Re: [Qemu-devel] [RFC 0/2] Attempt to implement the standby feature for assigned network devices

[Daniel P . Berrangé]

On Fri, Dec 07, 2018 at 02:36:07PM -0200, Eduardo Habkost wrote:
> On Thu, Dec 06, 2018 at 10:06:18AM +0000, Daniel P. Berrangé wrote:
> > On Wed, Dec 05, 2018 at 02:24:32PM -0600, Michael Roth wrote:
> > > Quoting Daniel P. Berrangé (2018-12-05 11:18:18)
> > > > On Thu, Oct 25, 2018 at 05:06:29PM +0300, Sameeh Jubran wrote:
> > > > > From: Sameeh Jubran <address@hidden>
> > > > > 
> > > > > Hi all,
> > > > > 
> > > > > Background:
> > > > > 
> > > > > There has been a few attempts to implement the standby feature for 
> > > > > vfio
> > > > > assigned devices which aims to enable the migration of such devices. 
> > > > > This
> > > > > is another attempt.
> > > > > 
> > > > > The series implements an infrastructure for hiding devices from the 
> > > > > bus
> > > > > upon boot. What it does is the following:
> > > > > 
> > > > > * In the first patch the infrastructure for hiding the device is added
> > > > >   for the qbus and qdev APIs. A "hidden" boolean is added to the 
> > > > > device
> > > > >   state and it is set based on a callback to the standby device which
> > > > >   registers itself for handling the assessment: "should the primary 
> > > > > device
> > > > >   be hidden?" by cross validating the ids of the devices.
> > > > > 
> > > > > * In the second patch the virtio-net uses the API to hide the vfio
> > > > >   device and unhides it when the feature is acked.
> > > > 
> > > > IIUC, the general idea is that we want to provide a pair of associated 
> > > > NIC
> > > > devices to the guest, one emulated, one physical PCI device. The guest 
> > > > would
> > > > put them in a bonded pair. Before migration the PCI device is unplugged 
> > > > & a
> > > > new PCI device plugged on target after migration. The guest traffic 
> > > > continues
> > > > without interuption due to the emulate device.
> > > > 
> > > > This kind of conceptual approach can already be implemented today by 
> > > > management
> > > > apps. The only hard problem that exists today is how the guest OS can 
> > > > figure
> > > > out that a particular pair of devices it has are intended to be used 
> > > > together. 
> > > > 
> > > > With this series, IIUC, the virtio-net device is getting a given 
> > > > property which
> > > > defines the qdev ID of the associated VFIO device. When the guest OS 
> > > > activates
> > > > the virtio-net device and acknowledges the STANDBY feature bit, qdev 
> > > > then
> > > > unhides the associated VFIO device.
> > > > 
> > > > AFAICT the guest has to infer that the device which suddenly appears is 
> > > > the one
> > > > associated with the virtio-net device it just initialized, for purposes 
> > > > of
> > > > setting up the NIC bonding. There doesn't appear to be any explicit 
> > > > assocation
> > > > between the devices exposed to the guest.
> > > > 
> > > > This feels pretty fragile for a guest needing to match up devices when 
> > > > there
> > > > are many pairs of devices exposed to a single guest.
> > > 
> > > The impression I get from 
> > > linux.git:Documentation/networking/net_failover.rst 
> > > is that the matching is done based on the primary/standby NICs having
> > > the same MAC address. In theory you pass both to a guest and based on
> > > MAC it essentially does automatic, and if you additionally add STANDBY
> > > it'll know to use a virtio-net device specifically for failover.
> > > 
> > > None of this requires any sort of hiding/plugging of devices from
> > > QEMU/libvirt (except for the VFIO unplug we'd need to initiate live 
> > > migration
> > > and the VFIO hotplug on the other end to switch back over).
> > > 
> > > That simplifies things greatly, but also introduces the problem of how
> > > an older guest will handle seeing 2 NICs with the same MAC, which IIUC
> > > is why this series is looking at hotplugging the VFIO device only after
> > > we confirm STANDBY is supported by the virtio-net device, and why it's
> > > being done transparent to management.
> > >
> > > > 
> > > > Unless I'm mis-reading the patches, it looks like the VFIO device 
> > > > always has
> > > > to be available at the time QEMU is started. There's no way to boot a 
> > > > guest
> > > > and then later hotplug a VFIO device to accelerate the existing 
> > > > virtio-net NIC.
> > > > Or similarly after migration there might not be any VFIO device 
> > > > available
> > > > initially when QEMU is started to accept the incoming migration. So it 
> > > > might
> > > > need to run in degraded mode for an extended period of time until one 
> > > > becomes
> > > > available for hotplugging. The use of qdev IDs makes this troublesome, 
> > > > as the
> > > > qdev ID of the future VFIO device would need to be decided upfront 
> > > > before it
> > > > even exists.
> > > 
> > > > 
> > > > So overall I'm not really a fan of the dynamic hiding/unhiding of 
> > > > devices. I
> > > > would much prefer to see some way to expose an explicit relationship 
> > > > between
> > > > the devices to the guest.
> > > 
> > > If we place the burden of determining whether the guest supports STANDBY
> > > on the part of users/management, a lot of this complexity goes away. For
> > > instance, one possible implementation is to simply fail migration and say
> > > "sorry your VFIO device is still there" if the VFIO device is still around
> > > at the start of migration (whether due to unplug failure or a
> > > user/management forgetting to do it manually beforehand).
> > > 
> > > So how important is it that setting F_STANDBY cap doesn't break older
> > > guests? If the idea is to support live migration with VFs then aren't
> > > we still dead in the water if the guest boots okay but doesn't have
> > > the requisite functionality to be migrated later? Shouldn't that all
> > > be sorted out as early as possible? Is a very clear QEMU error message
> > > in this case insufficient?
> > > 
> > > And if backward compatibility is important, are there alternative
> > > approaches? Like maybe starting off with a dummy MAC and switching over
> > > to the duplicate MAC only after F_STANDBY is negotiated? In that case
> > > we could still warn users/management about it but still have the guest
> > > be otherwise functional.
> > 
> > Relying on F_STANDBY negotiation to decide whether to activate the VFIO
> > device is a bad idea. PCI devices are precious, so if the guest OS does
> > not support this standby feature, we must never add the VFIO device to
> > QEMU in the first place.
> > 
> > We have the libosinfo project which provides metadata on what features
> > different guest OS versions support. This can be used to indicate whether
> > a guest OS version supports the standby NIC concept and thus avoid needing
> > to allocate PCI devices to guests that will never use them.
> > 
> > F_STANDBY is still useful as a flag to inform the guest OS that it should
> > pair up NICs with identical MACs, as opposed to configuring them separately.
> > It shouldn't be used to show/hide the device though, we should simply never
> > add the 2nd device if we know it won't be used by a given guest OS version.
> 
> The two mechanisms are not exclusive.  Not wasting a PCI device
> if the guest OS won't use it is a good idea.  Making the guest
> behave gracefully even when an older driver is loaded is also
> useful.

I'm not convinced it is useful enough to justify playing games in qdev
with dynamically hiding devices. This adds complexity to the code which
will make it harder to maintain and debug at runtime.


Regards,
Daniel
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