[PATCH v2 0/3] pcie/acs: configurable ACS capabilities for root and downstream ports

[Tushar Dave <tdave@nvidia.com>]

This series adds an 'acs-ctrl' property to PCIe root ports and downstream
ports, letting the hypervisor expose a specific PCIe ACS (Access Control
Services) configuration to the guest instead of QEMU always advertising
every ACS capability the port structurally supports.

This matters for P2P (peer-to-peer) topologies such as GPUDirect RDMA,
where the actual ACS configuration in effect must match what the host
side is doing -- otherwise the guest sees a topology/configuration that
doesn't reflect reality, which is undefined behavior and can fail either
in software or in hardware.

Patch 1 adds the missing ACS capability to xio3130 downstream ports
(previously only root ports exposed one), gated behind a new
'x-pcie-acs' property so older machine types can disable it via
hw_compat and preserve migration compatibility.

Patch 2 introduces the 'acs-ctrl' property itself on both root and
downstream ports, so the configured value -- not just the maximal
capability set -- is what gets exposed to the guest.

Patch 3 makes the ACS Control register read-only to the guest once
'acs-ctrl' has been explicitly configured, so a guest OS doing normal
PCIe programming can't silently override the host-configured ACS
state. An explicitly configured value of 0 is treated the same as any
other configured value (locked), distinct from the property never
being set at all (which stays guest-writable, unchanged from current
behavior).

Changes in v2:
- Patch 1: Limit x-pcie-acs to the xio3130 downstream port
- Patch 2: Error out when acs-ctrl is set on a port without ACS, reword
  the commit message and the unsupported-bits error
- Patch 3: Make pcie_acs_init() safe for devices that are not PCIe
  ports, clarify the acs-ctrl property description

Link to v1: https://lore.kernel.org/qemu-devel/20260922215008.2554402-1-tdave@nvidia.com/

Tushar Dave (2):
  xio3130_downstream: Add ACS support for downstream PCIe ports
  hw/pci: Make ACS Control read-only to guests

Yishai Hadas (1):
  hw/pci: Expose ACS ctrl on Guest as configured by the hypervisor

 hw/core/machine.c                  |  1 +
 hw/pci-bridge/pcie_root_port.c     | 14 ++++++-
 hw/pci-bridge/xio3130_downstream.c | 19 ++++++++++
 hw/pci/pcie.c                      | 61 ++++++++++++++++++++++++++++--
 include/hw/pci/pci.h               |  3 ++
 include/hw/pci/pcie.h              |  8 +++-
 include/hw/pci/pcie_port.h         |  2 +
 7 files changed, 100 insertions(+), 8 deletions(-)

-- 
2.34.1