Re: [libvirt] [PATCH 2/2] hw/vfio/display: add ramfb support
On Fri, 14 Sep 2018 16:19:07 +0200
Erik Skultety <eskultet(a)redhat.com> wrote:
On Fri, Sep 14, 2018 at 12:50:09PM +0200, Gerd Hoffmann wrote:
> Hi,
>
> > > Also libvirt manages hotpluggability per device *class*, not per device
> > > *instance*. So a device being hotpluggable or not depending on some
> > > device property is a problem for libvirt ...
> > >
> > > I'm open to suggestions how to handle this better, as long as the
> > > libvirt people are on board with the approach.
> >
> > Ok, so we need a new class to handle making a device non-hotpluggable,
> > but I'm still not sure whether we should make:
> >
> > -device vfio-pci-ramfb
> >
> > or
> >
> > -device vfio-pci-nohotplug,ramfb=on
> >
> > Where ramfb would be a property only available on the nohotplug class
> > variant.
>
> I'm fine with the latter.
>
> > The latter seems to provide a lot more flexibility, but which
> > is more practical for libvirt?
>
> Any comment from the libvirt camp?
We had a discussion about this a few months ago [1] where we spoke about
-device vfio-pci-ramfb.
Ah yes, probably my bad for not following up more thoroughly there.
However, as Alex has pointed out, the latter proposal
gives us more flexibility in terms of introduction of other device properties
which are unrelated to ramfb but still might require non-hotpluggable device.
Either way, libvirt needs a capability to test whether we should favour this
new device over plain vfio-pci if an mdev with display='on' is required.
What about new device properties (specifically mdev)? In the discussion below,
Gerd noted that apart from the ramfb stuff and the fact that one can be
hotplugged while the latter can not, these are identical (option-wise), is that
to stay, IOW are we going to keep these two device classes in sync when
introducing new vfio-pci device options or are these going to divert more? Is
it even possible? What I mean by that is that I'd like to avoid is a situation
where there are 2 disjunct sets of options which could potentially lead to
problems in decision making in libvirt and we don't like making decisions.
The vfio-pci device is the parent of this new device, so it should
automatically inherit any new properties of vfio-pci, it only modifies
the device class for non-hotpluggability and adds properties dependent
on non-hotpluggability. I'm not sure if libvirt would expose this as a
new model, ie. model="vfio-pci-nohotplug", or if it would be selected
via attribute, ie. nohotplug="on", or perhaps if enabling a property
only found on the nohotplug variant would select it, ie. ramfb="on".
The latter option alone makes it difficult for a user to select it for
any random device, for instance if they're trying to setup a kiosk VM
where they want to prevent even the guest OS admin from changing the VM
configuration. In any case, it seems that libvirt would never be
enabling this automatically.
Anyhow, I don't feel like any of the proposals has a strong
advantage/disadvantage in usage for libvirt, both will require a capability and
both would be special cased in our cmdline code depending on the 'display'
attribute. Luckily, we don't have mdev migration yet, so it's good we don't
have to worry about that at this point yet.
That's a good point that ramfb depends on display, it seems that
regardless of which route we take, using vfio-pci-ramfb or
vfio-pci-nohotplug,ramfb=on, it should fail without a display rather
than simply adding functionality if a display is present or in the
former case, being an obscure way to make a device non-hotpluggable.
Personally I prefer the non-hotplug variant of vfio-pci in hopes that
it provides more flexibility to users and we only need to tackle this
issue once rather than each device we invent with a non-hotplug
dependency. Thanks,
Alex