vfio: Improve error reporting when MMIO region mapping fails

When the IOMMU address space width is smaller than the physical
address width, a MMIO region of a device can fail to map because the
region is outside the supported IOVA ranges of the VM. In this case,
PCI peer-to-peer transactions on BARs are not supported. This can
occur with the 39-bit IOMMU address space width, as can be the case on
some Intel consumer processors, or when using a vIOMMU device with
default settings.

The current error message is unclear, improve it and also change the
error report to a warning because it is a non fatal condition for the
VM. To prevent excessive log messages, restrict these recurring DMA
mapping errors to a single warning at runtime.

Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250206131438.1505542-6-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/common.c

@@ -555,6 +555,18 @@ static bool vfio_get_section_iova_range(VFIOContainerBase *bcontainer,
     return true;
 }
 
+static void vfio_device_error_append(VFIODevice *vbasedev, Error **errp)
+{
+    /*
+     * MMIO region mapping failures are not fatal but in this case PCI
+     * peer-to-peer transactions are broken.
+     */
+    if (vbasedev && vbasedev->type == VFIO_DEVICE_TYPE_PCI) {
+        error_append_hint(errp, "%s: PCI peer-to-peer transactions "
+                          "on BARs are not supported.\n", vbasedev->name);
+    }
+}
+
 static void vfio_listener_region_add(MemoryListener *listener,
                                      MemoryRegionSection *section)
 {
@@ -670,7 +682,10 @@ static void vfio_listener_region_add(MemoryListener *listener,
                    strerror(-ret));
         if (memory_region_is_ram_device(section->mr)) {
             /* Allow unexpected mappings not to be fatal for RAM devices */
-            error_report_err(err);
+            VFIODevice *vbasedev =
+                vfio_get_vfio_device(memory_region_owner(section->mr));
+            vfio_device_error_append(vbasedev, &err);
+            warn_report_err_once(err);
             return;
         }
         goto fail;