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

[Laine Stump <laine@xxxxxxxxxx>]

(Sorry for any duplicates. I sent it from the wrong address the first time)

On 12/01/2016 11:18 PM, David Gibson wrote:

On Fri, Nov 25, 2016 at 02:46:21PM +0100, Andrea Bolognani wrote:

On Wed, 2016-11-23 at 16:00 +1100, David Gibson wrote:

Existing libvirt versions assume that pseries guests have
a legacy PCI root bus, and will base their PCI address
allocation / PCI topology decisions on that fact: they
will, for example, use legacy PCI bridges.

  Um.. yeah.. trouble is libvirt's PCI-E address allocation probably
won't work for spapr PCI-E either, because of the weird PCI-E without
root complex presentation we get in PAPR.

So, would the PCIe Root Bus in a pseries guest behave
differently than the one in a q35 or mach-virt guest?

Yes.  I had a long discussion with BenH and got a somewhat better idea
about this.

If only a single host PE (== iommu group) is passed through and there
are no emulated devices, the difference isn't too bad: basically on
pseries you'll see the subtree that would be below the root complex on
q35.

But if you pass through multiple groups, things get weird.  On q35,
you'd generally expect physically separate (different slot) devices to
appear under separate root complexes.  Whereas on pseries they'll
appear as siblings on a virtual bus (which makes no physical sense for
point-to-point PCI-E).

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.

Does it have a different number of slots, do we have to
plug different controllers into them, ...?

Regardless of how we decide to move forward with the
PCIe-enabled pseries machine type, libvirt will have to
know about this so it can behave appropriately.

So there are kind of two extremes of how to address this.  There are a
variety of options in between, but I suspect they're going to be even
more muddled and hideous than the extremes.

1) Give up.  You said there's already a flag that says a PCI-E bus is
able to accept vanilla-PCI devices.  We add a hack flag that says a
vanilla-PCI bus is able to accept PCI-E devices.  We keep address
allocation as it is now - the pseries topology really does resemble
vanilla-PCI much better than it does PCI-E. But, we allow PCI-E
devices, and PAPR has mechanisms for accessing the extended config
space.  PCI-E standard hotplug and error reporting will never work,
but PAPR provides its own mechanisms for those, so that should be ok.

2) Start exposing the PCI-E heirarchy for pseries guests much more
like q35, root complexes and all.  It's not clear that PAPR actually
*forbids* exposing the root complex, it just doesn't require it and
that's not what PowerVM does.  But.. there are big questions about
whether existing guests will cope with this or not.  When you start
adding in multiple passed through devices and particularly virtual
functions as well, things could get very ugly - we might need to
construct multiple emulated virtual root complexes or other messes.

In the short to medium term, I'm thinking option (1) seems pretty
compelling.

I believe after we introduced the very first
pseries-pcie-X.Y, we will just stop adding new pseries-X.Y.

  Isn't i440fx still being updated despite the fact that q35
exists? Granted, there are a lot more differences between
those two machine types than just the root bus type.

  Right, there are heaps of differences between i440fx and q35, and
reasons to keep both updated.  For pseries we have neither the impetus
nor the resources to maintain two different machine type variant,
where the only difference is between legacy PCI and weirdly presented
PCI-E.

Calling the PCIe machine type either pseries-2.8 or
pseries-pcie-2.8 would result in the very same amount of
work, and in both cases it would be understood that the
legacy PCI machine type is no longer going to be updated,
but can still be used to run existing guests.

So, I'm not sure if the idea of a new machine type has legs or not,
but let's think it through a bit further.  Suppose we have a new
machine type, let's call it 'papr'.  I'm thinking it would be (at
least with -nodefaults) basically a super-minimal version of pseries:
so each PHB would have to be explicitly created, the VIO bridge would
have to be explicitly created, likewise the NVRAM.  Not sure about the
"devices" which really represent firmware features - the RTC, RNG,
hypervisor event source and so forth.

Might have some advantages.  Then again, it doesn't really solve the
specific problem here.  It means libvirt (or the user) has to
explicitly choose a PCI or PCI-E PHB to put things on, but libvirt's
PCI-E address allocation will still be wrong in all probability.

