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Message-ID: <e50f810f-a770-47b2-b266-5701172c8041@arm.com>
Date: Thu, 6 Nov 2025 17:19:29 -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
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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