[PATCH v5 0/4] An API for Cache Stashing with TPH

[Wathsala Vithanage]

Hi,

Thank you for your enthusiasm :)
This patch is barred until kernel VFIO-TPH patch gets merged.

--wathsala

On 1/18/26 19:16, fengchengwen wrote:
> Hi Wathsala,
>
> Looking forward to your reply.
>
> Thanks
>
> On 1/8/2026 8:30 AM, fengchengwen wrote:
>> Hi Wathsala,
>>
>> Sorry to ask if this patchset is under development or stopped?
>>
>> PCIe Steer-tag provides a mechanism for precise data stash, which
>> delivers a positive performance gain and is therefore a valuable
>> feature I think.
>>
>> This patchset concludes with the statement: "the PMDs should only
>> enable TPH in device-specific mode", I don't think such restraints
>> should be made, the framework should be compatible with various
>> device capabilities:
>> 1. The PCIe protocol defines two modes: one is the interrupt-vector
>>     mode, and the other is the device-specific mode. A device may
>>     choose to support either one or both.
>> 2. If device support device-specific mode, it has a large degree of
>>     freedom to implement, such as locate ST table in self-defined
>>     place (just like '[PATCH v5 4/4] net/i40e: enable TPH in i40e'),
>>     and also support only stash part of data (e.g. only desc or header
>>     or even an offset data).
>> 3. If device only support interrupt-vector mode (which each TLP will
>>     use ST from an ST table entry), we could also support it, in this
>>     framework, it could only report basic stash capability.
>>
>> Thanks
>>
>> On 6/3/2025 6:38 AM, Wathsala Vithanage wrote:
>>> Today, DPDK applications benefit from Direct Cache Access (DCA) features
>>> like Intel DDIO and Arm's write-allocate-to-SLC. However, those features
>>> do not allow fine-grained control of direct cache access, such as
>>> stashing packets into upper-level caches (L2 caches) of a processor or
>>> the shared cache of a chiplet. PCIe TLP Processing Hints (TPH) addresses
>>> this need in a vendor-agnostic manner. TPH capability has existed since
>>> PCI Express Base Specification revision 3.0; today, numerous Network
>>> Interface Cards and interconnects from different vendors support TPH
>>> capability. TPH comprises a steering tag (ST) and a processing hint
>>> (PH). ST specifies the cache level of a CPU at which the data should be
>>> written to (or DCAed into), while PH is a hint provided by the PCIe
>>> requester to the completer on an upcoming traffic pattern. Some NIC
>>> vendors bundle TPH capability with fine-grained control over the type of
>>> objects that can be stashed into CPU caches, such as
>>>
>>> - Rx/Tx queue descriptors
>>> - Packet-headers
>>> - Packet-payloads
>>> - Data from a given offset from the start of a packet
>>>
>>> Note that stashable object types are outside the scope of the PCIe
>>> standard; therefore, vendors could support any combination of the above
>>> items as they see fit.
>>>
>>> To enable TPH and fine-grained packet stashing, this API extends the
>>> ethdev library and the PCI bus driver. In this design, the application
>>> provides hints to the PMD via the ethdev stashing API to indicate the
>>> underlying hardware at which CPU and cache level it prefers a packet to
>>> end up. Once the PMD receives a CPU and a cache-level combination (or a
>>> list of such combinations), it must extract the matching ST from the PCI
>>> bus driver for such combinations. The PCI bus driver implements the TPH
>>> functions in an OS specific way; for Linux, it depends on the TPH
>>> capabilities of the VFIO kernel driver.
>>>
>>> An application uses the cache stashing ethdev API by first calling the
>>> rte_eth_dev_stashing_capabilities_get() function to find out what object
>>> types can be stashed into a CPU cache by the NIC out of the object types
>>> in the bulleted list above. This function takes a port_id and a pointer
>>> to a uint16_t to report back the object type flags. PMD implements the
>>> stashing_capabilities_get function pointer in eth_dev_ops. If the
