Re: [Qemu-devel] [PATCH v5 3/4] migration: disallow migrate_add_blocker during migration

[Ashijeet Acharya]

On Mon, Jan 16, 2017 at 1:17 AM, Greg Kurz <address@hidden> wrote:
> On Thu, 12 Jan 2017 21:22:33 +0530
> Ashijeet Acharya <address@hidden> wrote:
>
>> If a migration is already in progress and somebody attempts
>> to add a migration blocker, this should rightly fail.
>>
>> Add an errp parameter and a retcode return value to migrate_add_blocker.
>>
>> Signed-off-by: John Snow <address@hidden>
>> Signed-off-by: Ashijeet Acharya <address@hidden>
>> ---
>>  block/qcow.c                  |  7 ++++++-
>>  block/vdi.c                   |  7 ++++++-
>>  block/vhdx.c                  | 16 ++++++++++------
>>  block/vmdk.c                  |  8 +++++++-
>>  block/vpc.c                   | 10 +++++++---
>>  block/vvfat.c                 | 20 ++++++++++++--------
>>  hw/9pfs/9p.c                  | 31 ++++++++++++++++++++-----------
>>  hw/display/virtio-gpu.c       | 31 ++++++++++++++++++-------------
>>  hw/intc/arm_gic_kvm.c         | 16 ++++++++++------
>>  hw/intc/arm_gicv3_its_kvm.c   | 19 ++++++++++++-------
>>  hw/intc/arm_gicv3_kvm.c       | 18 +++++++++++-------
>>  hw/misc/ivshmem.c             | 13 +++++++++----
>>  hw/scsi/vhost-scsi.c          | 24 ++++++++++++++++++------
>>  hw/virtio/vhost.c             |  7 ++++++-
>>  include/migration/migration.h |  7 ++++++-
>>  migration/migration.c         | 38 ++++++++++++++++++++++++++++++++++++--
>>  stubs/migr-blocker.c          |  3 ++-
>>  target-i386/kvm.c             | 15 ++++++++++++---
>>  18 files changed, 208 insertions(+), 82 deletions(-)
>>
>> diff --git a/block/qcow.c b/block/qcow.c
>> index 7540f43..90ebe78 100644
>> --- a/block/qcow.c
>> +++ b/block/qcow.c
>> @@ -104,6 +104,7 @@ static int qcow_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      unsigned int len, i, shift;
>>      int ret;
>>      QCowHeader header;
>> +    Error *local_err = NULL;
>>
>>      ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
>>      if (ret < 0) {
>> @@ -252,7 +253,11 @@ static int qcow_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      error_setg(&s->migration_blocker, "The qcow format used by node '%s' "
>>                 "does not support live migration",
>>                 bdrv_get_device_or_node_name(bs));
>> -    migrate_add_blocker(s->migration_blocker);
>> +    ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        goto fail;
>> +    }
>>
>>      qemu_co_mutex_init(&s->lock);
>>      return 0;
>> diff --git a/block/vdi.c b/block/vdi.c
>> index 96b78d5..2cb8ef0 100644
>> --- a/block/vdi.c
>> +++ b/block/vdi.c
>> @@ -361,6 +361,7 @@ static int vdi_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      VdiHeader header;
>>      size_t bmap_size;
>>      int ret;
>> +    Error *local_err = NULL;
>>
>>      logout("\n");
>>
>> @@ -471,7 +472,11 @@ static int vdi_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      error_setg(&s->migration_blocker, "The vdi format used by node '%s' "
>>                 "does not support live migration",
>>                 bdrv_get_device_or_node_name(bs));
>> -    migrate_add_blocker(s->migration_blocker);
>> +    ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        goto fail_free_bmap;
>> +    }
>>
>>      qemu_co_mutex_init(&s->write_lock);
>>
