[PATCH 1/3] vfio: selftests: Automatically detect the IOMMU mode in iommu_init()
From: David Matlack
Date: Thu Oct 29 2026 - 18:18:03 EST
Automatically detect the IOMMU mode that is most likely to work on the
current system when iommu_init() is passed a NULL mode, rather than always
using iommufd. Drop the global default_iommu_mode and convert its users to
pass NULL to iommu_init().
iommufd is an optional feature, and VFIO's iommufd support additionally
requires CONFIG_VFIO_DEVICE_CDEV. Hardcoding iommufd as the default causes
tests to fail with an unhelpful assert on otherwise functional kernels
that only support the legacy VFIO container:
tools/testing/selftests/vfio/lib/iommu.c:448: Assertion Failure
Expression: iommu->iommufd > 0
Observed: 0xffffffffffffffff > 0
[errno: 2 - No such file or directory]
Order iommu_modes[] by preference and pick the first mode that is
available. Consider a mode available if its container (or /dev/iommu) can
be opened and, for container modes, if VFIO_CHECK_EXTENSION reports
support for the mode's IOMMU type. iommufd mode additionally requires VFIO
device cdevs. If no mode is available, fail with a message that explains
why.
Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Link: https://lore.kernel.org/kvm/aiIG2L_KdPwuxYA1@xxxxxxxxxx/
Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
---
.../vfio/lib/include/libvfio/iommu.h | 6 +-
tools/testing/selftests/vfio/lib/iommu.c | 67 +++++++++++++++----
.../selftests/vfio/vfio_pci_device_test.c | 4 +-
.../selftests/vfio/vfio_pci_driver_test.c | 2 +-
4 files changed, 62 insertions(+), 17 deletions(-)
diff --git a/tools/testing/selftests/vfio/lib/include/libvfio/iommu.h b/tools/testing/selftests/vfio/lib/include/libvfio/iommu.h
index e9a3386a4719..fb0a681ecc72 100644
--- a/tools/testing/selftests/vfio/lib/include/libvfio/iommu.h
+++ b/tools/testing/selftests/vfio/lib/include/libvfio/iommu.h
@@ -15,8 +15,6 @@ struct iommu_mode {
unsigned long iommu_type;
};
-extern const char *default_iommu_mode;
-
struct dma_region {
struct list_head link;
void *vaddr;
@@ -32,6 +30,10 @@ struct iommu {
struct list_head dma_regions;
};
+/*
+ * Pass NULL for iommu_mode to automatically pick the mode most likely to work
+ * on the current system.
+ */
struct iommu *iommu_init(const char *iommu_mode);
void iommu_cleanup(struct iommu *iommu);
diff --git a/tools/testing/selftests/vfio/lib/iommu.c b/tools/testing/selftests/vfio/lib/iommu.c
index b6f3c5c84e01..535e6c7cf260 100644
--- a/tools/testing/selftests/vfio/lib/iommu.c
+++ b/tools/testing/selftests/vfio/lib/iommu.c
@@ -20,14 +20,15 @@
#include "../../../kselftest.h"
#include <libvfio.h>
-const char *default_iommu_mode = MODE_IOMMUFD;
-
-/* Reminder: Keep in sync with FIXTURE_VARIANT_ADD_ALL_IOMMU_MODES(). */
+/*
+ * Ordered by preference, i.e. iommu_init(NULL) picks the first mode in this
+ * list that is available on the current system.
+ *
+ * Reminder: Keep in sync with FIXTURE_VARIANT_ADD_ALL_IOMMU_MODES().
