[libvirt] How should libvirt apps enable virtio-pci for aarch64?

[Daniel P. Berrange]

On Mon, Dec 07, 2015 at 10:37:38AM -0500, Cole Robinson wrote:
> On 12/07/2015 07:27 AM, Daniel P. Berrange wrote:
> > On Sun, Dec 06, 2015 at 09:46:56PM -0500, Cole Robinson wrote:
> >> Hi all,
> >>
> >> I'm trying to figure out how apps should request virtio-pci for libvirt + qemu
> >> + arm/aarch64. Let me provide some background.
> >>
> >> qemu's arm/aarch64 original virtio support is via virtio-mmio, libvirt XML
> >> <address type='virtio-mmio'/>. Currently this is what libvirt sets as the
> >> address default for all arm/aarch64 virtio devices in the XML. Long term
> >> though all arm virt will likely be using virtio-pci: it's faster, enables
> >> hotplug, is more x86 like, etc.
> >>
> >> Support for virtio-pci is newer and not as widespread. qemu has had the
> >> necessary support since 2.4 at least, but the guest side isn't well
> >> distributed yet. For example, Fedora 23 and earlier don't work out of the box
> >> with virtio-pci. Internal RHELSA (RHEL Server for Aarch64) builds have it
> >> recently working AFAIK.
> >>
> >> Libvirt has some support for enabling virtio-pci with aarch64, commits added
> >> by Pavel Fedin in v1.2.19. (See e8d55172544c1fafe31a9e09346bdebca4f0d6f9). The
> >> patches add a PCIe controller automatically to the XML (and qemu commandline)
> >> if qemu-system-aarch64 supports it. However virtio-mmio is still used as the
> >> default virtio address, given the current lack of OS support.
> >>
> >> So we are at the point where libvirt apps want to enable this, but presently
> >> there isn't a good solution; the only option is to fully allocate <address
> >> type='pci' ...> for each virtio device in the XML. This is suboptimal for 2
> >> reasons:
> >>
> >> #1) apps need to duplicate libvirt's non-trivial address type=pci allocation logic
> >>
> >> #2) apps have to add an <address> block for every virtio device, which is less
> >> friendly than the x86 case where this is rarely required. Any XML device
> >> snippets that work for x86 likely won't give the desired result for aarch64,
> >> since they will default to virtio-mmio. Think virsh attach-device/attach-disk
> >> commands
> > 
> > Yeah this is very undesirable for a default out of the box config - we should
> > always strive to "do the best thing" when no address is given.
> > 
> >> Here are some possible solutions:
> >>
> >> * Drop the current behavior of adding a PCIe controller unconditionally, and
> >> instead require apps to specify it in the XML. Then, if libvirt sees a PCIe
> >> controller in the XML, default the virtio address type to pci. Apps will know
> >> if the OS they are installing supports virtio-pci (eventually via libosinfo),
> >> so this is the way we can implicitly ask libvirt 'allocate us pci addresses'
> > 
> > Yes, clearly we need to record in libosinfo whether an OS can do PCI vs
> > MMIO.
> > 
> >> Upsides:
> >> - Solves both the stated problems.
> >> - Simplest addition for applications IMO
> >>
> >> Downsides:
> >> - Requires a libvirt behavior change, no longer adding the PCIe controller by
> >> default. But in practice I don't think it will really affect anyone, since
> >> there isn't really any OS support for virtio-pci yet, and no apps support it
> >> either AFAIK.
> >> - The PCIe controller is not strictly about virtio-pci, it's for enabling
> >> plain emulated PCI devices as well. So there is a use case for using the PCIe
> >> controller for a graphics card even while your OS doesn't yet support
> >> virtio-pci. In the big picture though this is a small time window with current
> >> OS, and users can work around it by manually requesting <address
> >> type='virtio-mmio'/>, so medium/long term this isn't a big deal IMO
> >> - The PCIe controller XML is:
> >>     <controller type='pci' index='0' model='pcie-root'/>
> >>     <controller type='pci' index='1' model='dmi-to-pci-bridge'/>
> >>     <controller type='pci' index='2' model='pci-bridge'/>
> >> I have no idea if that's always going to be the expected XML, maybe it's not
> >> wise to hardcode that in apps. Laine?
> >>
> >>
> >> * Next idea: Users specify something like like <address type='pci'/> and
> >> libvirt fills in the address for us.
> >>
> >> Upsides:
> >> - We can stick with the current PCIe controller default and avoid some of the
> >> problems mentioned above.
> >> - An auto address feature may be useful in other contexts as well.
> >>
> >> Downsides:
> >> - Seems potentially tricky to implement in libvirt code. There's many places
> >> that check type=pci and key off that, seems like it would be easy to miss
> >> updating a check and cause regressions. Maybe we could add a new type like
> >> auto-pci to make it explicit. There's probably some implementation trick to
> >> make this safe, but at first glance it looked a little dicey.
> > 
> > I'm not sure it is actually all that hairy - it might be as simple as
> > updating only qemuAssignDevicePCISlots so that instread of:
> > 
> >     if (def->controllers[i]->info.type != VIR_DOMAIN_DEVICE_ADDRESS_TYPE_NONE)
> >                 continue;
> > 
> > It handles type=pci with no values set too
> > 
> 
> I think my fear was that there are other places in domain_conf that check for
> ADDRESS_TYPE_PCI before we even get to assigning PCI slots. But i'll poke at it.

Its always possible, but I'd rather hope nothing actually is. The only thing
that should care about specific addresses is QEMU itself - the specific choice
of bus/slot/func shouldn't be relevant to any logic inside libvirt except the
command line generator - at most code should care whether it is PCI or not.


Regards,
Daniel
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