Add shared memory BAR support to vhost-user-device-pci
to enable direct file mapping for VIRTIO Shared
Memory Regions.

The implementation creates a consolidated shared
memory BAR that contains all VIRTIO Shared
Memory Regions as subregions. Each region is
configured with its proper shmid, size, and
offset within the BAR. The number and size of
regions are retrieved via VHOST_USER_GET_SHMEM_CONFIG
message sent by vhost-user-base during realization
after virtio_init().

Specifiically, it uses BAR 3 to avoid conflicts, as
it is currently unused.

The shared memory BAR is only created when the
backend supports VHOST_USER_PROTOCOL_F_SHMEM and
has configured shared memory regions. This maintains
backward compatibility with backends that do not
support shared memory functionality.

Signed-off-by: Albert Esteve <aesteve@redhat.com>
---
 hw/virtio/vhost-user-base.c            | 47 ++++++++++++++++++++++++--
 hw/virtio/vhost-user-test-device-pci.c | 35 +++++++++++++++++--
 2 files changed, 77 insertions(+), 5 deletions(-)

diff --git a/hw/virtio/vhost-user-base.c b/hw/virtio/vhost-user-base.c
index ff67a020b4..82f49500e4 100644
--- a/hw/virtio/vhost-user-base.c
+++ b/hw/virtio/vhost-user-base.c
@@ -16,6 +16,7 @@
 #include "hw/virtio/virtio-bus.h"
 #include "hw/virtio/vhost-user-base.h"
 #include "qemu/error-report.h"
+#include "migration/blocker.h"
 
 static void vub_start(VirtIODevice *vdev)
 {
@@ -276,7 +277,9 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
 {
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
     VHostUserBase *vub = VHOST_USER_BASE(dev);
-    int ret;
+    uint64_t memory_sizes[VIRTIO_MAX_SHMEM_REGIONS];
+    g_autofree char *name = NULL;
+    int i, ret, nregions, regions_processed = 0;
 
     if (!vub->chardev.chr) {
         error_setg(errp, "vhost-user-base: missing chardev");
@@ -319,7 +322,7 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
 
     /* Allocate queues */
     vub->vqs = g_ptr_array_sized_new(vub->num_vqs);
-    for (int i = 0; i < vub->num_vqs; i++) {
+    for (i = 0; i < vub->num_vqs; i++) {
         g_ptr_array_add(vub->vqs,
                         virtio_add_queue(vdev, vub->vq_size,
                                          vub_handle_output));
@@ -333,11 +336,49 @@ static void vub_device_realize(DeviceState *dev, Error **errp)
                          VHOST_BACKEND_TYPE_USER, 0, errp);
 
     if (ret < 0) {
-        do_vhost_user_cleanup(vdev, vub);
+        goto err;
+    }
+
+    ret = vub->vhost_dev.vhost_ops->vhost_get_shmem_config(&vub->vhost_dev,
+                                                           &nregions,
+                                                           memory_sizes,
+                                                           errp);
+
+    if (ret < 0) {
+        goto err;
+    }
+
+    for (i = 0; i < VIRTIO_MAX_SHMEM_REGIONS && regions_processed < nregions; i++) {
+        if (memory_sizes[i]) {
+            regions_processed++;
+            if (vub->vhost_dev.migration_blocker == NULL) {
+                error_setg(&vub->vhost_dev.migration_blocker,
+                       "Migration disabled: devices with VIRTIO Shared Memory "
+                       "Regions do not support migration yet.");
+                ret = migrate_add_blocker_normal(
+                    &vub->vhost_dev.migration_blocker,
+                    errp);
+
+                if (ret < 0) {
+                    goto err;
+                }
+            }
+
+            if (memory_sizes[i] % qemu_real_host_page_size() != 0) {
+                error_setg(errp, "Shared memory %d size must be a power of 2 "
+                                 "no smaller than the page size", i);
+                goto err;
+            }
+
+            virtio_new_shmem_region(vdev, i, memory_sizes[i]);
+        }
     }
 
     qemu_chr_fe_set_handlers(&vub->chardev, NULL, NULL, vub_event, NULL,
                              dev, NULL, true);
+    return;
+err:
+    do_vhost_user_cleanup(vdev, vub);
 }
 
 static void vub_device_unrealize(DeviceState *dev)
diff --git a/hw/virtio/vhost-user-test-device-pci.c b/hw/virtio/vhost-user-test-device-pci.c
index b4ed0efb50..60e59f5b1c 100644
--- a/hw/virtio/vhost-user-test-device-pci.c
+++ b/hw/virtio/vhost-user-test-device-pci.c
@@ -8,14 +8,18 @@
  */
 
