Re: [PATCH v16 00/12] vfio/pci: Add PCIe TPH support

[Alex Williamson <alex@shazbot.org>]

On Mon, 15 Jun 2026 20:19:48 +0800
fengchengwen <fengchengwen@huawei.com> wrote:

> Gentle ping for v16 TPH series.
> 
> Would appreciate reviews from Alex and Jason. Let me know if major
> changes are needed.

The same changes that were still being discussed while v15 and v16 were
posted, I don't think vfio-pci is the right place to host a CPU to ST
translation interface.  Thanks,

Alex


> On 6/4/2026 9:27 PM, Chengwen Feng wrote:
> > This patchset enables full userspace configurable PCIe TPH support
> > for VFIO, which brings performance benefits for userspace workloads
> > such as DPDK and SPDK.
> > 
> > Currently VFIO only exposes read-only TPH capability registers to 
> > userspace, while all write operations are silently discarded. This 
> > prevents userspace from enabling and configuring TPH, limiting
> > performance optimization opportunities.
> > 
> > Per PCIe spec 7.5.3.15: TPH Completer support is applicable to Root
> > Ports and Endpoints, allowing Steering Tags to target host CPUs or
> > peer devices for P2P transactions.
> > 
> > TPH usage model can be divided into three fundamental parts:
> > 1. Retrieve Steering Tag:
> >    - Tags targeting host CPUs are obtained via platform methods
> > (ACPI _DSM) wrapped in pcie_tph_get_cpu_st(). Userspace requires a
> > generic interface to query these CPU-associated ST values.
> >    - Tags targeting peer devices are managed by userspace drivers.
> > 2. Program Steering Tag table:
> >    - For devices with standard ST table structures (in capability
> > space or MSI-X table), userspace needs a unified interface to
> > configure ST entries.
> >    - Devices without standard ST tables are handled by userspace
> > itself. 3. Toggle device TPH Requester enable/disable state.
> > 
> > To support the above scenarios, this series extends VFIO with
> > complete TPH virtualization features:
> > - New device feature TPH_CPU_ST: Batch query interface to resolve 
> >   CPU-specific ST tags.
> > - New device feature TPH_ST_CONFIG: Batch configure interface for
> > device ST table entries, with shadow cache and atomic rollback
> > support.
> > - Full TPH capability register virtualization: allow userspace to
> > toggle TPH Requester state via TPH_CTRL register writes.
> > 
> > To guarantee isolation and security, this patchset adopts a
> > two-level safety gate design with careful ABI considerations:
> > 1. Global unsafe gate:
> >    TPH caching behavior may cross isolation domains and impact
> > shared platform resources. A new module parameter
> > `enable_unsafe_tph` is introduced (default off) to globally gate
> > all TPH functionalities. 2. Per-device opt-in gate:
> >    To preserve strict ABI compatibility and avoid unexpected
> > hardware state changes for existing users, a new VFIO device
> > feature TPH_ENABLE is added. TPH capabilities are only available
> > after userspace explicitly enables it per-device.
> > 
> > Because Kernel PCI TPH implementation requires TPH Requester to be
> > enabled before programming ST entries. To support userspace
> > configuring ST table in arbitrary order, a shadow ST table is
> > introduced to buffer ST writes before TPH is enabled. All cached
> > entries are flushed to hardware when TPH Requester turns on. This
> > also provides atomic batch rollback capability for reliable
> > configuration.
> > 
> > The patchset is split into two logical parts: the first seven
> > patches fix and refactor core PCI/TPH kernel code to export
> > required helper interfaces, and the remaining five patches
> > implement corresponding VFIO TPH virtualization layer step by step.
> > 
> > Patch breakdown:
> >  1/12 PCI/TPH: Fix pcie_tph_get_st_table_loc() field extraction
> >  2/12 PCI/TPH: Fix tph_enabled concurrent update race by bitfield
> > packing 3/12 PCI/TPH: Cache TPH requester capability at probe time
> >  4/12 PCI/TPH: Refactor pcie_enable_tph & add explicit requester
> > variant 5/12 PCI/TPH: Refactor pcie_tph_get_cpu_st & add explicit
> > variant 6/12 PCI/TPH: expose the enabled TPH requester type
> >  7/12 PCI/TPH: Add pcie_tph_supported() helper to check TPH
> > capability attrs 8/12 vfio/pci: Hide TPH capability when TPH is
> > unsupported 9/12 vfio/pci: Add TPH_ENABLE feature skeleton and
> > unsafe module parameter 10/12 vfio/pci: Add TPH_CPU_ST to query
> > CPU's TPH steering tag 11/12 vfio/pci: Add TPH_ST_CONFIG for PCIe
> > TPH ST configuration 12/12 vfio/pci: Virtualize PCIe TPH capability
> > registers
> > 
> > Based on earlier RFC work by Wathsala Vithanage
> > 
> > ---
> > v16: 
> > - Supports opt-in at the device level which address Alex's comment.
> > - Split sub-commit: add hide TPH capability when TPH is unsupported.
> > - Optimize the tph fields layout of the pci_dev structure.
> > - Optimize virtualize PCIe TPH capability commit: support rollback
> >   when set fail.
> > - Reorder PCI/TPH commits: make fix commit ahead.
> > - Reorganized the cover letter to serve as the starting point for
> >   discussion.
> > v15: Address Alex's comments:
> > - Drop TPH capability when tph_cap=0
> > - Use _explicit postfix other than add policy parameter for enable
> >   TPH and get tph st.
> > - Make sure set st entry under D0
> > - Reimpl virtualize TPH capability register
> > - Other fix
> > v14:
> > - Return PCI_TPH_LOC_NONE when !CONFIG_PCIE_TPH accord Alex's
> > comment
> > - Fix Sashiko comments:
> >   - Clear ST shadow state across user session
> >   - Fix out-of-bounds byte masking in vfio_pci_tph_config_read
> > 
> > Chengwen Feng (11):
> >   PCI/TPH: Fix pcie_tph_get_st_table_loc() field extraction
> >   PCI/TPH: Fix tph_enabled concurrent update race by bitfield
> > packing PCI/TPH: Cache TPH requester capability at probe time
> >   PCI/TPH: Refactor pcie_enable_tph & add explicit requester variant
> >   PCI/TPH: Refactor pcie_tph_get_cpu_st & add explicit variant
> >   PCI/TPH: Add pcie_tph_supported() helper to check TPH capability
> >     attributes
> >   vfio/pci: Hide TPH capability when TPH is unsupported
> >   vfio/pci: Add TPH_ENABLE feature skeleton and unsafe module
> > parameter vfio/pci: Add TPH_CPU_ST to query CPU's TPH steering tag
> >   vfio/pci: Add TPH_ST_CONFIG for PCIe TPH ST configuration
> >   vfio/pci: Virtualize PCIe TPH capability registers
> > 
> > Zhiping Zhang (1):
> >   PCI/TPH: expose the enabled TPH requester type
> > 
> >  drivers/pci/tph.c                  | 220
> > ++++++++++++++++++++-------- drivers/vfio/pci/vfio_pci.c        |
> > 13 +- drivers/vfio/pci/vfio_pci_config.c | 111 ++++++++++++++
> >  drivers/vfio/pci/vfio_pci_core.c   | 228
> > ++++++++++++++++++++++++++++- include/linux/pci-tph.h            |
> > 22 +++ include/linux/pci.h                |   6 +-
> >  include/linux/vfio_pci_core.h      |   7 +-
> >  include/uapi/linux/vfio.h          |  60 ++++++++
> >  8 files changed, 600 insertions(+), 67 deletions(-)
> >   
>