Re: [PATCH v3 4/8] hw/vfio: eradicate CONFIG_IOMMU from sources

[Cédric Le Goater]

Hello Pierrick,


On 3/15/26 08:08, Pierrick Bouvier wrote:

As pointed in c1139fa4 "vfio/iommufd: Remove CONFIG_IOMMUFD usage",
iommufd presence can be detected at runtime (through /dev/iommufd)

Note that the device name is "/dev/iommu/".

The overall flow is a slightly more complex. If IOMMUFD is not
supported by the kernel, QEMU will fail when attemting to attach
to the device. This failure occurs before the device is connected
to the IOMMUFD host backend and QEMU will report an error such as :

  qemu-XYZ: -device vfio-pci,host=0000:01:00.0,iommufd=iommufd0: vfio 0000:01:00.0: 
vfio/sys/bus/pci/devices/0000:01:00.0/vfio-dev: failed to load 
"/sys/bus/pci/devices/0000:01:00.0/vfio-dev/vfio0/dev

But this is a kernel-side issue.

and
we don't need to isolate this at build time.

It's much (much) more simple to expose this inconditionally

we should better explain the motivation behind this approach.

compared to
build dynamically properties arrays.

and that would be an alternative that we don't want to explore.


Let's come back to the proposal. Currently, when the QEMU platform
doesn't support IOMMUFD (i.e no support for the backend), QEMU reports
at runtime :

  qemu-XYZ...: invalid object type: iommufd

As a consequence, the vfio-pci device doesn't expose the "iommufd"
property when the backend IOMMUFD is not available.

However, the additional filtering on the vfio-pci properties based
on CONFIG_IOMMUFD seems unnecessary. Instead the "iommufd" property
could always be exposed, whether or not the QEMU build includes IOMMUFD
support (kernel support is another topic). If the property is used
in an environment where IOMMUFD is unsupported, QEMU will simply
fail to create the IOMMUFD object.

Could you please rephrase the commit log with some of this information ?

Thanks,

C.

Signed-off-by: Pierrick Bouvier <pierrick.bouv...@linaro.org>
---
  hw/vfio/ap.c  |  9 ---------
  hw/vfio/ccw.c |  9 ---------
  hw/vfio/pci.c | 11 -----------
  3 files changed, 29 deletions(-)

diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
index e58a0169af9..856fa2678cd 100644
--- a/hw/vfio/ap.c
+++ b/hw/vfio/ap.c
@@ -11,7 +11,6 @@
   */

#include "qemu/osdep.h"

-#include CONFIG_DEVICES /* CONFIG_IOMMUFD */
  #include <linux/vfio.h>
  #include <sys/ioctl.h>
  #include "qapi/error.h"
@@ -279,10 +278,8 @@ static void vfio_ap_unrealize(DeviceState *dev)

static const Property vfio_ap_properties[] = {

      DEFINE_PROP_STRING("sysfsdev", VFIOAPDevice, vdev.sysfsdev),
-#ifdef CONFIG_IOMMUFD
      DEFINE_PROP_LINK("iommufd", VFIOAPDevice, vdev.iommufd,
                       TYPE_IOMMUFD_BACKEND, IOMMUFDBackend *),
-#endif
  };

static void vfio_ap_reset(DeviceState *dev)

@@ -320,21 +317,17 @@ static void vfio_ap_instance_init(Object *obj)
      vbasedev->mdev = true;
  }

-#ifdef CONFIG_IOMMUFD

  static void vfio_ap_set_fd(Object *obj, const char *str, Error **errp)
  {
      vfio_device_set_fd(&VFIO_AP_DEVICE(obj)->vdev, str, errp);
  }
-#endif

static void vfio_ap_class_init(ObjectClass *klass, const void *data)

  {
      DeviceClass *dc = DEVICE_CLASS(klass);

device_class_set_props(dc, vfio_ap_properties);

-#ifdef CONFIG_IOMMUFD
      object_class_property_add_str(klass, "fd", NULL, vfio_ap_set_fd);
-#endif
      dc->vmsd = &vfio_ap_vmstate;
      dc->desc = "VFIO-based AP device assignment";
      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
@@ -347,11 +340,9 @@ static void vfio_ap_class_init(ObjectClass *klass, const 
void *data)
      object_class_property_set_description(klass, /* 3.1 */
                                            "sysfsdev",
                                            "Host sysfs path of assigned 
device");
-#ifdef CONFIG_IOMMUFD
      object_class_property_set_description(klass, /* 9.0 */
                                            "iommufd",
                                            "Set host IOMMUFD backend device");
-#endif
  }

static const TypeInfo vfio_ap_info = {

diff --git a/hw/vfio/ccw.c b/hw/vfio/ccw.c
index 2251facb356..c66f42a13c2 100644
--- a/hw/vfio/ccw.c
+++ b/hw/vfio/ccw.c
@@ -15,7 +15,6 @@
   */

#include "qemu/osdep.h"

-#include CONFIG_DEVICES /* CONFIG_IOMMUFD */
  #include <linux/vfio.h>
  #include <linux/vfio_ccw.h>
  #include <sys/ioctl.h>
@@ -647,10 +646,8 @@ static void vfio_ccw_unrealize(DeviceState *dev)
  static const Property vfio_ccw_properties[] = {
      DEFINE_PROP_STRING("sysfsdev", VFIOCCWDevice, vdev.sysfsdev),
      DEFINE_PROP_BOOL("force-orb-pfch", VFIOCCWDevice, force_orb_pfch, false),
-#ifdef CONFIG_IOMMUFD
      DEFINE_PROP_LINK("iommufd", VFIOCCWDevice, vdev.iommufd,
                       TYPE_IOMMUFD_BACKEND, IOMMUFDBackend *),
-#endif
      DEFINE_PROP_CCW_LOADPARM("loadparm", CcwDevice, loadparm),
  };

