When a VM has been assigned a number of PCIe devices behind a common
PCIe switch and the VM intends to use peer-to-peer (P2P) transfers
between the devices, it is useful to be able to more report the
apparent PCIe bandwidth so it reflects the real handwidth of the
underlying hardware. This allows software in the VM (such as NCCL[1])
to make better decisions about whether to use P2P or not.

The future QEMU 11.2[1] will support new upstream and downstream port
models that have configurable speed and width properties and extend
the existing root port model to support the same options.

If supported by QEMU, prefer the new upstream and downstream port
models, and support new attributes to set the speed and width.

Patch 1 (tests: Update QEMU 11.2.0 x86_64 caps for generic PCIe switch
ports qemu: add capabilities for generic PCIe switch port devices)
should be replaced with a more comprehensive refresh of the QEMU
capabilities, and is only included here so tests continue to pass.

David

[1] See QEMU commits 432b3f707a3c (hw/pci-bridge: Make speed and width for
    PCIe ports non-experimental) and 73962cc5f06a (hw/pci-bridge: Add
    generic PCIe switch upstream and downstream ports).

David Vrabel (6):
  tests: Update QEMU 11.2.0 x86_64 caps for generic PCIe switch ports
  qemu: add capabilities for generic PCIe switch port devices
  qemu: support generic PCIe switch port models
  qemu: add capabilities for pcie-root-port speed and width properties
  qemu: add PCIe link speed and width attributes
  NEWS: mention generic PCIe switch ports and link speed/width

 NEWS.rst                                      | 16 ++++
 docs/formatdomain.rst                         | 28 ++++++
 src/conf/domain_conf.c                        | 42 +++++++++
 src/conf/domain_conf.h                        | 32 +++++++
 src/conf/schemas/domaincommon.rng             | 26 ++++++
 src/libvirt_private.syms                      |  4 +
 src/qemu/qemu_capabilities.c                  | 10 ++
 src/qemu/qemu_capabilities.h                  |  6 ++
 src/qemu/qemu_command.c                       | 32 +++++++
 src/qemu/qemu_domain_address.c                | 10 +-
 src/qemu/qemu_validate.c                      | 80 +++++++++++++++-
 .../caps_11.2.0_x86_64.replies                | 12 ++-
 .../caps_11.2.0_x86_64.xml                    |  4 +
 .../autoindex.x86_64-latest.args              | 22 ++---
 .../autoindex.x86_64-latest.xml               | 22 ++---
 .../pci-bridge-speed.x86_64-latest.err        |  1 +
 tests/qemuxmlconfdata/pci-bridge-speed.xml    | 18 ++++
 .../pcie-expander-bus.x86_64-latest.args      | 66 +++++++-------
 .../pcie-expander-bus.x86_64-latest.xml       | 66 +++++++-------
 ...cie-root-port-nohotplug.x86_64-latest.args |  8 +-
 ...pcie-root-port-nohotplug.x86_64-latest.xml |  8 +-
 ...-root-port-speed-ioh3420.x86_64-latest.err |  1 +
 .../pcie-root-port-speed-ioh3420.xml          | 19 ++++
 ...ie-root-port-speed-width.x86_64-11.0.0.err |  1 +
 ...e-root-port-speed-width.x86_64-latest.args | 37 ++++++++
 ...ie-root-port-speed-width.x86_64-latest.xml | 48 ++++++++++
 .../pcie-root-port-speed-width.xml            | 23 +++++
 ...tream-port-model-generic.x86_64-11.1.0.err |  1 +
 ...ream-port-model-generic.x86_64-latest.args | 47 ++++++++++
 ...tream-port-model-generic.x86_64-latest.xml | 91 +++++++++++++++++++
 ...e-switch-downstream-port-model-generic.xml | 44 +++++++++
 ...tream-port-width-xio3130.x86_64-latest.err |  1 +
 ...e-switch-downstream-port-width-xio3130.xml | 23 +++++
 ...-switch-downstream-port.x86_64-11.1.0.args | 47 ++++++++++
 ...e-switch-downstream-port.x86_64-11.1.0.xml | 91 +++++++++++++++++++
 ...-switch-downstream-port.x86_64-latest.args | 16 ++--
 ...e-switch-downstream-port.x86_64-latest.xml | 16 ++--
 ...tream-port-model-generic.x86_64-11.1.0.err |  1 +
 ...ream-port-model-generic.x86_64-latest.args | 42 +++++++++
 ...tream-port-model-generic.x86_64-latest.xml | 67 ++++++++++++++
 ...cie-switch-upstream-port-model-generic.xml | 38 ++++++++
 ...ie-switch-upstream-port.x86_64-11.1.0.args | 42 +++++++++
 ...cie-switch-upstream-port.x86_64-11.1.0.xml | 66 ++++++++++++++
 ...ie-switch-upstream-port.x86_64-latest.args |  2 +-
 ...cie-switch-upstream-port.x86_64-latest.xml |  2 +-
 tests/qemuxmlconftest.c                       | 11 +++
 46 files changed, 1170 insertions(+), 120 deletions(-)
 create mode 100644 tests/qemuxmlconfdata/pci-bridge-speed.x86_64-latest.err
 create mode 100644 tests/qemuxmlconfdata/pci-bridge-speed.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-ioh3420.x86_64-latest.err
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-ioh3420.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-11.0.0.err
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-latest.args
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-11.1.0.err
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-latest.args
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-width-xio3130.x86_64-latest.err
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-width-xio3130.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port.x86_64-11.1.0.args
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port.x86_64-11.1.0.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-11.1.0.err
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-latest.args
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-latest.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.xml
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port.x86_64-11.1.0.args
 create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port.x86_64-11.1.0.xml

