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
> - Doesn't really solve problem #2 mentioned up above... maybe we could change
> the address allocation logic to default to virtio-pci if there's already a
> virtio-pci device in the XML. But it's more work.
> - More work for apps, but nothing horrible.
> * Change the default address type from virtio-mmio to pci, if qemu supports
> it. I'm listing this for completeness. In the short term this doesn't make
> sense as there isn't any OS releases that will work with this default. However
> it might be worth considering for the future, maybe keying off a particular
> qemu version or machine type. I suspect 2 years from now no one is going to be
> using virtio-mmio so long term it's not an ideal default.
Yeah, when QEMU starts doing versioned machine types for AArch64 we could
do this, but then this just kind of flips the problem around - apps now
need to manually add <address type="mmio"> for every device if deploying
an OS that can't do PCI. Admittedly this is slightly easier, since address
rules for mmio are simpler than address rules for PCI.
> I think the first option is best (keying off the PCIe controller specified by
> the user), with a longer term plan to change the default from mmio to pci. But
> I'm not really sold on anything either way. So I'm interested if anyone else
> has ideas.
I guess I'd tend towards option 1 too - only adding PCI controller if we
actually want to use PCI with the guest.
Regards,
Daniel
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