vfio/pci: Expose setup ROM at ROM bar when needed

If ROM bar is missing for any reason, we can fallback to using pdev->rom
to expose the ROM content to the guest. This fixes some passthrough use
cases where the upstream bridge does not have enough address window.

Signed-off-by: Yunxiang Li <Yunxiang.Li@amd.com>
Link: https://lore.kernel.org/r/20250102185013.15082-3-Yunxiang.Li@amd.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: yunxiali

drivers/vfio/pci/vfio_pci_config.c

@@ -511,6 +511,10 @@ static void vfio_bar_fixup(struct vfio_pci_core_device *vdev)
 		mask = ~(pci_resource_len(pdev, PCI_ROM_RESOURCE) - 1);
 		mask |= PCI_ROM_ADDRESS_ENABLE;
 		*vbar &= cpu_to_le32((u32)mask);
+	} else if (pdev->rom && pdev->romlen) {
+		mask = ~(roundup_pow_of_two(pdev->romlen) - 1);
+		mask |= PCI_ROM_ADDRESS_ENABLE;
+		*vbar &= cpu_to_le32((u32)mask);
 	} else {
 		*vbar = 0;
 	}

drivers/vfio/pci/vfio_pci_core.c

@@ -1054,25 +1054,27 @@ static int vfio_pci_ioctl_get_region_info(struct vfio_pci_core_device *vdev,
 
 		info.offset = VFIO_PCI_INDEX_TO_OFFSET(info.index);
 		info.flags = 0;
+		info.size = 0;
 
-		/* Report the BAR size, not the ROM size */
-		info.size = pci_resource_len(pdev, info.index);
-		if (!info.size)
-			break;
-
-		/*
-		 * Is it really there?  Enable memory decode for implicit access
-		 * in pci_map_rom().
-		 */
-		cmd = vfio_pci_memory_lock_and_enable(vdev);
-		io = pci_map_rom(pdev, &size);
-		if (io) {
+		if (pci_resource_start(pdev, PCI_ROM_RESOURCE)) {
+			/*
+			 * Check ROM content is valid. Need to enable memory
+			 * decode for ROM access in pci_map_rom().
+			 */
+			cmd = vfio_pci_memory_lock_and_enable(vdev);
+			io = pci_map_rom(pdev, &size);
+			if (io) {
+				info.flags = VFIO_REGION_INFO_FLAG_READ;
+				/* Report the BAR size, not the ROM size. */
+				info.size = pci_resource_len(pdev, PCI_ROM_RESOURCE);
+				pci_unmap_rom(pdev, io);
+			}
+			vfio_pci_memory_unlock_and_restore(vdev, cmd);
+		} else if (pdev->rom && pdev->romlen) {
 			info.flags = VFIO_REGION_INFO_FLAG_READ;
-			pci_unmap_rom(pdev, io);
-		} else {
-			info.size = 0;
+			/* Report BAR size as power of two. */
+			info.size = roundup_pow_of_two(pdev->romlen);
 		}
-		vfio_pci_memory_unlock_and_restore(vdev, cmd);
 
 		break;
 	}

drivers/vfio/pci/vfio_pci_rdwr.c

@@ -237,6 +237,8 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 
 	if (pci_resource_start(pdev, bar))
 		end = pci_resource_len(pdev, bar);
+	else if (bar == PCI_ROM_RESOURCE && pdev->rom && pdev->romlen)
+		end = roundup_pow_of_two(pdev->romlen);
 	else
 		return -EINVAL;
 
@@ -251,11 +253,14 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 		 * excluded range at the end of the actual ROM.  This makes
 		 * filling large ROM BARs much faster.
 		 */
-		io = pci_map_rom(pdev, &x_start);
-		if (!io) {
-			done = -ENOMEM;
-			goto out;
+		if (pci_resource_start(pdev, bar)) {
+			io = pci_map_rom(pdev, &x_start);
+		} else {
+			io = ioremap(pdev->rom, pdev->romlen);
+			x_start = pdev->romlen;
 		}
+		if (!io)
+			return -ENOMEM;
 		x_end = end;
 	} else {
 		int ret = vfio_pci_core_setup_barmap(vdev, bar);
@@ -278,8 +283,13 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	if (done >= 0)
 		*ppos += done;
 
-	if (bar == PCI_ROM_RESOURCE)
-		pci_unmap_rom(pdev, io);
+	if (bar == PCI_ROM_RESOURCE) {
+		if (pci_resource_start(pdev, bar))
+			pci_unmap_rom(pdev, io);
+		else
+			iounmap(io);
+	}
+
 out:
 	return done;
 }