Add vfio_pci_find_p2p_pair() to locate an initiator device with memcpy
driver support and a distinct target device with a memory BAR of at
least min_bar_size bytes from an array of VFIO PCI devices.

Factor out vfio_pci_has_p2p_path() as the central place to validate PCI
topology and P2P DMA reachability between the two devices in the future.

Signed-off-by: David Matlack <dmatlack@google.com>
---
 .../lib/include/libvfio/vfio_pci_device.h     |  8 ++++
 .../selftests/vfio/lib/vfio_pci_device.c      | 48 +++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/tools/testing/selftests/vfio/lib/include/libvfio/vfio_pci_device.h b/tools/testing/selftests/vfio/lib/include/libvfio/vfio_pci_device.h
index fd130f680a2d..9bda93712859 100644
--- a/tools/testing/selftests/vfio/lib/include/libvfio/vfio_pci_device.h
+++ b/tools/testing/selftests/vfio/lib/include/libvfio/vfio_pci_device.h
@@ -42,6 +42,12 @@ struct vfio_pci_device {
 	struct vfio_pci_driver driver;
 };
 
+struct vfio_pci_p2p_pair {
+	struct vfio_pci_device *initiator;
+	struct vfio_pci_device *target;
+	struct vfio_pci_mem_bar mem_bar;
+};
+
 #define dev_info(_dev, _fmt, ...) printf("%s: " _fmt, (_dev)->bdf, ##__VA_ARGS__)
 #define dev_err(_dev, _fmt, ...) fprintf(stderr, "%s: " _fmt, (_dev)->bdf, ##__VA_ARGS__)
 
@@ -171,6 +177,8 @@ static inline bool vfio_pci_device_match(struct vfio_pci_device *device,
 
 bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
 			   struct vfio_pci_mem_bar *mem_bar);
+bool vfio_pci_find_p2p_pair(struct vfio_pci_device **devices, int nr_devices,
+			    u64 min_bar_size, struct vfio_pci_p2p_pair *pair);
 
 int __vfio_pci_dmabuf_create(struct vfio_pci_device *device, u32 region_index,
 			     u64 offset, u64 length, u32 open_flags);
diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
index eca6eb13d7f0..16fc98ed170b 100644
--- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
+++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
@@ -378,6 +378,54 @@ bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
 	return false;
 }
 
+static bool vfio_pci_has_p2p_path(struct vfio_pci_device *initiator,
+				  struct vfio_pci_device *target)
+{
+	/*
+	 * TODO: Check PCI topology / ACS / host bridge support to verify that
+	 * P2P DMA transactions from @initiator can reach @target.
+	 */
+	return initiator != target;
+}
+
+bool vfio_pci_find_p2p_pair(struct vfio_pci_device **devices, int nr_devices,
+			    u64 min_bar_size, struct vfio_pci_p2p_pair *pair)
+{
+	struct vfio_pci_mem_bar mem_bar;
+	int i, j;
+
+	for (i = 0; i < nr_devices; i++) {
+		struct vfio_pci_device *initiator = devices[i];
+		const struct vfio_pci_driver_ops *ops = initiator->driver.ops;
+
+		if (!ops || !ops->memcpy_start || !ops->memcpy_wait)
+			continue;
+
+		for (j = 0; j < nr_devices; j++) {
+			struct vfio_pci_device *target = devices[j];
+
+			if (i == j)
+				continue;
+
+			if (!vfio_pci_has_p2p_path(initiator, target))
+				continue;
+
+			if (!vfio_pci_find_mem_bar(target, min_bar_size,
+						   &mem_bar))
+				continue;
+
+			*pair = (struct vfio_pci_p2p_pair){
+				.initiator = initiator,
+				.target = target,
+				.mem_bar = mem_bar,
+			};
+			return true;
+		}
+	}
+
+	return false;
+}
+
 void vfio_pci_config_access(struct vfio_pci_device *device, bool write,
 			    size_t config, size_t size, void *data)
 {
-- 
2.56.0.567.gdb80ae8cd5-goog