On Tue, May 12, 2026 at 05:06:50PM -0600, Alex Williamson wrote: > On Tue, 12 May 2026 12:25:45 -0500 > Tushar Dave <[email protected]> wrote: > > > On 5/11/2026 6:43 AM, Ard Biesheuvel wrote: > > > Hello Tushar, > > > > > > On Fri, 8 May 2026, at 20:37, Tushar Dave via groups.io wrote: > > >> This RFC introduces a mechanism to specify Guest Physical Addresses > > >> (GPAs) for PCI BARs, allowing explicit placement of guest MMIO BAR > > >> addresses to match host physical addresses for assigned devices. > > >> > > >> On some platforms, P2P DMA is performed between devices within the same > > >> IOMMU group. The PCI fabric ACS is configured to permit direct P2P > > >> without going through the host bridge in order to achieve the required > > >> performance. > > >> > > >> To support this multi-device IOMMU group P2P scenario in virtualization, > > >> the VM may need to use the same MMIO BAR addresses as the host physical > > >> address layout. > > >> > > > > > > Did you consider implementing this using Enhanced Allocation (EA)? If so, > > > could you explain why it is not suitable here? > > > > I have not evaluated EA for this design. When I looked at EDK2, I > > chose PcdPciDisableBusEnumeration because it cleanly preserves fixed > > BAR programming established by the hypervisor — at the cost of QEMU > > performing PCI bus number and resource assignment. > > > > I did a quick search and do not see EA support in EDK2. Any pointers > > to EA being used in a similar fashion to achieve fixed BAR placement > > would be appreciated. > > EA wasn't on my radar either, but I did some research and chatted with > Tushar and I think it could work. I'll sketch out a rough idea of what > it might looks like. > > EA describes BAR equivalents (fixed base address, size, and type) in a > separate capability while the corresponding device BAR registers appear > unimplemented. Linux already consumes endpoint EA capabilities and > marks the resulting resources IORESOURCE_PCI_FIXED. EDK2 doesn't know > about EA (cap 0x14 isn't defined anywhere in MdePkg, and PciBusDxe > never consults it afaict), but that turns out to be useful here rather > than a problem. > > Starting at the QEMU device, for a vfio-pci device we'd need to > virtualize the real BARs as unimplemented and surface that information > via a synthesized EA capability instead. It's debatable whether this > is a generic PCI mechanism or vfio-pci specific, whether HPA is > automatically used as the base address for vfio-pci devices or > user-specified, and the capability offset in config space. None of > those fundamentally change the shape of the flow. > > For the absolute bare-minimum level of support (EA device on the root > complex, EA resources don't overlap the VM address space or MMIO range, > EDK2 firmware, Linux guest booted with pci=nocrs) I think this actually > works with just adding the EA capability above. Let's walk through > those constraints and how we relax them. > > At the firmware level we lean on the real BAR registers being > unimplemented for EA devices, so EDK2 allocates no MMIO or IO resources > for them. Only bus numbers get assigned if the EA device sits in a PCI > hierarchy. That's exactly what we want, EDK2 doing conventional bus > assignment but staying out of the EA resource flow entirely. > > Instead of firmware EA enlightenment we lean on the guest OS. Linux > reads endpoint EA today, but the bridge aperture sizing path ignores > those fixed resources. As Tushar's series demonstrates, generically > handling mixed "fixed-BAR" and programmable-BAR devices in one > hierarchy is hard. An incremental Linux enhancement that greatly > simplifies the problem space would be to program bridge apertures only > for hierarchies consisting entirely of fixed resources. The math > becomes trivial (window spans min..max of fixed children, aligned to > bridge granularity), and there's no regression risk, these hierarchies > currently fail silently. The sizer ignores fixed children and the > fixed-claim walk-up finds no containing parent. This enhancement, > plus the homogeneous-hierarchy constraint, removes the root-complex > constraint and lets us mirror the bare-metal topologies we need. > > Resource ranges are a bit messier. The extent of the EA device ranges > could be determined in QEMU and the VM address map adjusted to prevent > overlap. Tushar already has a similar user-specified machine option in > this series. That range also needs to reach the guest as a CRS (to > avoid pci=nocrs) but needs to stay distinct from the DT range passed to > EDK2 for programmable BAR devices so EDK2 won't place a programmable > BAR or bridge window into the EA region. So long as we keep EA and > programmable devices in separate hierarchies, EDK2 only needs the > programmable range via DT and we can add the EA range as additional CRS > ranges visible only to the guest. > > In practice, EDK2 programs all the programmable devices and the EA > devices live entirely in the additional CRS. A possibly cleaner > alternative is additional PXB host bridges for the EA devices, each > with its own CRS. That sidesteps the DT/CRS split entirely since the > EA PXB has nothing for EDK2 to allocate anyway. > > If we agree that homogeneous hierarchies (no mixing of EA and > programmable BARs) is a reasonable constraint, and possibly extend that > to homogeneous per host bridge to simplify the CRS mapping, we have the > following work items: > > * Extend Linux EA support to program bridge apertures for subordinate > homogeneous EA hierarchies. > > * Develop options to virtualize programmable BARs as EA for vfio-pci > devices, if not generically for the benefit of testing. > > * Implement a way to poke holes in the VM address space and plumb > through to account for addresses used by EA devices. > > * Provide those same ranges to the guest via CRS (but not via DT to > EDK2), or alternatively expose them through additional PXB host > bridges. > > Does that shape roughly seem accurate? Are there additional gaps I've > missed? Thanks, > > Alex
just one question why not do it in firmware so windows is thinkably also handled? -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#121944): https://edk2.groups.io/g/devel/message/121944 Mute This Topic: https://groups.io/mt/119221703/21656 Group Owner: [email protected] Unsubscribe: https://edk2.groups.io/g/devel/unsub [[email protected]] -=-=-=-=-=-=-=-=-=-=-=-