>> diff --git a/block/vhdx.c b/block/vhdx.c
>> index 0ba2f0a..77ced7c 100644
>> --- a/block/vhdx.c
>> +++ b/block/vhdx.c
>> @@ -991,6 +991,16 @@ static int vhdx_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>          }
>>      }
>>
>> +    /* Disable migration when VHDX images are used */
>> +    error_setg(&s->migration_blocker, "The vhdx format used by node '%s' "
>> +               "does not support live migration",
>> +               bdrv_get_device_or_node_name(bs));
>> +    ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        goto fail;
>> +    }
>> +
>>      if (flags & BDRV_O_RDWR) {
>>          ret = vhdx_update_headers(bs, s, false, NULL);
>>          if (ret < 0) {
>> @@ -1000,12 +1010,6 @@ static int vhdx_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>
>>      /* TODO: differencing files */
>>
>> -    /* Disable migration when VHDX images are used */
>> -    error_setg(&s->migration_blocker, "The vhdx format used by node '%s' "
>> -               "does not support live migration",
>> -               bdrv_get_device_or_node_name(bs));
>> -    migrate_add_blocker(s->migration_blocker);
>> -
>>      return 0;
>>  fail:
>>      vhdx_close(bs);
>> diff --git a/block/vmdk.c b/block/vmdk.c
>> index a11c27a..f1d75ae 100644
>> --- a/block/vmdk.c
>> +++ b/block/vmdk.c
>> @@ -941,6 +941,7 @@ static int vmdk_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      int ret;
>>      BDRVVmdkState *s = bs->opaque;
>>      uint32_t magic;
>> +    Error *local_err = NULL;
>>
>>      buf = vmdk_read_desc(bs->file, 0, errp);
>>      if (!buf) {
>> @@ -976,7 +977,12 @@ static int vmdk_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>      error_setg(&s->migration_blocker, "The vmdk format used by node '%s' "
>>                 "does not support live migration",
>>                 bdrv_get_device_or_node_name(bs));
>> -    migrate_add_blocker(s->migration_blocker);
>> +    ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        goto fail;
>> +    }
>> +
>>      g_free(buf);
>>      return 0;
>>
>> diff --git a/block/vpc.c b/block/vpc.c
>> index 8d5886f..37031fa 100644
>> --- a/block/vpc.c
>> +++ b/block/vpc.c
>> @@ -422,13 +422,17 @@ static int vpc_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>  #endif
>>      }
>>
>> -    qemu_co_mutex_init(&s->lock);
>> -
>>      /* Disable migration when VHD images are used */
>>      error_setg(&s->migration_blocker, "The vpc format used by node '%s' "
>>                 "does not support live migration",
>>                 bdrv_get_device_or_node_name(bs));
>> -    migrate_add_blocker(s->migration_blocker);
>> +    ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        goto fail;
>> +    }
>> +
>> +    qemu_co_mutex_init(&s->lock);
>>
>>      return 0;
>>
>> diff --git a/block/vvfat.c b/block/vvfat.c
>> index ded2109..d061d25 100644
>> --- a/block/vvfat.c
>> +++ b/block/vvfat.c
>> @@ -1185,22 +1185,26 @@ static int vvfat_open(BlockDriverState *bs, QDict 
>> *options, int flags,
>>
>>      s->sector_count = s->faked_sectors + 
>> s->sectors_per_cluster*s->cluster_count;
>>
>> -    if (s->first_sectors_number == 0x40) {
>> -        init_mbr(s, cyls, heads, secs);
>> -    }
>> -
>> -    //    assert(is_consistent(s));
>
> This line makes patchew unhappy every time, maybe you could remove it in
> a preparatory patch ?

