[libvirt] [Qemu-ppc] [RFC PATCH qemu] spapr_pci: Create PCI-express root bus by default

Andrea Bolognani abologna at redhat.com
Wed Dec 7 16:42:03 UTC 2016

[Added Marcel to CC]

On Wed, 2016-12-07 at 15:11 +1100, David Gibson wrote:
> > Is the difference between q35 and pseries guests with
> > respect to PCIe only relevant when it comes to assigned
> > devices, or in general? I'm asking this because you seem to
> > focus entirely on assigned devices.
> Well, in a sense that's up to us.  The only existing model we have is
> PowerVM, and PowerVM only does device passthrough, no emulated
> devices.  PAPR doesn't really distinguish one way or the other, but
> it's written from the perspective of assuming that all PCI devices
> correspond to physical devices on the host

Okay, that makes sense.

> > > On q35,
> > > you'd generally expect physically separate (different slot) devices to
> > > appear under separate root complexes.
>> > This part I don't get at all, so please bear with me.
>> > The way I read it you're claiming that eg. a SCSI controller
> > and a network adapter, being physically separate and assigned
> > to separate PCI slots, should have a dedicated PCIe Root
> > Complex each on a q35 guest.
> Right, my understanding was that if the devices were slotted, rather
> than integrated, each one would sit under a separate root complex, the
> root complex being a pseudo PCI to PCI bridge.

I assume "slotted" means "plugged into a slot that's not one
of those provided by pcie.0" or something along those lines.

More on the root complex bit later.

> > That doesn't match with my experience, where you would simply
> > assign them to separate slots of the default PCIe Root Bus
> > (pcie.0), eg. 00:01.0 and 00:02.0.
> The qemu default, or the libvirt default?

I'm talking about the libvirt default, which is supposed to
follows Marcel's PCIe Guidelines.

> I think this represents
> treating the devices as though they were integrated devices in the
> host bridge.  I believe on q35 they would not be hotpluggable

Yeah, that's indeed not quite what libvirt would do by
default: in reality, there would be a ioh3420 between the
pcie.0 slots and each device exactly to enable hotplug.

> but on
> pseries they would be (because we don't use the standard hot plug
> controller).

We can account for that in libvirt and avoid adding the
extra ioh3420s (or rather the upcoming generic PCIe Root
Ports) for pseries guests.

> > Maybe you're referring to the fact that you might want to
> > create multiple PCIe Root Complexes in order to assign the
> > host devices to separate guest NUMA nodes? How is creating
> > multiple PCIe Root Complexes on q35 using pxb-pcie different
> > than creating multiple PHBs using spapr-pci-host-bridge on
> > pseries?
> Uh.. AIUI the root complex is the PCI to PCI bridge under which PCI-E
> slots appear.  PXB is something different - essentially different host
> bridges as you say (though with some weird hacks to access config
> space, which make it dependent on the primary bus in a way which spapr
> PHBs are not).
> I'll admit I'm pretty confused myself about the exact distinction
> between root complex, root port and upstream and downstream ports.

I think we both need to get our terminology straight :)
I'm sure Marcel will be happy to point us in the right

My understanding is that the PCIe Root Complex is the piece
of hardware that exposes a PCIe Root Bus (pcie.0 in QEMU);
PXBs can be connected to slots in pcie.0 to create more buses
that behave, for the most part, like pcie.0 and are mostly
useful to bind devices to specific NUMA nodes. Same applies
to legacy PCI with the pxb (instead of pxb-pcie) device.

In a similar fashion, PHBs are the hardware thingies that
expose a PCI Root Bus (pci.0 and so on), the main difference
being that they are truly independent: so a q35 guest will
always have a "primary" PCIe Root Bus and (optionally) a
bunch of "secondary" ones, but the same will not be the case
for pseries guests.

I don't think the difference is that important though, at
least from libvirt's point of view: whether you're creating
a pseries guest with two PHBs, or a q35 guest with its
built-in PCIe Root Complex and an extra PCIe Expander Bus,
you will end up with two "top level" buses that you can plug
more devices into. If we had spapr-pcie-host-bridge, we
could treat them mostly the same - with caveats such as the
one described above, of course.

> > > Whereas on pseries they'll
> > > appear as siblings on a virtual bus (which makes no physical sense for
> > > point-to-point PCI-E).
>> > What is the virtual bus in question? Why would it matter
> > that they're siblings?
> On pseries it won't.  But my understanding is that libvirt won't
> create them that way on q35 - instead it will insert the RCs / P2P
> bridges to allow them to be hotplugged.  Inserting that bridge may
> confuse pseries guests which aren't expecting it.

libvirt will automatically add PCIe Root Ports to make the
devices hotpluggable on q35 guests, yes. But, as mentioned
above, we can teach it not to.

