[PATCH 1/3] vfio: selftests: Automatically detect the IOMMU mode in iommu_init()

[David Matlack <dmatlack@google.com>]

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@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      | 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