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Message-ID: <4bf8ba8f-57c3-4af2-9f2a-f4313121be87@arm.com>
Date: Mon, 27 Oct 2025 09:33:33 -0500
From: Wathsala Vithanage <wathsala.vithanage@....com>
To: Jeremy Linton <jeremy.linton@....com>, alex.williamson@...hat.com,
jgg@...pe.ca, pstanner@...hat.com
Cc: 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 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.
Thanks,
>
>> +
>> +
>> /**
>> * VFIO_DEVICE_FEATURE - _IOWR(VFIO_TYPE, VFIO_BASE + 17,
>> * struct vfio_device_feature)
>
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