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
+
 struct VHostUserDevicePCI {
     VirtIOPCIProxy parent_obj;
 
     VHostUserBase vub;
+    MemoryRegion shmembar;
 };
 
 #define TYPE_VHOST_USER_DEVICE_PCI "vhost-user-device-pci-base"
@@ -25,10 +29,36 @@ OBJECT_DECLARE_SIMPLE_TYPE(VHostUserDevicePCI, VHOST_USER_DEVICE_PCI)
 static void vhost_user_device_pci_realize(VirtIOPCIProxy *vpci_dev, Error **errp)
 {
     VHostUserDevicePCI *dev = VHOST_USER_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->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, 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.

On Mon, Sep 29, 2025 at 9:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> 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.

I see. But doing the same strategy would collide with
virtio-gpu-hostmem if both devices are used simultaneously? Would that
be ok?

The only other option (I think?) is to just use
`VIRTIO_DEVICE_PCI_SHMEM_BAR 2` as there is a comment in virtio-pci
that suggests that virtio modern io bar is off by default. But that
would also collide when used with virtio-gpu-hostmem :/

On Mon, Oct 13, 2025 at 05:37:52PM +0200, Albert Esteve wrote:
> On Mon, Sep 29, 2025 at 9:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > 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.
> 
> I see. But doing the same strategy would collide with
> virtio-gpu-hostmem if both devices are used simultaneously? Would that
> be ok?

virtio-gpu-* and vhost-user-device-pci are separate PCI devices with
their own BARs, how would they conflict?

On Tue, Oct 14, 2025 at 5:18 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Mon, Oct 13, 2025 at 05:37:52PM +0200, Albert Esteve wrote:
> > On Mon, Sep 29, 2025 at 9:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > >
> > > 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.
> >
> > I see. But doing the same strategy would collide with
> > virtio-gpu-hostmem if both devices are used simultaneously? Would that
> > be ok?
>
> virtio-gpu-* and vhost-user-device-pci are separate PCI devices with
> their own BARs, how would they conflict?

Ah! I see. I thought different devices registering the same IDs would
conflict. But you're right, it doesn't make sense. Understood. Then
I'll follow your initial suggestion.