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

Leon Romanovsky
2026-04-13T19:19:34Z

On Mon, Apr 13, 2026 at 08:04:10PM +0800, fengchengwen wrote:
> On 4/13/2026 6:01 PM, Leon Romanovsky wrote:
> > On Fri, Apr 10, 2026 at 10:30:52PM +0800, fengchengwen wrote:
> >> Hi all,
> >>
> >> I'm writing to propose adding a sysfs interface to expose and configure the
> >> PCIe TPH
> >> Steering Tag for PCIe devices, which is retrieved inside the kernel.
> >>
> >>
> >> 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.
> >>
> >>
> >> 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
> >>
> >>
> >> The design strictly follows PCI subsystem sysfs standards and has the
> >> following key properties:
> >>
> >> 1. Dynamic Visibility: The sysfs attributes will only be present for PCIe
> >> devices that
> >>    support TPH Steering Tag. Devices without TPH capability will not show
> >> these nodes,
> >>    avoiding unnecessary user confusion.
> >>
> >> 2. Permission Control: The attributes will use 0600 file permissions,
> >> ensuring only
> >>    privileged root users can read or write them, which satisfies security
> >> requirements
> >>    for hardware configuration interfaces.
> >>
> >> 3. Standard Implementation Location: The interface will be implemented in
> >>    drivers/pci/pci-sysfs.c, the canonical location for all PCI device sysfs
> >> attributes,
> >>    ensuring consistency and maintainability within the PCI subsystem.
> >>
> >>
> >> Why sysfs instead of alternatives like VFIO-PCI ioctl:
> >>
> >> - Universality: sysfs does not require binding the device to a special
> >> driver such as
> >>   vfio-pci. It is available to any privileged user-space component,
> >> including system
> >>   utilities, daemons, and monitoring tools.
> >>
> >> - Simplicity: Both user-space usage (cat/echo) and kernel implementation are
> >>   straightforward, reducing code complexity and long-term maintenance cost.
> >>
> >> - Design Alignment: TPH Steering Tag is a generic PCIe device feature, not
> >> specific to
> >>   user-space drivers like DPDK or VFIO. Exposing it via sysfs matches the
> >> kernel's
> >>   standard pattern for hardware capabilities.
> >>
> >>
> >> I look forward to your comments about this design before submitting the
> >> final patch.
> >
> > You need to explain more clearly why this write functionality is useful
> > and necessary outside the VFIO/RDMA context:
> > https://lore.kernel.org/all/20260324234615.3731237-1-zhipingz@meta.com/
> >
> > AFAIK, for non-VFIO TPH callers, kernel has enough knowledge to set
> > right ST values.
> >
> > There are several comments regarding the implementation, but those can wait
> > until the rationale behind the proposal is fully clarified.
>
> Thanks for your review and comments.
>
> Let me clarify the rationale behind this user-space sysfs interface:
>
> 1. VFIO is just one of the user-space device access frameworks.
> There are many other in-kernel frameworks that expose devices
> to user space, such as UIO, UACCE, etc., which may also require
> TPH Steering Tag support.
>
> 2. The kernel can automatically program Steering Tags only when
> the device provides a standard ST table in MSI-X or config space.
> However, many devices implement vendor-specific or platform-specific
> Steering Tag programming methods that cannot be fully handled
> by the generic kernel code.
>
> 3. For such devices, user-space applications or framework drivers
> need to retrieve and configure TPH Steering Tags directly.
> A unified sysfs interface allows all user-space frameworks
> (not just VFIO) to use a common, standard way to manage
> TPH Steering Tags, rather than implementing duplicated logic
> in each subsystem.
>
> This interface provides a uniform method for any user-space
> device access solution to work with TPH, which is why I believe
> it is useful and necessary beyond the VFIO/RDMA case.

I understand the rationale for providing a read interface, for example for
debugging, but I do not see any justification for a write interface.

TPH is defined by the PCI specification. If a device intends to support it,
then it should conform to the specification.

Thanks

>
> Thanks
>
> >
> > Thanks
> >
> >>
> >> Best regards,
> >> Chengwen Feng
> >>
>
>