Re: [edk2-devel] [RFC PATCH 0/8] hw/arm/virt, hw/pci: PCI pre-enumeration and fixed BAR allocation

[Tushar Dave <tdave@nvidia.com>]

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.

> 
> Also, I think I understand what the intent is here, but could you describe
> the topology in a bit more detail? These are assigned physical PCIe endpoints
> behind an emulated host bridge, right? And the BAR needs to reside at an
> a priori fixed address so that another PCIe endpoint behind the same emulated
> host bridge can DMA straight into it?

Yes, that is all correct.

      -[0000:00]-+-00.0  Host bridge
                 +-01.0  Root Port
                     \-[0000:02]
                          +-00.0 Switch Upstream Port
                          +-01.0 Switch Downstream Port A
                          |      \-[0000:04] Device A
                          +-02.0 Switch Downstream Port B
                                 \-[0000:05] Device B

> 
> Doing PCIe enumeration at yet another level is not a feasible approach imo,
> having UEFI and Linux play nice together is already a bit of a challenge.

I agree but to clarify, in this case QEMU performs PCI topology
initialization and resource assignment prior to firmware execution,
where EDK2 avoids full PCI bus re-enumeration. Linux sees a fully
enumerated bus from firmware just as it does today. There is no
duplicated enumeration step between firmware and Linux when we use
EDK2 with PcdPciDisableBusEnumeration.

> 
> Is there any way this could be handled by having special rules for inbound
> translation in the host bridge driver/implementation?

Not that I can think of.

Thanks.
-Tushar