Write and read back test patterns before returning a BAR from
vfio_pci_find_mem_bar(), restoring the original contents and
PCI_COMMAND_MEMORY state afterward.

QEMU's pci-testdev exposes a readable, writable, mmap-capable BAR2
whenever membar=<size> is passed, even if membar-backed=on was omitted
and accesses are dropped as unimplemented MMIO. Probing the BAR upfront
lets callers skip cleanly instead of failing later DMA or fault checks.

Suggested-by: Alex Williamson <alex@shazbot.org>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 .../selftests/vfio/lib/vfio_pci_device.c      | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
index d5308fe61490..9840463594ab 100644
--- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
+++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
@@ -13,6 +13,7 @@
 #include <sys/mman.h>
 
 #include <linux/align.h>
+#include <linux/io.h>
 #include <linux/iommufd.h>
 #include <linux/kernel.h>
 #include <linux/limits.h>
@@ -265,6 +266,33 @@ static const struct vfio_pci_mem_bar_desc mem_bar_descs[] = {
 	},
 };
 
+static bool vfio_pci_mem_bar_verify(struct vfio_pci_device *device, void *vaddr)
+{
+	const u64 pattern = 0x55aa55aa55aa55aaULL;
+	u16 orig_cmd;
+	u64 orig;
+	bool ok;
+
+	orig_cmd = vfio_pci_config_readw(device, PCI_COMMAND);
+	if (!(orig_cmd & PCI_COMMAND_MEMORY))
+		vfio_pci_config_writew(device, PCI_COMMAND,
+				       orig_cmd | PCI_COMMAND_MEMORY);
+
+	orig = readq(vaddr);
+	writeq(pattern, vaddr);
+	ok = (readq(vaddr) == pattern);
+	if (ok) {
+		writeq(~pattern, vaddr);
+		ok = (readq(vaddr) == ~pattern);
+	}
+	writeq(orig, vaddr);
+
+	if (!(orig_cmd & PCI_COMMAND_MEMORY))
+		vfio_pci_config_writew(device, PCI_COMMAND, orig_cmd);
+
+	return ok;
+}
+
 bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
 			   struct vfio_pci_mem_bar *mem_bar)
 {
@@ -273,6 +301,8 @@ bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
 				  VFIO_REGION_INFO_FLAG_WRITE;
 	size_t i;
 
+	min_size = max_t(u64, min_size, sizeof(u64));
+
 	for (i = 0; i < ARRAY_SIZE(mem_bar_descs); i++) {
 		const struct vfio_pci_mem_bar_desc *desc = &mem_bar_descs[i];
 		struct vfio_pci_bar *bar;
@@ -300,6 +330,9 @@ bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
 		if (size < min_size)
 			continue;
 
+		if (!vfio_pci_mem_bar_verify(device, bar->vaddr + desc->offset))
+			continue;
+
 		*mem_bar = (struct vfio_pci_mem_bar){
 			.bar = bar,
 			.vaddr = bar->vaddr + desc->offset,
-- 
2.56.0.567.gdb80ae8cd5-goog