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

On Fri, Nov 25, 2016 at 02:46:21PM +0100, Andrea Bolognani wrote: 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. 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. 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. 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. But the thing is libvirt fundamentally CAN NOT get this right. 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. 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. 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. In this regard libvirt doesn't just have a design flaw, it has design flaws which breed more design flaws like pestilent cancer. And what's worse the consequences of those design flaws are now making sane design decisions increasingly difficult in adjacent projects like qemu. 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). -- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson