Re: [PATCH v2 0/5] hw/pci, hw/arm/virt: fixed PCI BAR placement

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

On Mon, Nov 02, 2026 at 12:12:36PM -0600, Tushar Dave wrote:
> This patch series 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.


My understanding is that all this does it pass a property to
the firmware asking it to set the bar to a given value?

There are two standard ways to do this:
- EA capability in pci config space
- fixed BAR _DSM in ACPI


If none of these are appropriate, pls document why.



> On some platforms, P2P DMA between devices in the same IOMMU group is required
> for the desired data path and performance. The PCI fabric ACS is configured
> to permit direct P2P without routing traffic through the host bridge.
> 
> To support this multi-device IOMMU group P2P scenario in virtualization, the
> VM may need to preserve the host PCI BAR addresses in the guest.
> 
> A new fixed-bar-N property on a PCI endpoint device specifies the required
> guest address for BAR N (N = 0 through 5):
> 
>     -device some-device,fixed-bar-0=<addr>[,fixed-bar-1=<addr>]...
> 
> Once any fixed-bar-N property is set on a device, every memory BAR on that
> device must have a fixed address. Under a PCIe root port, fixed BAR placement
> is all-or-nothing within that root port's subtree. If one endpoint below the
> root port has a fixed BAR, every other endpoint with a memory BAR below that
> same root port must also have all of its memory BARs fixed.
> 
> QEMU validates the configuration when the machine is done initializing. Each
> address must be aligned to its BAR size, fall within the appropriate PCIe MMIO
> aperture, and not overlap any other fixed BAR. QEMU aborts on any validation
> failure. QEMU performs neither PCI enumeration nor BAR allocation; the
> validated configuration is exported to firmware through the "etc/fixed-bars"
> fw_cfg blob, which carries each PCI function's vendor/device identification,
> firmware device path, and, for fixed devices, the configured address, size,
> and type of each BAR. The blob format is documented in
> docs/specs/pci-fixed-bars.rst.
> 
> The fixed-bar-N property and its validation are generic and apply to any PCI
> device. A machine type opts in by setting the new pci_fixed_bar_supported
> MachineClass flag. Machine types that leave it unset reject fixed-bar-N. This
> series enables it on the virt machine only, by setting the flag and wiring up
> the fw_cfg export. Other machine types would need equivalent wiring to expose
> the same mechanism to firmware.
> 
> On the firmware side, the existing IncompatiblePciDeviceSupportDxe driver is
> extended to consume this blob. During device discovery, its CheckDevice()
> implementation returns ACPI address descriptors with _MIF|_MAF set for the
> fixed BARs found in the blob. PciBusDxe's resource allocator already supports
> fixed BARs, including sizing and pinning the bridge windows above them. It now
> treats a BAR as fixed when the descriptor carries _MIF|_MAF, and programs the
> fixed address into the BAR register.
> 
> Fixed BAR placement requires the guest to preserve the PCI resource
> configuration established by firmware. This is requested through the PCI boot
> configuration preservation mechanism using ACPI _DSM function 5. Fixed BAR
> configuration is rejected when firmware is not loaded or ACPI is disabled.
> 
> Example 1: VFIO passthrough device with fixed BARs and the PCIe MMIO
> aperture adjusted to accommodate the fixed addresses.
> 
>     -machine virt,...,highmem-mmio-base=4T,highmem-mmio-size=4T
>     -device pcie-root-port,id=pcie.port1,bus=pcie.1,chassis=1,io-reserve=0
>     -device x3130-upstream,id=upstream1,bus=pcie.port1
>     -device xio3130-downstream,id=downstream1_1,bus=upstream1,chassis=1,slot=2
>     -device vfio-pci-nohotplug,host=0018:06:00.0,bus=downstream1_1,id=dev0,\
>             fixed-bar-0=0x48000000000,fixed-bar-2=0x50000000000,fixed-bar-4=0x58000000000
> 
> Example 2: VFIO and emulated devices under the same root port with fixed BARs
> and the PCIe MMIO aperture adjusted using highmem-mmio-base.
> 
>     -machine virt,...,highmem-mmio-base=64T,highmem-mmio-size=64T
>     -device pxb-pcie,id=pcie.1,bus_nr=1,bus=pcie.0
