Re: [libvirt] Proposal PCI/PCIe device placement on PAPR guests

There was a discussion back in November on the qemu list which spilled onto the libvirt list about how to add support for PCIe devices to POWER VMs, specifically 'pseries' machine type PAPR guests. Here's a more concrete proposal for how to handle part of this in future from the libvirt side. Strictly speaking what I'm suggesting here isn't intrinsically linked to PCIe: it will make adding PCIe support sanely easier, as well as having a number of advantages for both PCIe and plain-PCI devices on PAPR guests. Background: * Currently the pseries machine type only supports vanilla PCI buses. * This is a qemu limitation, not something inherent - PAPR guests running under PowerVM (the IBM hypervisor) can use passthrough PCIe devices (PowerVM doesn't emulate devices though). * In fact the way PCI access is para-virtalized in PAPR makes the usual distinctions between PCI and PCIe largely disappear * Presentation of PCIe devices to PAPR guests is unusual * Unlike x86 - and other "bare metal" platforms, root ports are not made visible to the guest. i.e. all devices (typically) appear as though they were integrated devices on x86 * In terms of topology all devices will appear in a way similar to a vanilla PCI bus, even PCIe devices * However PCIe extended config space is accessible * This means libvirt's usual placement of PCIe devices is not suitable for PAPR guests * PAPR has its own hotplug mechanism * This is used instead of standard PCIe hotplug * This mechanism works for both PCIe and vanilla-PCI devices * This can hotplug/unplug devices even without a root port P2P bridge between it and the root "bus * Multiple independent host bridges are routine on PAPR * Unlike PC (where all host bridges have multiplexed access to configuration space) PCI host bridges (PHBs) are truly independent for PAPR guests (disjoint MMIO regions in system address space) * PowerVM typically presents a separate PHB to the guest for each host slot passed through The Proposal: I suggest that libvirt implement a new default algorithm for placing (i.e. assigning addresses to) both PCI and PCIe devices for (only) PAPR guests. The short summary is that by default it should assign each device to a separate vPHB, creating vPHBs as necessary. * For passthrough sometimes a group of host devices can't be safely isolated from each other - this is known as a (host) Partitionable Endpoint (PE). In this case, if any device in the PE is passed through to a guest, the whole PE must be passed through to the same vPHB in the guest. From the guest POV, each vPHB has exactly one (guest) PE. * To allow for hotplugged devices, libvirt should also add a number of additional, empty vPHBs (the PAPR spec allows for hotplug of PHBs, but this is not yet implemented in qemu). When hotplugging a new device (or PE) libvirt should locate a vPHB which doesn't currently contain anything. * libvirt should only (automatically) add PHBs - never root ports or other PCI to PCI bridges In order to handle migration, the vPHBs will need to be represented in the domain XML, which will also allow the user to override this topology if they want. Advantages: There are still some details I need to figure out w.r.t. handling PCIe devices (on both the qemu and libvirt sides). However the fact that

PAPR guests don't typically see PCIe root ports means that the normal libvirt PCIe allocation scheme won't work. This scheme has several advantages with or without support for PCIe devices: * Better performance for 32-bit devices With multiple devices on a single vPHB they all must share a (fairly small) 32-bit DMA/IOMMU window. With separate PHBs they each have a separate window. PAPR guests have an always-on guest visible IOMMU. * Better EEH handling for passthrough devices EEH is an IBM hardware-assisted mechanism for isolating and safely resetting devices experiencing hardware faults so they don't bring down other devices or the system at large. It's roughly similar to PCIe AER in concept, but has a different IBM specific interface, and works on both PCI and PCIe devices. Currently the kernel interfaces for handling EEH events on passthrough devices will only work if there is a single (host) iommu group in the vfio container. While lifting that restriction would be nice, it's quite difficult to do so (it requires keeping state synchronized between multiple host groups). That also means that an EEH error on one device could stop another device where that isn't required by the actual hardware. The unit of EEH isolation is a PE (Partitionable Endpoint) and currently there is only one guest PE per vPHB. Changing this might also be possible, but is again quite complex and may result in confusing and/or broken distinctions between groups for EEH isolation and IOMMU isolation purposes. Placing separate host groups in separate vPHBs sidesteps these problems. * Guest NUMA node assignment of devices PAPR does not (and can't reasonably) use the pxb device. Instead to allocate devices to different guest NUMA nodes they should be placed on different vPHBs. Placing them on different PHBs by default allows NUMA node to be assigned to those PHBs in a straightforward manner.