vfio-user: forward MSI-X PBA BAR accesses to server

For vfio-user, the server holds the pending IRQ state; set up an I/O
region for the MSI-X PBA so we can ask the server for this state on a
PBA read.

Originally-by: John Johnson <john.g.johnson@oracle.com>
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250625193012.2316242-11-john.levon@nutanix.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

2 files changed, 65 insertions, 0 deletions

diff --git a/hw/vfio-user/pci.c b/hw/vfio-user/pci.c
index b49f42b..c0f00f1 100644
--- a/hw/vfio-user/pci.c
+++ b/hw/vfio-user/pci.c
@@ -25,6 +25,62 @@ struct VFIOUserPCIDevice {
 };
 
 /*
+ * The server maintains the device's pending interrupts,
+ * via its MSIX table and PBA, so we treat these accesses
+ * like PCI config space and forward them.
+ */
+static uint64_t vfio_user_pba_read(void *opaque, hwaddr addr,
+                                   unsigned size)
+{
+    VFIOPCIDevice *vdev = opaque;
+    VFIORegion *region = &vdev->bars[vdev->msix->pba_bar].region;
+    uint64_t data;
+
+    /* server copy is what matters */
+    data = vfio_region_read(region, addr + vdev->msix->pba_offset, size);
+    return data;
+}
+
+static void vfio_user_pba_write(void *opaque, hwaddr addr,
+                                  uint64_t data, unsigned size)
+{
+    /* dropped */
+}
+
+static const MemoryRegionOps vfio_user_pba_ops = {
+    .read = vfio_user_pba_read,
+    .write = vfio_user_pba_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+};
+
+static void vfio_user_msix_setup(VFIOPCIDevice *vdev)
+{
+    MemoryRegion *vfio_reg, *msix_reg, *pba_reg;
+
+    pba_reg = g_new0(MemoryRegion, 1);
+    vdev->msix->pba_region = pba_reg;
+
+    vfio_reg = vdev->bars[vdev->msix->pba_bar].mr;
+    msix_reg = &vdev->pdev.msix_pba_mmio;
+    memory_region_init_io(pba_reg, OBJECT(vdev), &vfio_user_pba_ops, vdev,
+                          "VFIO MSIX PBA", int128_get64(msix_reg->size));
+    memory_region_add_subregion_overlap(vfio_reg, vdev->msix->pba_offset,
+                                        pba_reg, 1);
+}
+
+static void vfio_user_msix_teardown(VFIOPCIDevice *vdev)
+{
+    MemoryRegion *mr, *sub;
+
+    mr = vdev->bars[vdev->msix->pba_bar].mr;
+    sub = vdev->msix->pba_region;
+    memory_region_del_subregion(mr, sub);
+
+    g_free(vdev->msix->pba_region);
+    vdev->msix->pba_region = NULL;
+}
+
+/*
  * Incoming request message callback.
  *
  * Runs off main loop, so BQL held.
@@ -144,6 +200,10 @@ static void vfio_user_pci_realize(PCIDevice *pdev, Error **errp)
         goto out_teardown;
     }
 
+    if (vdev->msix != NULL) {
+        vfio_user_msix_setup(vdev);
+    }
+
     if (!vfio_pci_interrupt_setup(vdev, errp)) {
         goto out_teardown;
     }
@@ -192,6 +252,10 @@ static void vfio_user_instance_finalize(Object *obj)
     VFIOPCIDevice *vdev = VFIO_PCI_BASE(obj);
     VFIODevice *vbasedev = &vdev->vbasedev;
 
+    if (vdev->msix != NULL) {
+        vfio_user_msix_teardown(vdev);
+    }
+
     vfio_pci_put_device(vdev);
 
     if (vbasedev->proxy != NULL) {
diff --git a/hw/vfio/pci.h b/hw/vfio/pci.h
index d3dc227..5ba7330 100644
--- a/hw/vfio/pci.h
+++ b/hw/vfio/pci.h
@@ -116,6 +116,7 @@ typedef struct VFIOMSIXInfo {
     uint32_t pba_offset;
     unsigned long *pending;
     bool noresize;
+    MemoryRegion *pba_region;
 } VFIOMSIXInfo;
 
 /*