[PATCH v5 10/27] vfio/pci: Migrate MSI-X exclusion onto the generic excluded-range list

[mhonap]

From: Manish Honap <mhonap@xxxxxxxxxx>

The MSI-X table is virtualized in vfio_pci_bar_rw() by an open-coded
x_start/x_end window that fills reads with -1 and drops writes. Now that
a generic excluded-range list expresses the same fill/drop behavior,
register the MSI-X table as a read and write excluded range instead of
special-casing it in the read/write path.

Add the range when the MSI-X capability is parsed in
vfio_pci_core_enable() and clear the list in vfio_pci_core_disable()
alongside the config teardown. The read/write path now relies solely on
vfio_pci_bar_find_exclusion(), so MSI-X and a provider's (e.g. vfio-cxl)
trapped registers share one mechanism.

No behavioral change: an access to the MSI-X table still reads -1 and
drops writes.

Assisted-by: LLM
Signed-off-by: Manish Honap <mhonap@xxxxxxxxxx>
---
 drivers/vfio/pci/vfio_pci_core.c | 19 +++++++++++
 drivers/vfio/pci/vfio_pci_rdwr.c | 56 ++++++++++++++++++++++++--------
 2 files changed, 62 insertions(+), 13 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 9e4fa5d088a4..c5b7a59a4548 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -589,6 +589,8 @@ static const struct dev_pm_ops vfio_pci_core_pm_ops = {
 			   NULL)
 };
 
+static void vfio_pci_free_excluded_ranges(struct vfio_pci_core_device *vdev);
+
 int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
 {
 	struct pci_dev *pdev = vdev->pdev;
@@ -655,6 +657,22 @@ int vfio_pci_core_enable(struct vfio_pci_core_device *vdev)
 		vdev->msix_offset = table & PCI_MSIX_TABLE_OFFSET;
 		vdev->msix_size = ((flags & PCI_MSIX_FLAGS_QSIZE) + 1) * 16;
 		vdev->has_dyn_msix = pci_msix_can_alloc_dyn(pdev);
+
+		/*
+		 * Virtualize the MSI-X table through the excluded-range list:
+		 * reads fill -1 and writes are dropped so the guest never
+		 * reaches the hardware table directly.
+		 */
+		ret = vfio_pci_core_add_excluded_range(vdev, vdev->msix_bar,
+						       vdev->msix_offset,
+						       vdev->msix_size,
+						       VFIO_PCI_EXCLUDE_READ |
+						       VFIO_PCI_EXCLUDE_WRITE);
+		if (ret) {
+			vfio_pci_free_excluded_ranges(vdev);
+			vfio_config_free(vdev);
+			goto out_free_zdev;
+		}
 	} else {
 		vdev->msix_bar = 0xFF;
 		vdev->has_dyn_msix = false;
@@ -741,6 +759,7 @@ void vfio_pci_core_disable(struct vfio_pci_core_device *vdev)
 	vdev->region = NULL; /* don't krealloc a freed pointer */
 
 	vfio_config_free(vdev);
+	vfio_pci_free_excluded_ranges(vdev);
 
 	for (i = 0; i < PCI_STD_NUM_BARS; i++) {
 		bar = i + PCI_STD_RESOURCES;
diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
index 48da1cb08296..f8e5f94a2e8a 100644
--- a/drivers/vfio/pci/vfio_pci_rdwr.c
+++ b/drivers/vfio/pci/vfio_pci_rdwr.c
@@ -256,21 +256,51 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 		}
 	}
 
-	if (bar == vdev->msix_bar) {
-		x_start = vdev->msix_offset;
-		x_end = vdev->msix_offset + vdev->msix_size;
-	}
-
 	/*
-	 * A provider-excluded sub-range is filled with -1 on read and dropped on
-	 * write for the same reason: the guest reaches it only through the trap.
-	 * An access spans at most one exclusion window.
+	 * The MSI-X table and any provider-excluded sub-ranges (such as a CXL
+	 * HDM decoder block) are filled with -1 on read and dropped on write:
+	 * the guest reaches them only through the virtualized path, never the
+	 * hardware directly. A BAR can hold several such windows and a single
+	 * access may span more than one, so walk the access one window at a
+	 * time. The ROM BAR uses the single trailing window set above.
 	 */
-	vfio_pci_bar_find_exclusion(vdev, bar, pos, count, iswrite,
-				    &x_start, &x_end);
-
-	done = vfio_pci_core_do_io_rw(vdev, res->flags & IORESOURCE_MEM, io, buf, pos,
-				      count, x_start, x_end, iswrite, max_width);
+	if (bar == PCI_ROM_RESOURCE) {
+		done = vfio_pci_core_do_io_rw(vdev, res->flags & IORESOURCE_MEM,
+					      io, buf, pos, count, x_start, x_end,
+					      iswrite, max_width);
+	} else {
+		done = 0;
+		while (count) {
+			size_t chunk;
+			ssize_t ret;
+
+			x_start = 0;
+			x_end = 0;
+			if (vfio_pci_bar_find_exclusion(vdev, bar, pos, count,
+							iswrite, &x_start,
+							&x_end))
+				chunk = min(count, (size_t)(x_end - pos));
+			else
+				chunk = count;
+
+			ret = vfio_pci_core_do_io_rw(vdev,
+						     res->flags & IORESOURCE_MEM,
+						     io, buf, pos, chunk,
+						     x_start, x_end, iswrite,
+						     max_width);
+			if (ret < 0) {
+				if (!done)
+					done = ret;
+				break;
+			}
+			done += ret;
+			pos += ret;
+			buf += ret;
+			count -= ret;
+			if ((size_t)ret < chunk)
+				break;
+		}
+	}
 
 	if (done >= 0)
 		*ppos += done;
-- 
2.25.1