[PATCH v3 1/5] vfio: selftests: Automatically detect the IOMMU mode in iommu_init()

[David Matlack <dmatlack@google.com>]

Pick the most preferred IOMMU mode that is available 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: iommufd, then the VFIO container,
then iommufd's VFIO container compatibility modes, with type1v2 ahead of
type1, i.e. prefer iommufd, the native VFIO container over iommufd's
emulation of it, and type1v2 over the older type1. Consider a mode
available if its container (or /dev/iommu) can be opened and supports
the mode's IOMMU type. Also require VFIO device cdevs for iommufd mode
and VFIO groups for container modes, since that is how the selftests
access devices in those modes. If no mode is available, fail with a
message suggesting that VFIO or IOMMUFD may not be loaded and enabled.

Suggested-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/kvm/aiIG2L_KdPwuxYA1@google.com/
Signed-off-by: David Matlack <dmatlack@google.com>
---
 .../vfio/lib/include/libvfio/iommu.h          |  6 +-
 tools/testing/selftests/vfio/lib/iommu.c      | 83 ++++++++++++++++---
 .../selftests/vfio/vfio_pci_device_test.c     |  4 +-
 .../selftests/vfio/vfio_pci_driver_test.c     |  2 +-
 4 files changed, 79 insertions(+), 16 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..53e38130c334 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 use the most preferred IOMMU mode that is
+ * available 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..ecdfbb62ac5e 100644
--- a/tools/testing/selftests/vfio/lib/iommu.c
+++ b/tools/testing/selftests/vfio/lib/iommu.c
@@ -20,14 +20,18 @@
 #include "../../../kselftest.h"
 #include <libvfio.h>
 
-const char *default_iommu_mode = MODE_IOMMUFD;
+#define NO_IOMMU_MODE_AVAILABLE_MSG \
+	"No IOMMU mode is available. Is VFIO (or IOMMUFD) loaded and enabled?"
 
-/* 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 +39,8 @@ 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 +49,72 @@ 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 iommufd_probe(void)
+{
+	/* iommufd mode requires VFIO device cdevs (CONFIG_VFIO_DEVICE_CDEV). */
+	return !access("/dev/vfio/devices", F_OK) &&
+	       !access("/dev/iommu", R_OK | W_OK);
+}
+
+static bool vfio_container_probe(const struct iommu_mode *mode)
+{
+	bool available;
+	int fd;
+
+	/*
+	 * Container modes, including the iommufd_compat modes, access devices
+	 * through VFIO groups (CONFIG_VFIO_GROUP). Check for them explicitly,
+	 * since iommufd provides the compat container even without VFIO groups.
+	 */
+	if (access("/sys/class/vfio", F_OK))
+		return false;
+
+	fd = open(mode->container_path, O_RDWR);
+	if (fd < 0)
+		return false;
+
+	available = ioctl(fd, VFIO_CHECK_EXTENSION, mode->iommu_type) > 0;
+	close(fd);
+	return available;
+}
+
+static bool iommu_mode_probe(const struct iommu_mode *mode)
+{
+	if (mode->container_path)
+		return vfio_container_probe(mode);
+
+	return iommufd_probe();
+}
+
+/* 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 +485,8 @@ 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_MSG);
 
 	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;
-- 
2.56.0.506.g506a7eabc5-goog