Okay

>> -    qemu_co_mutex_init(&s->lock);
>> -
>>      /* Disable migration when vvfat is used rw */
>>      if (s->qcow) {
>>          error_setg(&s->migration_blocker,
>>                     "The vvfat (rw) format used by node '%s' "
>>                     "does not support live migration",
>>                     bdrv_get_device_or_node_name(bs));
>> -        migrate_add_blocker(s->migration_blocker);
>> +        ret = migrate_add_blocker(s->migration_blocker, &local_err);
>> +        if (local_err) {
>> +            error_propagate(errp, local_err);
>> +            goto fail;
>> +        }
>> +    }
>> +
>> +    if (s->first_sectors_number == 0x40) {
>> +        init_mbr(s, cyls, heads, secs);
>>      }
>>
>> +    //    assert(is_consistent(s));
>> +    qemu_co_mutex_init(&s->lock);
>> +
>>      ret = 0;
>>  fail:
>>      qemu_opts_del(opts);
>> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
>> index faebd91..9bb749c 100644
>> --- a/hw/9pfs/9p.c
>> +++ b/hw/9pfs/9p.c
>> @@ -979,6 +979,7 @@ static void coroutine_fn v9fs_attach(void *opaque)
>>      size_t offset = 7;
>>      V9fsQID qid;
>>      ssize_t err;
>> +    Error *local_err;
>
>    Error *local_err = NULL;

Yes, I will fix this everywhere else as well.

>
>>
>>      v9fs_string_init(&uname);
>>      v9fs_string_init(&aname);
>> @@ -1007,26 +1008,34 @@ static void coroutine_fn v9fs_attach(void *opaque)
>>          clunk_fid(s, fid);
>>          goto out;
>>      }
>> -    err = pdu_marshal(pdu, offset, "Q", &qid);
>> -    if (err < 0) {
>> -        clunk_fid(s, fid);
>> -        goto out;
>> -    }
>> -    err += offset;
>> -    memcpy(&s->root_qid, &qid, sizeof(qid));
>> -    trace_v9fs_attach_return(pdu->tag, pdu->id,
>> -                             qid.type, qid.version, qid.path);
>> +
>>      /*
>>       * disable migration if we haven't done already.
>>       * attach could get called multiple times for the same export.
>>       */
>>      if (!s->migration_blocker) {
>> -        s->root_fid = fid;
>>          error_setg(&s->migration_blocker,
>>                     "Migration is disabled when VirtFS export path '%s' is 
>> mounted in the guest using mount_tag '%s'",
>>                     s->ctx.fs_root ? s->ctx.fs_root : "NULL", s->tag);
>> -        migrate_add_blocker(s->migration_blocker);
>> +        err = migrate_add_blocker(s->migration_blocker, &local_err);
>> +        if (local_err) {
>
> Since we don't do anything with local_err, it should be freed.

Right.

>
> Also, it is awkward to allocate the migration_blocker error here but
> it gets freed in migrate_add_blocker() on failure...

I don't remember quite clearly, but this was a suggestion on the list
or the IRC. I am not sure who suggested it though, but this is why I
was removing all the error_free() lines added in 3/4 again in 4/4
earlier.
Should I just add back the error_free() line at all callsites?

>
>> +            s->migration_blocker = NULL;
>> +            clunk_fid(s, fid);
>> +            goto out;
>> +        }
>> +        s->root_fid = fid;
>> +    }
>> +
>> +    err = pdu_marshal(pdu, offset, "Q", &qid);
>> +    if (err < 0) {
>> +        clunk_fid(s, fid);
>> +        goto out;
>>      }
>> +    err += offset;
>> +
>> +    memcpy(&s->root_qid, &qid, sizeof(qid));
>> +    trace_v9fs_attach_return(pdu->tag, pdu->id,
>> +                             qid.type, qid.version, qid.path);
>>  out:
>>      put_fid(pdu, fidp);
>>  out_nofid:
