On Tue, Nov 11, 2025 at 10:10:58AM +0100, Albert Esteve wrote:
> 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.
>
> Reviewed-by: Stefan Hajnoczi <[email protected]>
> Signed-off-by: Albert Esteve <[email protected]>
> ---
> hw/virtio/vhost-user-base.c | 47 ++++++++++++++++++++++++--
> hw/virtio/vhost-user-test-device-pci.c | 39 +++++++++++++++++++--
> 2 files changed, 81 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;
This add a local variable "name" ...
> + 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);
> }
>
which is never used.
this results in a build error:
https://gitlab.com/mstredhat/qemu/-/jobs/12974866799
> 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..d010cb3205 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 +29,41 @@ 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) {
> + 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;
> + }
> + 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
