-----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");
+ }
