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 <[email protected]> > --- > 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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