+ */
static const struct iommu_mode iommu_modes[] = {
{
- .name = MODE_VFIO_TYPE1_IOMMU,
- .container_path = "/dev/vfio/vfio",
- .iommu_type = VFIO_TYPE1_IOMMU,
+ .name = MODE_IOMMUFD,
},
{
.name = MODE_VFIO_TYPE1V2_IOMMU,
@@ -35,8 +36,8 @@ static const struct iommu_mode iommu_modes[] = {
.iommu_type = VFIO_TYPE1v2_IOMMU,
},
{
- .name = MODE_IOMMUFD_COMPAT_TYPE1,
- .container_path = "/dev/iommu",
+ .name = MODE_VFIO_TYPE1_IOMMU,
+ .container_path = "/dev/vfio/vfio",
.iommu_type = VFIO_TYPE1_IOMMU,
},
{
@@ -45,16 +46,56 @@ static const struct iommu_mode iommu_modes[] = {
.iommu_type = VFIO_TYPE1v2_IOMMU,
},
{
- .name = MODE_IOMMUFD,
+ .name = MODE_IOMMUFD_COMPAT_TYPE1,
+ .container_path = "/dev/iommu",
+ .iommu_type = VFIO_TYPE1_IOMMU,
},
};
-static const struct iommu_mode *lookup_iommu_mode(const char *iommu_mode)
+static bool iommu_mode_probe(const struct iommu_mode *mode)
+{
+ const char *path = mode->container_path ?: "/dev/iommu";
+ bool available;
+ int fd;
+
+ /* iommufd mode requires VFIO device cdevs (CONFIG_VFIO_DEVICE_CDEV). */
+ if (!mode->container_path && access("/dev/vfio/devices", F_OK))
+ return false;
+
+ fd = open(path, O_RDWR);
+ if (fd < 0)
+ return false;
+
+ available = !mode->container_path ||
+ ioctl(fd, VFIO_CHECK_EXTENSION, mode->iommu_type) > 0;
+
+ close(fd);
+ return available;
+}
+
+/* Returns the most preferred IOMMU mode that is available, or NULL if none. */
+static const struct iommu_mode *iommu_mode_detect(void)
+{
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(iommu_modes); i++) {
+ if (iommu_mode_probe(&iommu_modes[i]))
+ return &iommu_modes[i];
+ }
+
+ return NULL;
+}
+
+/*
+ * Returns the IOMMU mode with the given name, or if iommu_mode is NULL, the
+ * most preferred IOMMU mode that is available (or NULL if none).
+ */
+static const struct iommu_mode *iommu_mode_lookup(const char *iommu_mode)
{
int i;
if (!iommu_mode)
- iommu_mode = default_iommu_mode;
+ return iommu_mode_detect();
for (i = 0; i < ARRAY_SIZE(iommu_modes); i++) {
if (strcmp(iommu_mode, iommu_modes[i].name))
@@ -425,7 +466,9 @@ struct iommu *iommu_init(const char *iommu_mode)
INIT_LIST_HEAD(&iommu->dma_regions);
- iommu->mode = lookup_iommu_mode(iommu_mode);
+ iommu->mode = iommu_mode_lookup(iommu_mode);
+ VFIO_ASSERT_NOT_NULL(iommu->mode,
+ "No IOMMU mode available. Is VFIO (or IOMMUFD) loaded and enabled?\n");
container_path = iommu->mode->container_path;
if (container_path) {
diff --git a/tools/testing/selftests/vfio/vfio_pci_device_test.c b/tools/testing/selftests/vfio/vfio_pci_device_test.c
index 93c11fd5e081..59a7c1f71804 100644
--- a/tools/testing/selftests/vfio/vfio_pci_device_test.c
+++ b/tools/testing/selftests/vfio/vfio_pci_device_test.c
@@ -29,7 +29,7 @@ FIXTURE(vfio_pci_device_test) {
FIXTURE_SETUP(vfio_pci_device_test)
{
- self->iommu = iommu_init(default_iommu_mode);
+ self->iommu = iommu_init(NULL);
self->device = vfio_pci_device_init(device_bdf, self->iommu);
}
@@ -121,7 +121,7 @@ FIXTURE_VARIANT_ADD(vfio_pci_irq_test, msix) {
FIXTURE_SETUP(vfio_pci_irq_test)
{
- self->iommu = iommu_init(default_iommu_mode);
+ self->iommu = iommu_init(NULL);
self->device = vfio_pci_device_init(device_bdf, self->iommu);
}
diff --git a/tools/testing/selftests/vfio/vfio_pci_driver_test.c b/tools/testing/selftests/vfio/vfio_pci_driver_test.c
index 9538a3a436a6..f8f99ee6bb1a 100644
--- a/tools/testing/selftests/vfio/vfio_pci_driver_test.c
+++ b/tools/testing/selftests/vfio/vfio_pci_driver_test.c
@@ -255,7 +255,7 @@ static bool device_has_selftests_driver(const char *bdf)
struct iommu *iommu;
bool has_driver;
- iommu = iommu_init(default_iommu_mode);
+ iommu = iommu_init(NULL);
device = vfio_pci_device_init(device_bdf, iommu);
has_driver = !!device->driver.ops;
--
2.56.0.317.g454f8302e1-goog