On Tue, Feb 3, 2026 at 10:46 PM Michael S. Tsirkin <[email protected]> wrote:
>
> 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
>
>
Thanks. I will send a new version fixing the build errors you
mentioned. I am not sure why I did not see them myself when building
locally :/
>
>
> > 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
>