>> diff --git a/hw/display/virtio-gpu.c b/hw/display/virtio-gpu.c
>> index 5f32e1a..503b23e 100644
>> --- a/hw/display/virtio-gpu.c
>> +++ b/hw/display/virtio-gpu.c
>> @@ -1101,6 +1101,7 @@ static void virtio_gpu_device_realize(DeviceState 
>> *qdev, Error **errp)
>>      VirtIODevice *vdev = VIRTIO_DEVICE(qdev);
>>      VirtIOGPU *g = VIRTIO_GPU(qdev);
>>      bool have_virgl;
>> +    Error *local_err = NULL;
>>      int i;
>>
>>      if (g->conf.max_outputs > VIRTIO_GPU_MAX_SCANOUTS) {
>> @@ -1108,14 +1109,6 @@ static void virtio_gpu_device_realize(DeviceState 
>> *qdev, Error **errp)
>>          return;
>>      }
>>
>> -    g->config_size = sizeof(struct virtio_gpu_config);
>> -    g->virtio_config.num_scanouts = g->conf.max_outputs;
>> -    virtio_init(VIRTIO_DEVICE(g), "virtio-gpu", VIRTIO_ID_GPU,
>> -                g->config_size);
>> -
>> -    g->req_state[0].width = 1024;
>> -    g->req_state[0].height = 768;
>> -
>>      g->use_virgl_renderer = false;
>>  #if !defined(CONFIG_VIRGL) || defined(HOST_WORDS_BIGENDIAN)
>>      have_virgl = false;
>> @@ -1127,6 +1120,23 @@ static void virtio_gpu_device_realize(DeviceState 
>> *qdev, Error **errp)
>>      }
>>
>>      if (virtio_gpu_virgl_enabled(g->conf)) {
>> +        error_setg(&g->migration_blocker, "virgl is not yet migratable");
>> +        migrate_add_blocker(g->migration_blocker, &local_err);
>> +        if (local_err) {
>> +            error_propagate(errp, local_err);
>> +            return;
>> +        }
>> +    }
>> +
>> +    g->config_size = sizeof(struct virtio_gpu_config);
>> +    g->virtio_config.num_scanouts = g->conf.max_outputs;
>> +    virtio_init(VIRTIO_DEVICE(g), "virtio-gpu", VIRTIO_ID_GPU,
>> +                g->config_size);
>> +
>> +    g->req_state[0].width = 1024;
>> +    g->req_state[0].height = 768;
>> +
>> +    if (virtio_gpu_virgl_enabled(g->conf)) {
>>          /* use larger control queue in 3d mode */
>>          g->ctrl_vq   = virtio_add_queue(vdev, 256, 
>> virtio_gpu_handle_ctrl_cb);
>>          g->cursor_vq = virtio_add_queue(vdev, 16, 
>> virtio_gpu_handle_cursor_cb);
>> @@ -1152,11 +1162,6 @@ static void virtio_gpu_device_realize(DeviceState 
>> *qdev, Error **errp)
>>              dpy_gfx_replace_surface(g->scanout[i].con, NULL);
>>          }
>>      }
>> -
>> -    if (virtio_gpu_virgl_enabled(g->conf)) {
>> -        error_setg(&g->migration_blocker, "virgl is not yet migratable");
>> -        migrate_add_blocker(g->migration_blocker);
>> -    }
>>  }
>>
>>  static void virtio_gpu_device_unrealize(DeviceState *qdev, Error **errp)
>> diff --git a/hw/intc/arm_gic_kvm.c b/hw/intc/arm_gic_kvm.c
>> index 11729ee..9c33af9 100644
>> --- a/hw/intc/arm_gic_kvm.c
>> +++ b/hw/intc/arm_gic_kvm.c
>> @@ -510,6 +510,16 @@ static void kvm_arm_gic_realize(DeviceState *dev, Error 
>> **errp)
>>          return;
>>      }
>>
>> +    if (!kvm_arm_gic_can_save_restore(s)) {
>> +        error_setg(&s->migration_blocker, "This operating system kernel 
>> does "
>> +                                          "not support vGICv2 migration");
>> +        migrate_add_blocker(s->migration_blocker, &local_err);
>> +        if (local_err) {
>> +            error_propagate(errp, local_err);
>> +            return;
>> +        }
>> +    }
>> +
>>      gic_init_irqs_and_mmio(s, kvm_arm_gicv2_set_irq, NULL);
>>
>>      for (i = 0; i < s->num_irq - GIC_INTERNAL; i++) {
>> @@ -558,12 +568,6 @@ static void kvm_arm_gic_realize(DeviceState *dev, Error 