> > I'm possibly missing the point entirely, but so far it
> > looks to me like there are different configurations you
> > might want to use depending on your goal, and both q35
> > and pseries give you comparable tools to achieve such
> > configurations.
> > > 
> > > I suppose we could try treating all devices on pseries as though they
> > > were chipset builtin devices on q35, which will appear on the root
> > > PCI-E bus without root complex.  But I suspect that's likely to cause
> > > trouble with hotplug, and it will certainly need different address
> > > allocation from libvirt.
>> > PCIe Integrated Endpoint Devices are not hotpluggable on
> > q35, that's why libvirt will follow QEMU's PCIe topology
> > recommendations and place a PCIe Root Port between them;
> > I assume the same could be done for pseries guests as
> > soon as QEMU grows support for generic PCIe Root Ports,
> > something Marcel has already posted patches for.
> Here you've hit on it.  No, we should not do that for pseries,
> AFAICT.  PAPR doesn't really have the concept of integrated endpoint
> devices, and all devices can be hotplugged via the PAPR mechanisms
> (and none can via the PCI-E standard hotplug mechanism).

Cool, I get it now.

> > Again, sorry for clearly misunderstanding your explanation,
> > but I'm still not seeing the issue here. I'm sure it's very
> > clear in your mind, but I'm afraid you're going to have to
> > walk me through it :(
> I wish it were entirely clear in my mind.  Like I say I'm still pretty
> confused by exactly the root complex entails.

Same here, but this back-and-forth is helping! :)

> > What about virtio devices, which present themselves either
> > as legacy PCI or PCIe depending on the kind of slot they
> > are plugged into? Would they show up as PCIe or legacy PCI
> > on a PCIe-enabled pseries guest?
> That we'd have to address on the qemu side with some 

Unfinished sentence?

> > Is the Root Complex not currently exposed? The Root Bus
> > certainly is,
> Like I say, I'm fairly confused myself, but I'm pretty sure that Root
> Complex != Root Bus.  The RC sits under the root bus IIRC.. or
> possibly it consists of the root bus plus something under it as well.
> Now... from what Laine was saying it sounds like more of the
> differences between PCI-E placement and PCI placement may be
> implemented by libvirt than qemu than I realized.  So possibly we do
> want to make the bus be PCI-E on the qemu side, but have libvirt use
> the vanilla-PCI placement guidelines rather than PCI-E for pseries.

Basically the special casing I was mentioning earlier.

> > Maybe I just don't quite get the relationship between Root
> > Complexes and Root Buses, but I guess my question is: what
> > is preventing us from simply doing whatever a
> > spapr-pci-host-bridge is doing in order to expose a legacy
> > PCI Root Bus (pci.*) to the guest, and create a new
> > spapr-pcie-host-bridge that exposes a PCIe Root Bus (pcie.*)
> > instead?
> Hrm, the suggestion of providing both a vanilla-PCI and PCI-E host
> bridge came up before.  I think one of us spotted a problem with that,
> but I don't recall what it was now.  I guess one is how libvirt would
> map it's stupid-fake-domain-numbers to which root bus to use.

That issue is relevant whether or nor we have different PHB
flavors, isn't it? As soon as multiple PHBs are present in
a pseries guest, multiple PCI domains will be there as well,
and we need to handle that somehow.

On q35, on the other hand, I haven't been able to find a way
to create extra PCI domains: adding a pxb-pcie certainly
didn't work the same as adding an extra spapr-pci-host-bridge
in that regard.

> > Maybe we should have a different model, specific to
> > pseries guests, instead, so that all PHBs would look the
> > same in the guest XML, something like
>> >   <controller type='pci' model='phb-pcie'/>
>> > It would require shuffling libvirt's PCI address allocation
> > code around quite a bit, but it should be doable. And if it
> > makes life easier for our users, then it's worth it.
> Hrm.  So my first inclination would be to stick with the generic
> names, and map those to creating new pseries host bridges on pseries
> guests.  I would have thought that would be the easier option for
> users.  But I may not have realized all the implications yet.

You're probably right, but I can't immediately see how we
would make the user aware of which PHB is which. Maybe we
could add some sub-element or extra attribute...

Anyway, we should not focus too much on this specific bit
at the moment, deciding on a specific XML is mostly
bikeshedding :)

> > * Eduardo's work, which you mentioned, is going to be very
> >   beneficial in the long run; in the short run, Marcel's
> >   PCIe device placement guidelines, a document that has seen
> >   contributions from QEMU, OVMF and libvirt developers, have
> >   been invaluable to improve libvirt's PCI address allocation
> >   logic. So we're already doing better, and more improvements
> >   are on the way :)
> Right.. so here's the thing, I strongly suspect that Marcel's
> guidelines will not be correct for pseries.  I'm not sure if they'll
> be definitively wrong, or just different enough from PowerVM that it
> might confuse guests, but either way.

Those guidelines have been developed with q35/mach-virt in
mind[1], so I wouldn't at all be surprised if they didn't
apply to pseries guests. And in fact, we just found out
that they don't!

My point is that we could easily create a similar document
for pseries guests, and then libvirt will be able to pick
up whatever recommendations we come up with just like it
did for q35/mach-virt.

> Can you send me a link to that
> document though, which might help me figure this out.

It's docs/pcie.txt in QEMU's git repository.

[1] Even though I now realize that this is not immediately
    clear by looking at the document itself
Andrea Bolognani / Red Hat / Virtualization

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