That's a broad statement. Why? If qemu reports the default devices and characteristics of the devices properly (and libvirt uses that information) there's no reason for it to make the wrong decision.

Guh.


As an aside, here's a RANT.

libvirt address allocation.  Seriously, W. T. F!

libvirt insists on owning address allocation.  That's so it can
recreate the exact same machine at the far end of a migration.  So far
so good, except it insists on recording that information in the domain
XML in kinda-sorta-but-not-really back end independent form.

Explain "kinda-sorta-but-not-really". If there's a deficiency in the model maybe it can be fixed.

  But the
thing is libvirt fundamentally CAN NOT get this right.

True, but not for the reasons you think. If qemu is able to respond to queries with adequate details about the devices available for a machinetype (and what buses are in place by default), there's no reason that libvirt can't add devices addressed such that all the connections are legal; what libvirt *can't* get right is the policy requested in the next higher layer of management (and ultimately of the user) - does this device need to be hotpluggable? Does the user want to keep all devices on the root complex to avoid extra PCI controllers?

And qemu fundamentally CAN NOT get it right either. qemu knows what is possible and what is allowed, but it doesn't know what the user *wants* (beyond "they want device X", which is only 1/2 the story), and has no way of being told what the user wants other than with a PCI address.

To back up for a minute, some background info: once a device has been added to a domain, at *any* time in the future (not just during a migration, but forever more until the end of time) that device must always have the same PCI address as it had that first time. In order to guarantee that, libvirt needs to either:

a) keep track of the order the devices were added and always put the devices in the same order on the commandline (assuming that qemu guarantees that it actually assigns addresses based on the order of the devices' appearance on the commandline, which has never been stated anywhere as an API guarantee of qemu),

or

b) remember the address of each device as it is added and specify that on the commandline in the future. libvirt chooses (b). And where is the logical place to store that address? In the config.

So we've established that the PCI address of a device needs to be stored in the config. So why does libvirt need to choose it the first time?

1) Because qemu doesn't have (and CAN NOT have) all the information about what are the user's plans for that device:

  a) It has no idea if the user wants that device to be hotpluggable (on a root-port)
      or not (on root complex as an integrated device)

  b) it doesn't know if the user wants the device to be placed on an expander bus
      so that its NUMA status can be discovered by the guest OS.

If there is a choice, there must be a way to make that choice. The way that qemu provides to make the choice is by specifying an address. So libvirt must specify an address in its config.

2) Because qemu is unable/unwilling to automatically add PCIe root ports when necessary, it's *not even possible* (on PCIe machinetypes) for it to place a device on a hotpluggable port without libvirt specifying a PCI address the very first time the device is added (and also adding in a root-port), but libvirt's default policy is that (almost) all devices should be hotpluggable. If we were to follow your recommendation ("libvirt never specifies PCI addresses, but instead allows qemu to assign them"), hotplug on PCIe-based machinetypes would not be possible, though.

There have even been mentions that even libvirt is too *low* in the stack to be specifying the PCI address of devices (i.e. that all PCI address decisions should be up to higher level management applications) - I had posted a patch that would allow specifying "hotpluggable='yes|no'" in the XML rather than forcing the call to specify an address, and this was NACKed because it was seen as libvirt dictating policy. (In the end, libvirt *does* dictate a default policy, (it's just that the only way to modify that policy is by manually specifying addresses) - libvirt's default PCI address policy is that (almost) all devices will be assigned an address that makes them hotpluggable, and will not be placed on a non-0 NUMA node.

So, in spite of libvirt's effort, in the end we *still* need to expose address configuration to higher level management applications, since they may want to force all devices onto the root complex (e.g. libguestfs, which does it to reduce PCI controller count, and thus startup time) or force certain devices to be on a non-0 NUMA node (e.g. OpenStack when it wants to place a VFIO assigned device on the same NUMA node in the guest as it is in the host).

With all of that, I fail to see how it would be at all viable to simply leave PCI address assignment up to qemu.