>> **errp)
>>                              KVM_VGIC_V2_ADDR_TYPE_CPU,
>>                              s->dev_fd);
>>
>> -    if (!kvm_arm_gic_can_save_restore(s)) {
>> -        error_setg(&s->migration_blocker, "This operating system kernel 
>> does "
>> -                                          "not support vGICv2 migration");
>> -        migrate_add_blocker(s->migration_blocker);
>> -    }
>> -
>>      if (kvm_has_gsi_routing()) {
>>          /* set up irq routing */
>>          kvm_init_irq_routing(kvm_state);
>> diff --git a/hw/intc/arm_gicv3_its_kvm.c b/hw/intc/arm_gicv3_its_kvm.c
>> index fc246e0..a20cace 100644
>> --- a/hw/intc/arm_gicv3_its_kvm.c
>> +++ b/hw/intc/arm_gicv3_its_kvm.c
>> @@ -56,6 +56,18 @@ static int kvm_its_send_msi(GICv3ITSState *s, uint32_t 
>> value, uint16_t devid)
>>  static void kvm_arm_its_realize(DeviceState *dev, Error **errp)
>>  {
>>      GICv3ITSState *s = ARM_GICV3_ITS_COMMON(dev);
>> +    Error *local_err;
>
>    Error *local_err = NULL;
>
>> +
>> +    /*
>> +     * Block migration of a KVM GICv3 ITS device: the API for saving and
>> +     * restoring the state in the kernel is not yet available
>> +     */
>> +    error_setg(&s->migration_blocker, "vITS migration is not implemented");
>> +    migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        return;
>> +    }
>>
>>      s->dev_fd = kvm_create_device(kvm_state, KVM_DEV_TYPE_ARM_VGIC_ITS, 
>> false);
>>      if (s->dev_fd < 0) {
>> @@ -73,13 +85,6 @@ static void kvm_arm_its_realize(DeviceState *dev, Error 
>> **errp)
>>
>>      gicv3_its_init_mmio(s, NULL);
>>
>> -    /*
>> -     * Block migration of a KVM GICv3 ITS device: the API for saving and
>> -     * restoring the state in the kernel is not yet available
>> -     */
>> -    error_setg(&s->migration_blocker, "vITS migration is not implemented");
>> -    migrate_add_blocker(s->migration_blocker);
>> -
>>      kvm_msi_use_devid = true;
>>      kvm_gsi_direct_mapping = false;
>>      kvm_msi_via_irqfd_allowed = kvm_irqfds_enabled();
>> diff --git a/hw/intc/arm_gicv3_kvm.c b/hw/intc/arm_gicv3_kvm.c
>> index 199a439..a4cfebb 100644
>> --- a/hw/intc/arm_gicv3_kvm.c
>> +++ b/hw/intc/arm_gicv3_kvm.c
>> @@ -103,6 +103,17 @@ static void kvm_arm_gicv3_realize(DeviceState *dev, 
>> Error **errp)
>>
>>      gicv3_init_irqs_and_mmio(s, kvm_arm_gicv3_set_irq, NULL);
>>
>> +    /* Block migration of a KVM GICv3 device: the API for saving and 
>> restoring
>> +     * the state in the kernel is not yet finalised in the kernel or
>> +     * implemented in QEMU.
>> +     */
>> +    error_setg(&s->migration_blocker, "vGICv3 migration is not 
>> implemented");
>> +    migrate_add_blocker(s->migration_blocker, &local_err);
>> +    if (local_err) {
>> +        error_propagate(errp, local_err);
>> +        return;
>> +    }
>> +
>>      /* Try to create the device via the device control API */
>>      s->dev_fd = kvm_create_device(kvm_state, KVM_DEV_TYPE_ARM_VGIC_V3, 
>> false);
>>      if (s->dev_fd < 0) {
>> @@ -122,13 +133,6 @@ static void kvm_arm_gicv3_realize(DeviceState *dev, 
>> Error **errp)
>>      kvm_arm_register_device(&s->iomem_redist, -1, KVM_DEV_ARM_VGIC_GRP_ADDR,
>>                              KVM_VGIC_V3_ADDR_TYPE_REDIST, s->dev_fd);
>>
>> -    /* Block migration of a KVM GICv3 device: the API for saving and 
>> restoring
>> -     * the state in the kernel is not yet finalised in the kernel or
>> -     * implemented in QEMU.
