vfio/pci: introduce config_offset field in VFIOConfigMirrorQuirk

Device may only expose a specific portion of PCI config space through a
region in a BAR, such behavior is seen in igd GGC and BDSM mirrors in
BAR0. To handle these, config_offset is introduced to allow mirroring
arbitrary region in PCI config space.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20250104154219.7209-3-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

hw/vfio/pci-quirks.c

@@ -162,6 +162,7 @@ static uint64_t vfio_generic_quirk_mirror_read(void *opaque,
     (void)vfio_region_read(&vdev->bars[mirror->bar].region,
                            addr + mirror->offset, size);
 
+    addr += mirror->config_offset;
     data = vfio_pci_read_config(&vdev->pdev, addr, size);
     trace_vfio_quirk_generic_mirror_read(vdev->vbasedev.name,
                                          memory_region_name(mirror->mem),
@@ -175,6 +176,7 @@ static void vfio_generic_quirk_mirror_write(void *opaque, hwaddr addr,
     VFIOConfigMirrorQuirk *mirror = opaque;
     VFIOPCIDevice *vdev = mirror->vdev;
 
+    addr += mirror->config_offset;
     vfio_pci_write_config(&vdev->pdev, addr, data, size);
     trace_vfio_quirk_generic_mirror_write(vdev->vbasedev.name,
                                           memory_region_name(mirror->mem),

hw/vfio/pci-quirks.h

@@ -60,7 +60,8 @@ extern const MemoryRegionOps vfio_generic_window_data_quirk;
  */
 typedef struct VFIOConfigMirrorQuirk {
     struct VFIOPCIDevice *vdev;
-    uint32_t offset;
+    uint32_t offset; /* Offset in BAR */
+    uint32_t config_offset; /* Offset in PCI config space */
     uint8_t bar;
     MemoryRegion *mem;
     uint8_t data[];