a new device with its related sysfs entries, and it may
> > > look at it and configure it. IOW, if I (as a userspace application) get
> > > the ADD uevent, I expect to be able to look at the device's sysfs
> > > entries and find all the files/directories that are usually there,
> > > without having to wait.
> > >
> > > This expectation is broken if a device is first registered with the
> > > driver core (generating the ADD uevent) and the driver adds sysfs
> > > attributes later. To fix this, the driver core added a way to specify
> > > default attribute groups for the device, which are registered by the
> > > driver core itself before it generates the ADD uevent. Unfortunately, I
> > > did not see a way to do this here (which does not mean there isn't).
> > > The alternative was to prevent the driver core from sending the ADD
> > > uevent and do it from the mdev code when it was ready.
> > >
> > > The 'CHANGE' uevent, on the other hand, tells userspace that something
> > > has changed for the device (that already existed). I (as a userspace
> > > application) would expect to see it if, for example, the information
> > > exposed via sysfs has changed, or maybe even if new, optional, entries
> > > have appeared and I might want to rescan. With Zongyong's patch,
> > > userspace gets the CHANGE uevent for something that was already
> > > expected to be there, and is now _really_ there. It does give userspace
> > > an indication that it can now work with the device (which certainly
> > > improves things), but I would prefer to get rid of the too-early uevent
> > > completely so that userspace does not get notified at all before the
> > > device is completely present in sysfs.
> > >
> > > So, in short, my patch does 'don't tell userspace until we're really
> > > done', and Zongyong's patch does 'tell userspace again when we're
> > > really done'.
> >
> > This all sounds reasonable, but don't we have this synchronization
> > problem _everywhere_? I apologize for referencing this bug because it's
> > not public (https://bugzilla.redhat.com/show_bug.cgi?id=1376907) but
> > the gist of it is that soft-unplugging PCI devices using the remove
> > entry in sysfs and re-adding with rescan sysfs entry results in libvirt
> > seeing the ADD uevent before the PCI config attribute is created and it
> > balks on the device.
>
> Yikes. So it seems that _any_ PCI device is incomplete when the ADD
> uevent is sent? Or does this just apply to an unfortunately large
> number of drivers?

What do you mean by "incomplete"? The device is added to the bus at
that point in time, yes. BUT, if a driver decides to add their own
attributes to the sysfs node, then that will happen afterward, unless
the bus has properly set that up (there are default attributes it can
use for that type of thing.)

It's a common problem that has been there since the beginning, and now
there's a new uevent that is "KOBJ_BIND" that is emitted when the driver
is finished being "bound" to the device. Is that what you are looking
for here?

> > So at the PCI core we have this same issue and
> > developers are saying that there's no guarantee that sysfs entries
> > won't be added and removed at any time in the lifecycle of the device
> > and it's not the kernel's responsibility to provide that
> > synchronization.
>
> I think this is mixing up two things:
> - sysfs entries that are there by default. These not being in place
> when the ADD uevent is sent sounds broken.

Yes it is, fix that in your bus code, default device attributes should
always be set up before ADD happens. Driver specific ones will be there
by the time BIND happens.

> - Optional sysfs entries that might change. There may be a case for
> those to be added/removed later on (probably paired with a CHANGE
> uevent.)

CHANGE is pretty rare, and best used for things that are polled. Or
major system events, like docking station changes. Not for things that
happen all the time (like power state changes.)

does that help?

thanks,

greg k-h

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