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Message-ID: <2e228546-0250-4181-8cef-7b6f990daf7a@arm.com>
Date: Thu, 29 Jan 2026 08:06:24 -0600
From: Wathsala Vithanage <wathsala.vithanage@....com>
To: Alex Williamson <alex@...zbot.org>
Cc: Jeremy Linton <jeremy.linton@....com>, alex.williamson@...hat.com,
jgg@...pe.ca, pstanner@...hat.com, kvm@...r.kernel.org,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] vfio/pci: add PCIe TPH device ioctl
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@....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@....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
>
>
>
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