> -----Original Message-----
> From: Nathan Chen <[email protected]>
> Sent: 23 June 2026 03:36
> To: [email protected]
> Cc: Yi Liu <[email protected]>; Eric Auger <[email protected]>;
> Zhenzhong Duan <[email protected]>; Alex Williamson
> <[email protected]>; Cédric Le Goater <[email protected]>; Matt Ochs
> <[email protected]>; Nicolin Chen <[email protected]>; Shameer
> Kolothum Thodi <[email protected]>; Nathan Chen
> <[email protected]>
> Subject: [PATCH v3 2/2] vfio/pci: Add ats property
>
> From: Nathan Chen <[email protected]>
>
> Add an "ats" OnOffAuto property to vfio-pci. When the device has an ATS
> extended capability in config space but we should not expose it (ats=off,
> or ats=auto and kernel reports
> IOMMU_HW_CAP_PCI_ATS_NOT_SUPPORTED), mask
> the capability so the guest does not see it.
>
> If ATS is explicitly requested but not supported by the kernel, fail
> device realize.
>
> This aligns with the kernel's per-device effective ATS reporting and allows
> vfio-pci to mask ATS when the host kernel reports ATS as unsupported.
>
> Emit a warning when ats=on is requested but the physical device does not
> advertise ATS, since ATS cannot be exposed to the guest in this case.
>
> Suggested-by: Shameer Kolothum <[email protected]>
> Signed-off-by: Nathan Chen <[email protected]>
> ---
> hw/vfio/pci.h | 1 +
> hw/vfio/pci.c | 88
> ++++++++++++++++++++++++++++++++++++++++++++++++---
> 2 files changed, 85 insertions(+), 4 deletions(-)
>
> diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
> index c3a1f53d35..f2934f2d84 100644
> --- a/hw/vfio/pci.h
> +++ b/hw/vfio/pci.h
> @@ -188,6 +188,7 @@ struct VFIOPCIDevice {
> bool clear_parent_atomics_on_exit;
> bool skip_vsc_check;
> uint16_t vpasid_cap_offset;
> + OnOffAuto ats;
> VFIODisplay *dpy;
> Notifier irqchip_change_notifier;
> VFIOPCICPR cpr;
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 9c06b25e63..c0436b4c04 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -2546,10 +2546,58 @@ static bool
> vfio_pci_synthesize_pasid_cap(VFIOPCIDevice *vdev, Error **errp)
> return true;
> }
>
> -static void vfio_add_ext_cap(VFIOPCIDevice *vdev)
> +/*
> + * Determine whether ATS capability should be advertised for @vdev, based
> on
> + * whether it was enabled on the command line and whether it is supported
> + * according to the kernel.
> + *
> + * Store whether ATS capability should be advertised in @ats_needed.
> + *
> + * Returns false only when ats=on is explicitly requested but the kernel
> + * reports it is not supported. Returns true in all other cases.
> + */
> +static bool vfio_pci_ats_requested_and_supported(VFIOPCIDevice *vdev,
> + bool *ats_needed, Error
> **errp)
> +{
> + HostIOMMUDevice *hiod = vdev->vbasedev.hiod;
> + HostIOMMUDeviceClass *hiodc;
> + bool ats_supported;
> + *ats_needed = false;
> +
> + if (vdev->ats == ON_OFF_AUTO_OFF) {
> + return true;
> + }
> +
> + *ats_needed = true;
> + if (!hiod) {
> + return true;
> + }
> + hiodc = HOST_IOMMU_DEVICE_GET_CLASS(hiod);
> + if (!hiodc || !hiodc->support_ats) {
> + return true;
> + }
> +
> + ats_supported = hiodc->support_ats(hiod);
> + if (vdev->ats == ON_OFF_AUTO_ON && !ats_supported) {
> + error_setg(errp, "vfio-pci: ATS requested but not supported by
> kernel");
> + *ats_needed = false;
> + return false;
> + }
> +
> + if (vdev->ats == ON_OFF_AUTO_AUTO && !ats_supported) {
> + warn_report("vfio-pci: host kernel reports ATS unsupported; "
> + "ATS capability will be masked");
> + }
This will be slightly misleading if dev doesn't have the ATS CAP at all.
Isn't it? I think, the best place to do this warning is below where we know
ats_cap_present is set or not.