@@ -679,12 +676,10 @@ static void vfio_ccw_instance_init(Object *obj)

                       DEVICE(vcdev), true);
  }

-#ifdef CONFIG_IOMMUFD

  static void vfio_ccw_set_fd(Object *obj, const char *str, Error **errp)
  {
      vfio_device_set_fd(&VFIO_CCW(obj)->vdev, str, errp);
  }
-#endif

static void vfio_ccw_class_init(ObjectClass *klass, const void *data)

  {
@@ -692,9 +687,7 @@ static void vfio_ccw_class_init(ObjectClass *klass, const 
void *data)
      S390CCWDeviceClass *cdc = S390_CCW_DEVICE_CLASS(klass);

device_class_set_props(dc, vfio_ccw_properties);

-#ifdef CONFIG_IOMMUFD
      object_class_property_add_str(klass, "fd", NULL, vfio_ccw_set_fd);
-#endif
      dc->vmsd = &vfio_ccw_vmstate;
      dc->desc = "VFIO-based subchannel assignment";
      set_bit(DEVICE_CATEGORY_MISC, dc->categories);
@@ -713,11 +706,9 @@ static void vfio_ccw_class_init(ObjectClass *klass, const 
void *data)
      object_class_property_set_description(klass, /* 3.0 */
                                            "force-orb-pfch",
                                            "Force unlimited prefetch");
-#ifdef CONFIG_IOMMUFD
      object_class_property_set_description(klass, /* 9.0 */
                                            "iommufd",
                                            "Set host IOMMUFD backend device");
-#endif
      object_class_property_set_description(klass, /* 9.2 */
                                            "loadparm",
                                            "Define which devices that can be used 
for booting");
diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
index 94c174a773f..df617f1fe46 100644
--- a/hw/vfio/pci.c
+++ b/hw/vfio/pci.c
@@ -19,7 +19,6 @@
   */

#include "qemu/osdep.h"

-#include CONFIG_DEVICES /* CONFIG_IOMMUFD */
  #include <linux/vfio.h>
  #include <sys/ioctl.h>

@@ -3471,9 +3470,7 @@ static void vfio_pci_realize(PCIDevice *pdev, Error **errp)

                ~vdev->host.slot || ~vdev->host.function)) {
              error_setg(errp, "No provided host device");
              error_append_hint(errp, "Use -device vfio-pci,host=DDDD:BB:DD.F "
-#ifdef CONFIG_IOMMUFD
                                "or -device vfio-pci,fd=DEVICE_FD "
-#endif
                                "or -device 
vfio-pci,sysfsdev=PATH_TO_DEVICE\n");
              return;
          }
@@ -3816,22 +3813,18 @@ static const Property vfio_pci_properties[] = {
                                     qdev_prop_nv_gpudirect_clique, uint8_t),
      DEFINE_PROP_OFF_AUTO_PCIBAR("x-msix-relocation", VFIOPCIDevice, msix_relo,
                                  OFF_AUTO_PCIBAR_OFF),
-#ifdef CONFIG_IOMMUFD
      DEFINE_PROP_LINK("iommufd", VFIOPCIDevice, vbasedev.iommufd,
                       TYPE_IOMMUFD_BACKEND, IOMMUFDBackend *),
-#endif
      DEFINE_PROP_BOOL("skip-vsc-check", VFIOPCIDevice, skip_vsc_check, true),
      DEFINE_PROP_UINT16("x-vpasid-cap-offset", VFIOPCIDevice,
                         vpasid_cap_offset, 0),
  };

-#ifdef CONFIG_IOMMUFD

  static void vfio_pci_set_fd(Object *obj, const char *str, Error **errp)
  {
      VFIOPCIDevice *vdev = VFIO_PCI_DEVICE(obj);
      vfio_device_set_fd(&vdev->vbasedev, str, errp);
  }
-#endif

static void vfio_pci_class_init(ObjectClass *klass, const void *data)

  {
@@ -3840,9 +3833,7 @@ static void vfio_pci_class_init(ObjectClass *klass, const 
void *data)

device_class_set_legacy_reset(dc, vfio_pci_reset);

      device_class_set_props(dc, vfio_pci_properties);
-#ifdef CONFIG_IOMMUFD
      object_class_property_add_str(klass, "fd", NULL, vfio_pci_set_fd);
-#endif
      dc->vmsd = &vfio_cpr_pci_vmstate;
      dc->desc = "VFIO-based PCI device assignment";
      pdc->realize = vfio_pci_realize;
@@ -3944,11 +3935,9 @@ static void vfio_pci_class_init(ObjectClass *klass, 
const void *data)
                                            "vf-token",
                                            "Specify UUID VF token. Required for VF 
when PF is owned "
                                            "by another VFIO driver");
-#ifdef CONFIG_IOMMUFD
      object_class_property_set_description(klass, /* 9.0 */
                                            "iommufd",
                                            "Set host IOMMUFD backend device");
-#endif
      object_class_property_set_description(klass, /* 9.1 */
                                            "x-device-dirty-page-tracking",
                                            "Disable device dirty page tracking and 
use "