-- 
2.43.0

On 11/4/26 16:32, David Vrabel wrote:
> When a VM has been assigned a number of PCIe devices behind a common
> PCIe switch and the VM intends to use peer-to-peer (P2P) transfers
> between the devices, it is useful to be able to more report the
> apparent PCIe bandwidth so it reflects the real handwidth of the
> underlying hardware. This allows software in the VM (such as NCCL[1])
> to make better decisions about whether to use P2P or not.
> 
> The future QEMU 11.2[1] will support new upstream and downstream port
> models that have configurable speed and width properties and extend
> the existing root port model to support the same options.
> 
> If supported by QEMU, prefer the new upstream and downstream port
> models, and support new attributes to set the speed and width.
> 
> Patch 1 (tests: Update QEMU 11.2.0 x86_64 caps for generic PCIe switch
> ports qemu: add capabilities for generic PCIe switch port devices)
> should be replaced with a more comprehensive refresh of the QEMU
> capabilities, and is only included here so tests continue to pass.
> 
> David
> 
> [1] See QEMU commits 432b3f707a3c (hw/pci-bridge: Make speed and width for
>     PCIe ports non-experimental) and 73962cc5f06a (hw/pci-bridge: Add
>     generic PCIe switch upstream and downstream ports).
> 
> David Vrabel (6):
>   tests: Update QEMU 11.2.0 x86_64 caps for generic PCIe switch ports
>   qemu: add capabilities for generic PCIe switch port devices
>   qemu: support generic PCIe switch port models
>   qemu: add capabilities for pcie-root-port speed and width properties
>   qemu: add PCIe link speed and width attributes
>   NEWS: mention generic PCIe switch ports and link speed/width

You earn bonus points for remembering to add NEWS entries :-)

>  46 files changed, 1170 insertions(+), 120 deletions(-)
>  create mode 100644 tests/qemuxmlconfdata/pci-bridge-speed.x86_64-latest.err
>  create mode 100644 tests/qemuxmlconfdata/pci-bridge-speed.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-ioh3420.x86_64-latest.err
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-ioh3420.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-11.0.0.err
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-latest.args
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.x86_64-latest.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-root-port-speed-width.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-11.1.0.err
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-latest.args
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.x86_64-latest.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-model-generic.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-width-xio3130.x86_64-latest.err
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port-width-xio3130.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port.x86_64-11.1.0.args
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-downstream-port.x86_64-11.1.0.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-11.1.0.err
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-latest.args
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.x86_64-latest.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port-model-generic.xml
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port.x86_64-11.1.0.args
>  create mode 100644 tests/qemuxmlconfdata/pcie-switch-upstream-port.x86_64-11.1.0.xml
> 

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

and will merge after Peter refreshes caps.

Michal

On 11/5/26 13:34, Michal Prívozník wrote:
> On 11/4/26 16:32, David Vrabel wrote:

> 
> Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
> 
> and will merge after Peter refreshes caps.

Merged now. Congratulations on your first libvirt contribution!

Michal