Thanks,
Shameer
> +
> + *ats_needed = ats_supported;
> + return true;
> +}
> +
> +static void vfio_add_ext_cap(VFIOPCIDevice *vdev, bool ats_needed)
> {
> PCIDevice *pdev = PCI_DEVICE(vdev);
> bool pasid_cap_added = false;
> + bool ats_cap_present = false;
> Error *err = NULL;
> uint32_t header;
> uint16_t cap_id, next, size;
> @@ -2635,7 +2683,19 @@ static void vfio_add_ext_cap(VFIOPCIDevice
> *vdev)
> */
> case PCI_EXT_CAP_ID_PASID:
> pasid_cap_added = true;
> - /* fallthrough */
> + pcie_add_capability(pdev, cap_id, cap_ver, next, size);
> + break;
> + case PCI_EXT_CAP_ID_ATS:
> + ats_cap_present = true;
> + /*
> + * If ATS is requested and supported according to the kernel, add
> + * the ATS capability. If not supported according to the kernel
> or
> + * disabled on the qemu command line, omit the ATS cap.
> + */
> + if (ats_needed) {
> + pcie_add_capability(pdev, cap_id, cap_ver, next, size);
> + }
> + break;
> default:
> pcie_add_capability(pdev, cap_id, cap_ver, next, size);
> }
> @@ -2646,6 +2706,11 @@ static void vfio_add_ext_cap(VFIOPCIDevice
> *vdev)
> error_report_err(err);
> }
>
> + if (ats_needed && !ats_cap_present) {
> + warn_report("vfio-pci: ats=on requested, but host device has no "
> + "ATS extended capability");
> + }
> +
> /* Cleanup chain head ID if necessary */
> if (pci_get_word(pdev->config + PCI_CONFIG_SPACE_SIZE) == 0xFFFF) {
> pci_set_word(pdev->config + PCI_CONFIG_SPACE_SIZE, 0);
> @@ -2657,6 +2722,7 @@ static void vfio_add_ext_cap(VFIOPCIDevice
> *vdev)
> bool vfio_pci_add_capabilities(VFIOPCIDevice *vdev, Error **errp)
> {
> PCIDevice *pdev = PCI_DEVICE(vdev);
> + bool ats_needed = false;
>
> if (!(pdev->config[PCI_STATUS] & PCI_STATUS_CAP_LIST) ||
> !pdev->config[PCI_CAPABILITY_LIST]) {
> @@ -2667,7 +2733,11 @@ bool vfio_pci_add_capabilities(VFIOPCIDevice
> *vdev, Error **errp)
> return false;
> }
>
> - vfio_add_ext_cap(vdev);
> + if (!vfio_pci_ats_requested_and_supported(vdev, &ats_needed, errp)) {
> + return false;
> + }
> +
> + vfio_add_ext_cap(vdev, ats_needed);
> return true;
> }
>
> @@ -3815,6 +3885,7 @@ static const Property vfio_pci_properties[] = {
> DEFINE_PROP_BOOL("skip-vsc-check", VFIOPCIDevice, skip_vsc_check,
> true),
> DEFINE_PROP_UINT16("x-vpasid-cap-offset", VFIOPCIDevice,
> vpasid_cap_offset, 0),
> + DEFINE_PROP_ON_OFF_AUTO("ats", VFIOPCIDevice, ats,
> ON_OFF_AUTO_AUTO),
> };
>
> static void vfio_pci_set_fd(Object *obj, const char *str, Error **errp)
> @@ -3966,13 +4037,22 @@ static void vfio_pci_class_init(ObjectClass *klass,
> const void *data)
> "destination when doing live "
> "migration of device state via "
> "multifd channels");
> - object_class_property_set_description(klass, /* 11.0 */
> + object_class_property_set_description(klass, /* 11.0 */
> "x-vpasid-cap-offset",
> "PCIe extended configuration space
> offset at which to
> place a "
> "synthetic PASID extended
> capability when PASID is
> enabled via "
> "a vIOMMU. A value of 0 (default)
> places the capability
> at the "
> "end of the extended configuration
> space. The offset
> must be "
> "4-byte aligned and within the
> PCIe extended
> configuration space");
> + object_class_property_set_description(klass, /* 11.1 */
> + "ats",
> + "Control guest visibility of the
> ATS PCIe extended
> capability. "
> + "Valid values are on, off, and
> auto (default). "
> + "'off' always masks ATS. "
> + "'on' requires ATS support for the
> device and fails realize
> if the "
> + "host kernel reports ATS as
> unavailable for this device. "
> + "'auto' masks ATS only when the
> host kernel reports "
> + "ATS as unavailable");
> }
>
> static const TypeInfo vfio_pci_info = {
> --
> 2.43.0
