Re: [PATCH v8 7/7] vhost-user-device: Add shared memory BAR

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Sep 10, 2025 at 01:54:20PM +0200, 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.
> 
> Signed-off-by: Albert Esteve <aesteve@redhat.com>
> ---
>  hw/virtio/vhost-user-base.c       | 47 +++++++++++++++++++++++++++++--
>  hw/virtio/vhost-user-device-pci.c | 34 ++++++++++++++++++++--
>  2 files changed, 76 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-device-pci.c b/hw/virtio/vhost-user-device-pci.c
> index f10bac874e..bac99e7c60 100644
> --- a/hw/virtio/vhost-user-device-pci.c
> +++ b/hw/virtio/vhost-user-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 3

64-bit PCI memory BARs require two contiguous 32-bit BARs. Choosing BAR
3 won't work because modern_mem_bar_idx = 4 (and 5) and there is a
collision.

virtio-gpu-hostmem uses BAR 2 and 3 for VIRTIO Shared Memory Regions. In
order to do this, it rearranges the other VIRTIO PCI BARs in
virtio_vga_base_realize(). Following that approach should be safe.

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