Re: [PATCH 1/1] vfio/pci: add PCIe TPH device ioctl

[Wathsala Vithanage <wathsala.vithanage@arm.com>]

Hi Feng,

> We also hope to support this TPH feature in the Kunpeng (ARM server) user-space, and hope that this patch can continue.
>
> @Wathsala, I don't have the previous email, could you please send v2 again? I reviewed v2 [1] and found a few issues that I'd like to discuss.
>
> [1] https://lkml.org/lkml/2025/6/2/1116

This is the last patch  I sent here. This is a more simplified version 
of the previous RFCs (which I see you have commented on)
However, I haven't heard form Alex, since my last follow up.
Happy to discuss if you are still interested.

--wathsala

>
> Thanks
>
> On 1/29/2026 10:06 PM, Wathsala Vithanage wrote:
>> Hi Alex,
>>
>> Just checking back on the VFIO PCI TPH patch below. You’d mentioned wanting more time to evaluate the implications, so I wanted to see if you had any remaining concerns or if you’d like me to rework this in a different direction.
>>
>> Thanks,
>> Wathsala
>>
>> On 11/6/25 17:19, Wathsala Vithanage wrote:
>>> On 11/5/25 13:15, Alex Williamson wrote:
>>>> On Mon, 27 Oct 2025 09:33:33 -0500
>>>> Wathsala Vithanage <wathsala.vithanage@arm.com> wrote:
>>>>
>>>>> On 10/16/25 16:41, Jeremy Linton wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On 10/13/25 11:35 AM, Wathsala Vithanage wrote:
>>>>>>> TLP Processing Hints (TPH) let a requester provide steering hints that
>>>>>>> can enable direct cache injection on supported platforms and PCIe
>>>>>>> devices. The PCIe core already exposes TPH handling to kernel drivers.
>>>>>>>
>>>>>>> This change adds the VFIO_DEVICE_PCI_TPH ioctl and exposes TPH control
>>>>>>> to user space to reduce memory latency and improve throughput for
>>>>>>> polling drivers (e.g., DPDK poll-mode drivers). Through this interface,
>>>>>>> user-space drivers can:
>>>>>>>      - enable or disable TPH for the device function
>>>>>>>      - program steering tags in device-specific mode
>>>>>>>
>>>>>>> The ioctl is available only when the device advertises the TPH
>>>>>>> Capability. Invalid modes or tags are rejected. No functional change
>>>>>>> occurs unless the ioctl is used.
>>>>>>>
>>>>>>> Signed-off-by: Wathsala Vithanage <wathsala.vithanage@arm.com>
>>>>>>> ---
>>>>>>>     drivers/vfio/pci/vfio_pci_core.c | 74 ++++++++++++++++++++++++++++++++
>>>>>>>     include/uapi/linux/vfio.h        | 36 ++++++++++++++++
>>>>>>>     2 files changed, 110 insertions(+)
>>>>>>>
>>>>>>> diff --git a/drivers/vfio/pci/vfio_pci_core.c
>>>>>>> b/drivers/vfio/pci/vfio_pci_core.c
>>>>>>> index 7dcf5439dedc..0646d9a483fb 100644
>>>>>>> --- a/drivers/vfio/pci/vfio_pci_core.c
>>>>>>> +++ b/drivers/vfio/pci/vfio_pci_core.c
>>>>>>> @@ -28,6 +28,7 @@
>>>>>>>     #include <linux/nospec.h>
>>>>>>>     #include <linux/sched/mm.h>
>>>>>>>     #include <linux/iommufd.h>
>>>>>>> +#include <linux/pci-tph.h>
>>>>>>>     #if IS_ENABLED(CONFIG_EEH)
>>>>>>>     #include <asm/eeh.h>
>>>>>>>     #endif
>>>>>>> @@ -1443,6 +1444,77 @@ static int vfio_pci_ioctl_ioeventfd(struct
>>>>>>> vfio_pci_core_device *vdev,
>>>>>>>                       ioeventfd.fd);
