Re: [PATCH v3 0/5] vfio/pci: Add PCIe TPH support

[Leon Romanovsky <leon@kernel.org>]

On Thu, Apr 23, 2026 at 09:08:46AM +0800, Chengwen Feng wrote:
> This series adds support for PCIe TLP Processing Hints (TPH) to
> vfio-pci, allowing userspace to manage device steering tags for
> improved performance and QoS in virtualized deployments.
> 
> The implementation follows a clean incremental structure:
> - Patch 1: Export pcie_tph_get_st_modes()
> - Patch 2: Introduce UAPI ABI and implement capability query to
>   let userspace discover supported TPH modes, ST table presence
>   and size. Also add module parameter enable_unsafe_tph_ds to
>   guard unsafe usage of TPH Device-Specific mode with no ST table.
> - Patch 3: Add TPH enable/disable with mode selection. Restrict
>   unsafe Device-Specific mode without ST table to be allowed only
>   when the module parameter is enabled.
> - Patch 4: Add interface to batch get per-CPU steering tags for
>   device-specific mode without standard ST table. This interface
>   is only available when the unsafe module parameter is enabled.
> - Patch 5: Add interface to batch program steering tag table
>   entries for standard TPH modes.
> 
> All user API definitions are finalized in the first patch and
> remain stable across the series. The design follows existing
> VFIO conventions and relies on kernel pcie-tph infrastructure.
> 
> To avoid potential security risks in virtualization environments,
> TPH Device-Specific mode without a standard ST table is blocked
> by default. It can only be enabled by administrators via the
> enable_unsafe_tph_ds module parameter for trusted bare-metal
> userspace.
> 
> This series addresses the TPH management requirements discussed
> in the RFC "Proposal: Add sysfs interface for PCIe TPH Steering
> Tag retrieval and configuration".

As in the RFC, this patch series still lacks an explanation of *why* it is
needed. Please describe the scenario that cannot be handled without these
changes.

Thanks