[PATCH 3/3] hw/display/vhost-user-media: add shared memory cache BAR

[Albert Esteve]

Register a 4 GiB shared memory PCI BAR (BAR 2) so the guest driver
can use it as a DMA cache for video buffers. The memory region is
allocated on device realize and exposed to the guest through a VirtIO
shared memory capability (shmem ID 0).

The host must be started with a memfd-backed memory object covering
at least the shared region size, e.g.:

    -object memory-backend-memfd,id=mem,size=4G,share=on \
    -numa node,memdev=mem

Signed-off-by: Albert Esteve <aest...@redhat.com>
---
 hw/display/vhost-user-media-pci.c | 28 ++++++++++++++++++++++++++++
 hw/display/vhost-user-media.c     |  4 ++++
 2 files changed, 32 insertions(+)

diff --git a/hw/display/vhost-user-media-pci.c 
b/hw/display/vhost-user-media-pci.c
index ad9e3900a4..838a9da71d 100644
--- a/hw/display/vhost-user-media-pci.c
+++ b/hw/display/vhost-user-media-pci.c
@@ -12,12 +12,18 @@
 #include "hw/virtio/vhost-user-media.h"
 #include "hw/virtio/virtio-pci.h"
 
+/* BAR 2 is used for the shared memory cache region exposed to the guest */
+#define VIRTIO_MEDIA_PCI_CACHE_BAR 2
+
+#define VIRTIO_MEDIA_PCI_SHMCAP_ID_CACHE 0
+
 #define TYPE_VHOST_USER_MEDIA_PCI "vhost-user-media-pci-base"
 OBJECT_DECLARE_SIMPLE_TYPE(VHostUserMEDIAPCI, VHOST_USER_MEDIA_PCI)
 
 struct VHostUserMEDIAPCI {
     VirtIOPCIProxy parent_obj;
     VHostUserMEDIA vdev;
+    MemoryRegion cachebar;
 };
 
 static const Property vumedia_pci_properties[] = {
@@ -31,12 +37,34 @@ static void vumedia_pci_realize(VirtIOPCIProxy *vpci_dev, 
Error **errp)
 {
     VHostUserMEDIAPCI *dev = VHOST_USER_MEDIA_PCI(vpci_dev);
     DeviceState *dev_state = DEVICE(&dev->vdev);
+    VirtIODevice *vdev = VIRTIO_DEVICE(dev_state);
+    VirtioSharedMemory *shmem;
+    uint64_t cache_size;
 
     if (vpci_dev->nvectors == DEV_NVECTORS_UNSPECIFIED) {
         vpci_dev->nvectors = 1;
     }
 
     qdev_realize(dev_state, BUS(&vpci_dev->bus), errp);
+    if (*errp) {
+        return;
+    }
+
+    shmem = QSIMPLEQ_LAST(&vdev->shmem_list, VirtioSharedMemory, entry);
+    cache_size = memory_region_size(&shmem->mr);
+
+    memory_region_init(&dev->cachebar, OBJECT(vpci_dev),
+                       "vhost-media-pci-cachebar", cache_size);
+    memory_region_add_subregion(&dev->cachebar, 0, &shmem->mr);
+    virtio_pci_add_shm_cap(vpci_dev, VIRTIO_MEDIA_PCI_CACHE_BAR, 0,
+                            cache_size, VIRTIO_MEDIA_PCI_SHMCAP_ID_CACHE);
+
+    /* After 'realized' so the memory region exists */
+    pci_register_bar(&vpci_dev->pci_dev, VIRTIO_MEDIA_PCI_CACHE_BAR,
+                     PCI_BASE_ADDRESS_SPACE_MEMORY |
+                     PCI_BASE_ADDRESS_MEM_PREFETCH |
+                     PCI_BASE_ADDRESS_MEM_TYPE_64,
+                     &dev->cachebar);
 }
 
 static void vumedia_pci_class_init(ObjectClass *klass, const void *data)
diff --git a/hw/display/vhost-user-media.c b/hw/display/vhost-user-media.c
index 5c627437e7..bbb64ea4d3 100644
--- a/hw/display/vhost-user-media.c
+++ b/hw/display/vhost-user-media.c
@@ -20,6 +20,8 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/vhost-user-media.h"
 
+#define CACHE_SIZE (1ull << 32)
+
 static const int feature_bits[] = {
     VIRTIO_F_RING_RESET,
     VHOST_INVALID_FEATURE_BIT
@@ -288,6 +290,8 @@ static void vhost_user_media_device_realize(DeviceState 
*dev, Error **errp)
 
     virtio_init(vdev, VIRTIO_ID_MEDIA, sizeof(struct virtio_media_config));
 
+    virtio_new_shmem_region(vdev, 0, CACHE_SIZE);
+
     /* one command queue and one event queue */
     media->vhost_dev.nvqs = 2;
     media->vhost_dev.vqs = g_new0(struct vhost_virtqueue,
-- 
2.54.0