>>>>>>>     }
>>>>>>>     +static int vfio_pci_tph_set_st(struct vfio_pci_core_device *vdev,
>>>>>>> +                   const struct vfio_pci_tph_entry *ent)
>>>>>>> +{
>>>>>>> +    int ret, mem_type;
>>>>>>> +    u16 st;
>>>>>>> +    u32 cpu_id = ent->cpu_id;
>>>>>>> +
>>>>>>> +    if (cpu_id >= nr_cpu_ids || !cpu_present(cpu_id))
>>>>>>> +        return -EINVAL;
>>>>>>> +
>>>>>>> +    if (!cpumask_test_cpu(cpu_id, current->cpus_ptr))
>>>>>>> +        return -EINVAL;
>>>>>>> +
>>>>>>> +    switch (ent->mem_type) {
>>>>>>> +    case VFIO_TPH_MEM_TYPE_VMEM:
>>>>>>> +        mem_type = TPH_MEM_TYPE_VM;
>>>>>>> +        break;
>>>>>>> +    case VFIO_TPH_MEM_TYPE_PMEM:
>>>>>>> +        mem_type = TPH_MEM_TYPE_PM;
>>>>>>> +        break;
>>>>>>> +    default:
>>>>>>> +        return -EINVAL;
>>>>>>> +    }
>>>>>>> +    ret = pcie_tph_get_cpu_st(vdev->pdev, mem_type,
>>>>>>> topology_core_id(cpu_id),
>>>>>>> +                  &st);
>>>>>>> +    if (ret)
>>>>>>> +        return ret;
>>>>>>> +    /*
>>>>>>> +     * PCI core enforces table bounds and disables TPH on error.
>>>>>>> +     */
>>>>>>> +    return pcie_tph_set_st_entry(vdev->pdev, ent->index, st);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int vfio_pci_tph_enable(struct vfio_pci_core_device *vdev,
>>>>>>> int mode)
>>>>>>> +{
>>>>>>> +    /* IV mode is not supported. */
>>>>>>> +    if (mode == PCI_TPH_ST_IV_MODE)
>>>>>>> +        return -EINVAL;
>>>>>>> +    /* PCI core validates 'mode' and returns -EINVAL on bad values. */
>>>>>>> +    return pcie_enable_tph(vdev->pdev, mode);
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int vfio_pci_tph_disable(struct vfio_pci_core_device *vdev)
>>>>>>> +{
>>>>>>> +    pcie_disable_tph(vdev->pdev);
>>>>>>> +    return 0;
>>>>>>> +}
>>>>>>> +
>>>>>>> +static int vfio_pci_ioctl_tph(struct vfio_pci_core_device *vdev,
>>>>>>> +                  void __user *uarg)
>>>>>>> +{
>>>>>>> +    struct vfio_pci_tph tph;
>>>>>>> +
>>>>>>> +    if (copy_from_user(&tph, uarg, sizeof(struct vfio_pci_tph)))
>>>>>>> +        return -EFAULT;
>>>>>>> +
>>>>>>> +    if (tph.argsz != sizeof(struct vfio_pci_tph))
>>>>>>> +        return -EINVAL;
>>>>>>> +
>>>>>>> +    switch (tph.op) {
>>>>>>> +    case VFIO_DEVICE_TPH_ENABLE:
>>>>>>> +        return vfio_pci_tph_enable(vdev, tph.mode);
>>>>>>> +    case VFIO_DEVICE_TPH_DISABLE:
>>>>>>> +        return vfio_pci_tph_disable(vdev);
>>>>>>> +    case VFIO_DEVICE_TPH_SET_ST:
>>>>>>> +        return vfio_pci_tph_set_st(vdev, &tph.ent);
>>>>>>> +    default:
>>>>>>> +        return -EINVAL;
>>>>>>> +    }
>>>>>>> +}
>>>>>>> +
>>>>>>>     long vfio_pci_core_ioctl(struct vfio_device *core_vdev, unsigned
>>>>>>> int cmd,
>>>>>>>                  unsigned long arg)
>>>>>>>     {
>>>>>>> @@ -1467,6 +1539,8 @@ long vfio_pci_core_ioctl(struct vfio_device
>>>>>>> *core_vdev, unsigned int cmd,
>>>>>>>             return vfio_pci_ioctl_reset(vdev, uarg);
>>>>>>>         case VFIO_DEVICE_SET_IRQS:
>>>>>>>             return vfio_pci_ioctl_set_irqs(vdev, uarg);
>>>>>>> +    case VFIO_DEVICE_PCI_TPH:
>>>>>>> +        return vfio_pci_ioctl_tph(vdev, uarg);
