On Mon, Sep 29, 2025 at 9:00 PM Stefan Hajnoczi <[email protected]> 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 <[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.
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 :/
