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Message-ID: <20251105121541.4d383694.alex@shazbot.org>
Date: Wed, 5 Nov 2025 12:15:41 -0700
From: Alex Williamson <alex@...zbot.org>
To: Wathsala Vithanage <wathsala.vithanage@....com>
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 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
another for steering tags. I still need to fully grasp the
implications of this support though. Thanks,
Alex
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