>>> underlying platform or the NIC does not support TPH, this function
>>> returns -ENOTSUP, and the application should consider any values stored
>>> in the object invalid.
>>>
>>> Once the application knows the supported object types that can be
>>> stashed, the next step is to set the steering tags for the packets
>>> associated with Rx and Tx queues via
>>> rte_eth_dev_stashing_{rx,tx}_config_set() ethdev library functions. Both
>>> functions have an identical signature, a port_id, a queue_id, and a
>>> config object. The port_id and the queue_id are used to locate the
>>> device and the queue. The config object is of type struct
>>> rte_eth_stashing_config, which specifies the lcore_id and the
>>> cache_level, indicating where objects from this queue should be stashed.
>>> The 'objects' field in the config sets the types of objects the
>>> application wishes to stash based on the capabilities found earlier.
>>> Note that if the 'objects' field includes the flag
>>> RTE_ETH_DEV_STASH_OBJECT_OFFSET, the 'offset' field must be used to set
>>> the desired offset. These functions invoke PMD implementations of the
>>> stashing functionality via the stashing_{rx,tx}_hints_set function
>>> callbacks in the eth_dev_ops, respectively.
>>>
>>> The PMD's implementation of the stashing_rx_hints_set() and
>>> stashing_tx_hints_set() functions is ultimately responsible for
>>> extracting the ST via the API provided by the PCI bus driver. Before
>>> extracting STs, the PMD should enable the TPH capability in the endpoint
>>> device by calling the rte_pci_tph_enable() function.  The application
>>> begins the ST extraction process by calling the rte_pci_tph_st_get()
>>> function in drivers/bus/pci/rte_bus_pci.h, which returns STs via the
>>> same rte_tph_info objects array passed into it as an argument.  Once PMD
>>> acquires ST, the stashing_{rx,tx}_hints_set callbacks implemented in the
>>> PMD are ready to set the ST as per the rte_eth_stashing_config object
>>> passed to them by the higher-level ethdev functions
>>> ret_eth_dev_stashing_{rx,tx}_hints(). As per the PCIe specification, STs
>>> can be placed on the MSI-X tables or in a device-specific location. For
>>> PMDs, setting the STs on queue contexts is the only viable way of using
>>> TPH. Therefore, the PMDs should only enable TPH in device-specific mode.
>>>
>>> V4->V5:
>>>   * Enable stashing-hints (TPH) in Intel i40e driver.
>>>   * Update exported symbol version from 25.03 to 25.07.
>>>   * Add TPH mode macros.
>>>
>>> V3->V4:
>>>   * Add VFIO IOCTL based ST extraction mechanism to Linux PCI bus driver
>>>   * Remove ST extraction via direct access to ACPI _DSM
>>>   * Replace rte_pci_extract_tph_st() with rte_pci_tph_st_get() in PCI
>>>     bus driver.
>>>
>>> Wathsala Vithanage (4):
>>>    pci: add non-merged Linux uAPI changes
>>>    bus/pci: introduce the PCIe TLP Processing Hints API
>>>    ethdev: introduce the cache stashing hints API
>>>    net/i40e: enable TPH in i40e
>>>
>>>   drivers/bus/pci/bsd/pci.c            |  43 +++++++
>>>   drivers/bus/pci/bus_pci_driver.h     |  52 ++++++++
>>>   drivers/bus/pci/linux/pci.c          | 100 ++++++++++++++++
>>>   drivers/bus/pci/linux/pci_init.h     |  14 +++
>>>   drivers/bus/pci/linux/pci_vfio.c     | 170 +++++++++++++++++++++++++++
>>>   drivers/bus/pci/private.h            |   8 ++
>>>   drivers/bus/pci/rte_bus_pci.h        |  67 +++++++++++
>>>   drivers/bus/pci/windows/pci.c        |  43 +++++++
>>>   drivers/net/intel/i40e/i40e_ethdev.c | 127 ++++++++++++++++++++
>>>   kernel/linux/uapi/linux/vfio_tph.h   | 102 ++++++++++++++++
>>>   lib/ethdev/ethdev_driver.h           |  66 +++++++++++
>>>   lib/ethdev/rte_ethdev.c              | 149 +++++++++++++++++++++++
>>>   lib/ethdev/rte_ethdev.h              | 158 +++++++++++++++++++++++++
>>>   lib/pci/rte_pci.h                    |  15 +++
>>>   14 files changed, 1114 insertions(+)
>>>   create mode 100644 kernel/linux/uapi/linux/vfio_tph.h
>>>