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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, May 11, 2026 at 01:10:43PM -0500, Tushar Dave wrote:
> 
> 
> 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


I see no advantage in performing pci enumeration in qemu when firmware
is already doing an adequate job of it. If you want firmware to map
specific devices at specific addresses, pass that info along to it.

-- 
MST