>>>>>>>         default:
>>>>>>>             return -ENOTTY;
>>>>>>>         }
>>>>>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>>>>>> index 75100bf009ba..cfdee851031e 100644
>>>>>>> --- a/include/uapi/linux/vfio.h
>>>>>>> +++ b/include/uapi/linux/vfio.h
>>>>>>> @@ -873,6 +873,42 @@ struct vfio_device_ioeventfd {
>>>>>>>       #define VFIO_DEVICE_IOEVENTFD        _IO(VFIO_TYPE, VFIO_BASE + 16)
>>>>>>>     +/**
>>>>>>> + * VFIO_DEVICE_PCI_TPH - _IO(VFIO_TYPE, VFIO_BASE + 22)
>>>>>>> + *
>>>>>>> + * Control PCIe TLP Processing Hints (TPH) on a PCIe device.
>>>>>>> + *
>>>>>>> + * Supported operations:
>>>>>>> + * - VFIO_DEVICE_TPH_ENABLE: enable TPH in no-steering-tag (NS) or
>>>>>>> + *   device-specific (DS) mode. IV mode is not supported via this ioctl
>>>>>>> + *   and returns -EINVAL.
>>>>>>> + * - VFIO_DEVICE_TPH_DISABLE: disable TPH on the device.
>>>>>>> + * - VFIO_DEVICE_TPH_SET_ST: program an entry in the device TPH
>>>>>>> Steering-Tag
>>>>>>> + *   (ST) table. The kernel derives the ST from cpu_id and mem_type;
>>>>>>> the
>>>>>>> + *   value is not returned to userspace.
>>>>>>> + */
>>>>>>> +struct vfio_pci_tph_entry {
>>>>>>> +    __u32 cpu_id;            /* CPU logical ID */
>>>>>>> +    __u8  mem_type;
>>>>>>> +#define VFIO_TPH_MEM_TYPE_VMEM        0   /* Request volatile memory
>>>>>>> ST */
>>>>>>> +#define VFIO_TPH_MEM_TYPE_PMEM        1   /* Request persistent
>>>>>>> memory ST */
>>>>>>> +    __u8  rsvd[1];
>>>>>>> +    __u16 index;            /* ST-table index */
>>>>>>> +};
>>>>>>> +
>>>>>>> +struct vfio_pci_tph {
>>>>>>> +    __u32 argsz;            /* Size of vfio_pci_tph */
>>>>>>> +    __u32 mode;            /* NS and DS modes; IV not supported */
>>>>>>> +    __u32 op;
>>>>>>> +#define VFIO_DEVICE_TPH_ENABLE        0
>>>>>>> +#define VFIO_DEVICE_TPH_DISABLE        1
>>>>>>> +#define VFIO_DEVICE_TPH_SET_ST        2
>>>>>>> +    struct vfio_pci_tph_entry ent;
>>>>>>> +};
>>>>>>> +
>>>>>>> +#define VFIO_DEVICE_PCI_TPH    _IO(VFIO_TYPE, VFIO_BASE + 22)
>>>>>> A quick look at this, it seems its following the way the existing vfio
>>>>>> IOCTls are defined, yet two of them (ENABLE and DISABLE) won't likely
>>>>>> really change their structure, or don't need a structure in the case
>>>>>> of disable. Why not use IOW() and let the kernel error handling deal
>>>>>> with those two as independent ioctls?
>>>>>>
>>>>>>
>>>>>> Thanks,
>>>>> It will require two IOCTLs. I’m ok with having two IOCTLs for this
>>>>> feature if the maintainers are fine with it.
>>>> TBH, I'm not sure why we didn't use a DEVICE_FEATURE for this. Seems
>>>> like we could implement a SET operation that does enable/disable and
>>> Thanks Alex, it was implemented as a DEVICE_FEATURE in RFC v1,
>>> except it had a GET operation to get the tag to the user; which we
>>> decided to drop.
>>>
>>>> another for steering tags.  I still need to fully grasp the
>>>> implications of this support though.  Thanks,
>>> This is now same as the already merged RDMA TPH feature.
>>> https://lore.kernel.org/linux-rdma/cover.1751907231.git.leon@kernel.org/
>>>
>>> --wathsala
>>>
>>>
>>>
>>