Re: [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 4:09 AM, Michael S. Tsirkin wrote:
> On Fri, May 08, 2026 at 01:37:09PM -0500, Tushar Dave 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.
> 
> Pass this info to guest firmware, let it set bars any way it wants?

We are using firmware, relying on the existing EDK2-supported mode
enabled by PcdPciDisableBusEnumeration, where firmware is expected
to preserve the PCI topology and BAR programming established by
the hypervisor.

In our case, the hypervisor is QEMU, which performs PCI enumeration
and resource assignment before handing control to firmware. EDK2
then explicitly refrains from re-enumerating or reallocating PCI
BARs, as this is already a supported firmware behavior.

-Tushar