There are all
sorts of possible machine specific address allocation constraints that
can exist - from simply which devices are already created by default
(for varying values of "default") to complicated constraints depending
on details of board wiring.  The back end has to know about these - it
implements them.

And since qemu knows about them, it should be able to report them. Which is what Eduardo's work is doing. And then libvirt will know about all the constraints in an programmatic manner (rather than the horrible (tedious, error prone) hardcoding of all those details that we've had to suffer with until now).

  The ONLY way libvirt can get this (temporarily)
right is by duplicating a huge chunk of the back end's allocation
logic, which will inevitably get out of date causing problems just
like this.

Basically the back end will *always* have better information about how
to place devices than libvirt can.

And since no matter how hard qemu might try to come up with a policy for address assignment that will satisfy the needs of 100% of the users 100% of the time, it will fail (because different users have different needs). Because qemu will be unable to properly place all devices all the time, libvirt (and higher level management) will still need to do it. Even in the basic case qemu doesn't provide what libvirt requires as default - that devices be hotpluggable.

   So, libvirt should be allowing the
back end to do the allocation, then snapshotting that in a back end
specific format which can be used for creating migration
destinations. But that breaks libvirt's the-domain-XML-is-everything
model.

No, that doesn't work because qemu would in many situations place the devices at the wrong address / on the wrong controller, because there are many possible topologies that are legal, and the user may (for perfectly valid reasons) want something different from what qemu would have chosen.

(An example of two differing (and valid) policies - libguestfs needs guests to startup as quickly as possible, and thus wants as few PCI controllers as possible (this makes a noticeable difference in Linux boot time), so it wants all devices to be integrated on the root complex. On the other hand, a generic guest in libvirt should make all devices hotpluggable just in case the user wants to unplug them, so by default it tries to place all devices on a pcie-root-port. You can't support both of these if addressing is all left up to qemu)

In this regard libvirt doesn't just have a design flaw, it has design
flaws which breed more design flaws like pestilent cancer.

It may make you feel good to say that, but the facts don't back it up. Any project makes design mistakes, but in the specific case you're discussing here, I think you haven't looked from a wide enough viewpoint to see the necessity of what libvirt is doing and why it can't be done by qemu (certainly not all the time anyway).

   And what's
worse the consequences of those design flaws are now making sane
design decisions increasingly difficult in adjacent projects like
qemu.

libvirt has always done the best that could be done with the information provided by qemu. The problem isn't that libvirt is creating new problems for qemu out of thin air, it's that qemu is unable to automatically address PCI devices for all possible situations and user policy preferences, so higher levels need to make the decisions about addressing to satisfy their policies (ie what they *want*, eg hotpluggable, integrated on root complex), and qemu hasn't (until Eduardo's patches) been able to provide adequate information about what is *legal* (e.g which devices can be plugged into which model of pci controller, what slots are available on each type of controller, whether those slots are hotpluggable) in a programmatic way, so libvirt has had to hardcode rules about bus-device compatibility and capabilities, slot ranges, etc in order to make proper decisions itself when possible, and to sanity-check decisions about addresses made by higher level management when not. I don't think that's a design flaw. I think that's making the best of a "less than ideal" situation.

I'd feel better about this if there seemed to be some recognition of
it, and some necessarily long term plan to improve it, but if there is
I haven't heard of it.  Or at least the closest thing seems to be
coming from the qemu side (witness Eduardo's talk at the last KVM
forum, and mine at the one before).

Eduardo's work isn't being done to make up for some mythical design flaw in libvirt. It is being done in order to give libvirt the (previously unavailable) information it needs to do a necessary job, and is being done at least partly at the request of libvirt (we've certainly been demanding some of that stuff for a long time!)

The summary is that it's impossible for qemu to correctly decide where to put new devices, especially in a PCIe hierarchy for a few reasons (at least); because of this, libvirt (and higher level management) needs to be able to assign addresses to devices, and in order for us/them to be able to do that properly, qemu needs to provide detailed and accurate information about what buses/controllers/devices are in each machinetype, what controllers/devices are available to add, and what are the legal ways of connecting those devices and controllers.

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