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

On 01/05/2017 12:46 AM, David Gibson wrote: > 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 It's a bit unconventional to leave all but one slot of a controller unused,

but your thinking makes sense. I don't think this will be as large/disruptive of a change as you might be expecting - we already have different addressing rules to automatically addressed vs. manually addressed, as well as a framework in place to behave differently for different PCI controllers (e.g. some support hotplug and others don't), and to modify behavior based on machinetype / root bus model, so it should be straightforward to do make things behave as you outline above.

(The first item in your list sounds exactly like VFIO iommu groups. Is that how it's exposed on PPC?

If so, libvirt already takes care of guaranteeing that any devices in the same group aren't used by other guests or the host during the time a guest is using a device.

It doesn't automatically assign the other devices to the guest though, since this could have unexpected effects on host operation (the example that kept coming up when this was originally discussed wrt vfio device assignment was the case where a disk device in use on the host was attached to a controller in the same iommu group as a USB controller that was going to be assigned to a guest - silently assigning the disk controller to the guest would cause the host's disk to suddenly become unusable).)

> 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. Well, the "normal libvirt PCIe allocation scheme" assumes "normal PCIe" :-).

> 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. So far libvirt doesn't try to assign PCI addresses to devices according to NUMA node, but assumes that the management application will manually address devices that need to be put on a particular pxb (it's only since the recent advent of the pxb that guests have become aware of multiple NUMA nodes). Possibly in the future libvirt will attempt to automatically place devices on a pxb that matches its NUMA node (if it exists). We don't want to force use of pxb for all guests on a host that has multiple NUMA nodes though. This might make more sense on PCC though since all devices are on a PHB, and each PHB can have a NUMA node set.