>     -device pcie-root-port,id=pcie.port1,bus=pcie.1,chassis=1,io-reserve=0
>     -device x3130-upstream,id=upstream1,bus=pcie.port1
>     -device xio3130-downstream,id=downstream1_1,bus=upstream1,chassis=1,slot=1
>     -device vfio-pci-nohotplug,host=0018:06:00.0,bus=downstream1_1,rombar=0,id=dev0,iommufd=iommufd0,\
>             fixed-bar-0=0x6b4000000000,fixed-bar-2=0x6b8000000000,fixed-bar-4=0x6c0000000000
>     -device xio3130-downstream,id=downstream1_2,bus=upstream1,chassis=1,slot=2
>     -device vfio-pci,host=0012:03:00.1,bus=downstream1_2,id=nic1,iommufd=iommufd0,fixed-bar-0=0x6c8000000000
>     -device xio3130-downstream,id=downstream1_3,bus=upstream1,chassis=1,slot=3
>     -device e1000,netdev=net0,bus=downstream1_3,id=nic_emu,fixed-bar-0=0x20000000
> 
> Limitations:
> 
> * I/O BARs and the expansion ROM BAR are not covered by fixed-bar-N.
> * SR-IOV VF BARs are not covered.
> 
> Testing:
> 
> The series was tested on the AArch64 virt machine with a range of PCIe
> configurations, including emulated and VFIO passthrough devices, PCIe
> switches, single and multiple fixed-BAR endpoints behind a shared root port,
> devices attached directly to the host bridge, and endpoints behind nested
> bridges. All rejection paths were also exercised, with the corresponding
> error messages verified. These included unsupported machine types,
> hot-plugged devices, non-existent BARs, I/O BARs, unaligned, overlapping, or
> out-of-aperture BARs, incomplete BAR configurations, highmem-mmio=off,
> missing firmware, and highmem-mmio-base values that cause address-space
> overflow.
> 
> Changes since v1:
> 
> * Add a pci_fixed_bar_supported MachineClass flag; fixed-bar-N is rejected
>   on machine types that do not set it. virt sets it.
> * Keep the fixed BAR claims in FixedBarsInfo instead of a static array, and
>   report validation failures through Error ** to the machine code instead of
>   calling exit(1).
> * Allow a 64-bit non-prefetchable BAR in either aperture when the device is
>   attached to the host bridge; behind a PCI bridge it must lie below 4 GiB.
>   A 64-bit prefetchable BAR must still lie in the 64-bit aperture.
> * Treat the high PCIe MMIO aperture as absent with highmem-mmio=off, and
>   reject fixed BAR configuration unless firmware is loaded and ACPI is
>   enabled.
> * Check highmem-mmio-base + highmem-mmio-size against the PA space without
>   overflow, and document highmem-mmio-base in docs/system/arm/virt.rst.
> * Document the etc/fixed-bars format in docs/specs/pci-fixed-bars.rst, and
>   note that the firmware device path is carried for future matching.
> * Move pci-fixed-bar.h to include/hw/pci/.
> 
> v1:
> https://lore.kernel.org/qemu-arm/20260916185554.917526-1-tdave@nvidia.com/
> 
> A git branch with this series applied is available at:
> https://github.com/tdavenvidia/upstream-qemu/tree/Upstream-fixed-bar-Oct-29-V2
> 
> The related EDK2 change is available at:
> https://github.com/tianocore/edk2/pull/13183
> 
> Tushar Dave (5):
>   hw/pci: introduce PCI fixed BAR properties
>   hw/pci: add validation for fixed BAR configuration
>   hw/pci: add fixed BAR fw_cfg blob export
>   hw/arm/virt: export fixed BAR metadata via fw_cfg
>   hw/arm/virt: add highmem-mmio-base property
> 
>  docs/specs/index.rst            |   1 +
>  docs/specs/pci-fixed-bars.rst   |  75 +++++++
>  docs/system/arm/virt.rst        |   4 +
>  hw/arm/virt.c                   |  82 ++++++-
>  hw/pci/meson.build              |   2 +
>  hw/pci/pci-fixed-bar-blob.c     | 308 +++++++++++++++++++++++++++
>  hw/pci/pci-fixed-bar-validate.c | 365 ++++++++++++++++++++++++++++++++
>  hw/pci/pci-fixed-bar-validate.h |  22 ++
>  hw/pci/pci.c                    |  71 +++++++
>  include/hw/core/boards.h        |   3 +
>  include/hw/pci/pci-fixed-bar.h  |  63 ++++++
>  include/hw/pci/pci_device.h     |   7 +
>  12 files changed, 1002 insertions(+), 1 deletion(-)
>  create mode 100644 docs/specs/pci-fixed-bars.rst
>  create mode 100644 hw/pci/pci-fixed-bar-blob.c
>  create mode 100644 hw/pci/pci-fixed-bar-validate.c
>  create mode 100644 hw/pci/pci-fixed-bar-validate.h
>  create mode 100644 include/hw/pci/pci-fixed-bar.h
> 
> -- 
> 2.34.1