From: Mina Almasry <almasrymina@google.com>

Add vfio_pci_find_mem_bar() to libvfio to locate a BAR (or range within
a BAR) of at least min_size bytes that behaves like ordinary memory and
can be safely read and written without device-specific side effects.

Detection is backed by a table of known devices, starting with QEMU's
pci-testdev (1b36:0005) BAR 2 when configured with membar=<size> and
membar-backed=on.

Suggested-by: David Matlack <dmatlack@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: Matt Evans <matt@ozlabs.org>
Signed-off-by: Mina Almasry <almasrymina@google.com>

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 .../lib/include/libvfio/vfio_pci_device.h     | 10 +++
 .../selftests/vfio/lib/vfio_pci_device.c      | 67 +++++++++++++++++++
 2 files changed, 77 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 007adc322c1e..19d458adba1a 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
@@ -15,6 +15,13 @@ struct vfio_pci_bar {
 	void *vaddr;
 };
 
+struct vfio_pci_mem_bar {
+	struct vfio_pci_bar *bar;
+	void *vaddr;
+	u64 offset;
+	u64 size;
+};
+
 struct vfio_pci_device {
 	const char *bdf;
 	int fd;
@@ -162,6 +169,9 @@ static inline bool vfio_pci_device_match(struct vfio_pci_device *device,
 		(device_id == vfio_pci_config_readw(device, PCI_DEVICE_ID));
 }
 
+bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
+			   struct vfio_pci_mem_bar *mem_bar);
+
 const char *vfio_pci_get_cdev_path(const char *bdf);
 
 void vfio_pci_group_setup(struct vfio_pci_device *device, const char *bdf);
diff --git a/tools/testing/selftests/vfio/lib/vfio_pci_device.c b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
index 4063a0e2b3df..d5308fe61490 100644
--- a/tools/testing/selftests/vfio/lib/vfio_pci_device.c
+++ b/tools/testing/selftests/vfio/lib/vfio_pci_device.c
@@ -19,6 +19,7 @@
 #include <linux/log2.h>
 #include <linux/mman.h>
 #include <linux/overflow.h>
+#include <linux/pci_ids.h>
 #include <linux/sizes.h>
 #include <linux/types.h>
 #include <linux/vfio.h>
@@ -245,6 +246,72 @@ static void vfio_pci_bar_unmap_all(struct vfio_pci_device *device)
 	}
 }
 
+#define PCI_DEVICE_ID_REDHAT_TEST	0x0005
+
+struct vfio_pci_mem_bar_desc {
+	u16 vendor_id;
+	u16 device_id;
+	u8 bar_index;
+	u64 offset;
+	u64 size;
+};
+
+static const struct vfio_pci_mem_bar_desc mem_bar_descs[] = {
+	/* QEMU pci-testdev with membar=<size>,membar-backed=on */
+	{
+		.vendor_id = PCI_VENDOR_ID_REDHAT,
+		.device_id = PCI_DEVICE_ID_REDHAT_TEST,
+		.bar_index = 2,
+	},
+};
+
+bool vfio_pci_find_mem_bar(struct vfio_pci_device *device, u64 min_size,
+			   struct vfio_pci_mem_bar *mem_bar)
+{
+	const u32 rw_mmap_flags = VFIO_REGION_INFO_FLAG_MMAP |
+				  VFIO_REGION_INFO_FLAG_READ |
+				  VFIO_REGION_INFO_FLAG_WRITE;
+	size_t i;
+
+	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;
+		u64 size;
+
+		if (!vfio_pci_device_match(device, desc->vendor_id,
+					   desc->device_id))
+			continue;
+
+		VFIO_ASSERT_LT(desc->bar_index, PCI_STD_NUM_BARS);
+		bar = &device->bars[desc->bar_index];
+
+		if ((bar->info.flags & rw_mmap_flags) != rw_mmap_flags)
+			continue;
+
+		VFIO_ASSERT_NOT_NULL(bar->vaddr);
+
+		if (bar->info.size <= desc->offset)
+			continue;
+
+		size = bar->info.size - desc->offset;
+		if (desc->size)
+			size = min(size, desc->size);
+
+		if (size < min_size)
+			continue;
+
+		*mem_bar = (struct vfio_pci_mem_bar){
+			.bar = bar,
+			.vaddr = bar->vaddr + desc->offset,
+			.offset = desc->offset,
+			.size = size,
+		};
+		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