Re: [RFC] Proposal: Add sysfs interface for PCIe TPH Steering Tag retrieval and configuration

[Jason Gunthorpe <jgg@nvidia.com>]

On Fri, Apr 10, 2026 at 10:30:52PM +0800, fengchengwen wrote:

> Background: The TPH Steering Tag is tightly coupled with both a PCIe
> device (identified by its BDF) and a CPU core. It can only be
> obtained in kernel mode. To allow user-space applications to fetch
> and set this value securely and conveniently, we need a standard
> kernel-to-user interface.

There is no reason for userspace to have this value outside VFIO.

> Proposed Solution: Add several sysfs attributes under each PCIe device's
> sysfs directory:
> 1. /sys/bus/pci/devices/<BDF>/tph_mode to query the TPH mode (interrupt or
> device specific)
> 2. /sys/bus/pci/devices/<BDF>/tph_enable to control the TPH feature
> 3. /sys/bus/pci/devices/<BDF>/tph_st to support both read and write
> operations, e.g.:
>    Read operation:
>      echo "cpu=3" > /sys/bus/pci/devices/0000:01:00.0/tph_st
>      cat /sys/bus/pci/devices/0000:01:00.0/tph_st
>    Write operation:
>      echo "index=10 st=123" > /sys/bus/pci/devices/0000:01:00.0/tph_st

NAK

This is all low level details controled exclusively by the kernel
driver. Allowing userspace to mess it up will break drivers.

There is no justification to give userspace this kind of write access.

Jason