 #include "qemu/osdep.h"
+#include "qapi/error.h"
 #include "hw/qdev-properties.h"
 #include "hw/virtio/vhost-user-base.h"
 #include "hw/virtio/virtio-pci.h"
 
+#define VIRTIO_DEVICE_PCI_SHMEM_BAR 4
+
 struct VHostUserDevicePCI {
     VirtIOPCIProxy parent_obj;
 
     VHostUserBase vub;
+    MemoryRegion shmembar;
 };
 
 #define TYPE_VHOST_USER_TEST_DEVICE_PCI "vhost-user-test-device-pci-base"
@@ -25,10 +29,37 @@ OBJECT_DECLARE_SIMPLE_TYPE(VHostUserDevicePCI, VHOST_USER_TEST_DEVICE_PCI)
 static void vhost_user_device_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VHostUserDevicePCI *dev = VHOST_USER_TEST_DEVICE_PCI(vpci_dev);
-    DeviceState *vdev = DEVICE(&dev->vub);
+    DeviceState *dev_state = DEVICE(&dev->vub);
+    VirtIODevice *vdev = VIRTIO_DEVICE(dev_state);
+    VirtioSharedMemory *shmem, *next;
+    uint64_t offset = 0, shmem_size = 0;
 
+    vpci_dev->modern_mem_bar_idx = 2;
     vpci_dev->nvectors = 1;
-    qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
+    qdev_realize(dev_state, BUS(&vpci_dev->bus), errp);
+
+    QSIMPLEQ_FOREACH_SAFE(shmem, &vdev->shmem_list, entry, next) {
+        if (shmem->mr.size > UINT64_MAX - shmem_size) {
+            error_setg(errp, "Total shared memory required overflow");
+            return;
+        }
+        shmem_size = shmem_size + shmem->mr.size;
+    }
+    if (shmem_size) {
+        memory_region_init(&dev->shmembar, OBJECT(vpci_dev),
+                           "vhost-device-pci-shmembar", shmem_size);
+        QSIMPLEQ_FOREACH_SAFE(shmem, &vdev->shmem_list, entry, next) {
+            memory_region_add_subregion(&dev->shmembar, offset, &shmem->mr);
+            virtio_pci_add_shm_cap(vpci_dev, VIRTIO_DEVICE_PCI_SHMEM_BAR,
+                                   offset, shmem->mr.size, shmem->shmid);
+            offset = offset + shmem->mr.size;
+        }
+        pci_register_bar(&vpci_dev->pci_dev, VIRTIO_DEVICE_PCI_SHMEM_BAR,
+                        PCI_BASE_ADDRESS_SPACE_MEMORY |
+                        PCI_BASE_ADDRESS_MEM_PREFETCH |
+                        PCI_BASE_ADDRESS_MEM_TYPE_64,
+                        &dev->shmembar);
+    }
 }
 
 static void vhost_user_device_pci_class_init(ObjectClass *klass,
-- 
2.49.0

On Wed, Oct 15, 2025 at 10:59:30AM +0200, Albert Esteve wrote:
> @@ -25,10 +29,37 @@ OBJECT_DECLARE_SIMPLE_TYPE(VHostUserDevicePCI, VHOST_USER_TEST_DEVICE_PCI)
>  static void vhost_user_device_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
>  {
>      VHostUserDevicePCI *dev = VHOST_USER_TEST_DEVICE_PCI(vpci_dev);
> -    DeviceState *vdev = DEVICE(&dev->vub);
> +    DeviceState *dev_state = DEVICE(&dev->vub);
> +    VirtIODevice *vdev = VIRTIO_DEVICE(dev_state);
> +    VirtioSharedMemory *shmem, *next;
> +    uint64_t offset = 0, shmem_size = 0;
>  
> +    vpci_dev->modern_mem_bar_idx = 2;
>      vpci_dev->nvectors = 1;
> -    qdev_realize(vdev, BUS(&vpci_dev->bus), errp);
> +    qdev_realize(dev_state, BUS(&vpci_dev->bus), errp);
> +
> +    QSIMPLEQ_FOREACH_SAFE(shmem, &vdev->shmem_list, entry, next) {
> +        if (shmem->mr.size > UINT64_MAX - shmem_size) {
> +            error_setg(errp, "Total shared memory required overflow");
> +            return;
> +        }
> +        shmem_size = shmem_size + shmem->mr.size;
> +    }
> +    if (shmem_size) {

Let's reject invalid BAR configurations when VIRTIO Shared Memory
Regions are in use:

  if (vpci_dev->flags & VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY) {
      error_setg(errp, "modern-pio-notify is not supported due to PCI BAR layout limitations");
      return;
  }

Otherwise:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>