>> -     */
>> -    error_setg(&s->migration_blocker, "vGICv3 migration is not 
>> implemented");
>> -    migrate_add_blocker(s->migration_blocker);
>> -
>>      if (kvm_has_gsi_routing()) {
>>          /* set up irq routing */
>>          kvm_init_irq_routing(kvm_state);
>> diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
>> index abeaf3d..64283da 100644
>> --- a/hw/misc/ivshmem.c
>> +++ b/hw/misc/ivshmem.c
>> @@ -840,6 +840,7 @@ static void ivshmem_common_realize(PCIDevice *dev, Error 
>> **errp)
>>      uint8_t *pci_conf;
>>      uint8_t attr = PCI_BASE_ADDRESS_SPACE_MEMORY |
>>          PCI_BASE_ADDRESS_MEM_PREFETCH;
>> +    Error *local_err = NULL;
>>
>>      /* IRQFD requires MSI */
>>      if (ivshmem_has_feature(s, IVSHMEM_IOEVENTFD) &&
>> @@ -903,9 +904,6 @@ static void ivshmem_common_realize(PCIDevice *dev, Error 
>> **errp)
>>          }
>>      }
>>
>> -    vmstate_register_ram(s->ivshmem_bar2, DEVICE(s));
>> -    pci_register_bar(PCI_DEVICE(s), 2, attr, s->ivshmem_bar2);
>> -
>>      if (s->master == ON_OFF_AUTO_AUTO) {
>>          s->master = s->vm_id == 0 ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
>>      }
>> @@ -913,8 +911,15 @@ static void ivshmem_common_realize(PCIDevice *dev, 
>> Error **errp)
>>      if (!ivshmem_is_master(s)) {
>>          error_setg(&s->migration_blocker,
>>                     "Migration is disabled when using feature 'peer mode' in 
>> device 'ivshmem'");
>> -        migrate_add_blocker(s->migration_blocker);
>> +        migrate_add_blocker(s->migration_blocker, &local_err);
>> +        if (local_err) {
>> +            error_propagate(errp, local_err);
>> +            return;
>> +        }
>>      }
>> +
>> +    vmstate_register_ram(s->ivshmem_bar2, DEVICE(s));
>> +    pci_register_bar(PCI_DEVICE(s), 2, attr, s->ivshmem_bar2);
>>  }
>>
>>  static void ivshmem_exit(PCIDevice *dev)
>> diff --git a/hw/scsi/vhost-scsi.c b/hw/scsi/vhost-scsi.c
>> index 5b26946..0d3ad47 100644
>> --- a/hw/scsi/vhost-scsi.c
>> +++ b/hw/scsi/vhost-scsi.c
>> @@ -238,8 +238,15 @@ static void vhost_scsi_realize(DeviceState *dev, Error 
>> **errp)
>>                                 vhost_dummy_handle_output);
>>      if (err != NULL) {
>>          error_propagate(errp, err);
>> -        close(vhostfd);
>> -        return;
>> +        goto close_fd;
>> +    }
>> +
>> +    error_setg(&s->migration_blocker,
>> +               "vhost-scsi does not support migration");
>> +    migrate_add_blocker(s->migration_blocker, &err);
>> +    if (err) {
>> +        error_propagate(errp, err);
>> +        goto close_fd;
>>      }
>>
>>      s->dev.nvqs = VHOST_SCSI_VQ_NUM_FIXED + vs->conf.num_queues;
>> @@ -252,7 +259,7 @@ static void vhost_scsi_realize(DeviceState *dev, Error 
>> **errp)
>>      if (ret < 0) {
>>          error_setg(errp, "vhost-scsi: vhost initialization failed: %s",
>>                     strerror(-ret));
>> -        return;
>> +        goto free_vqs;
>>      }
>>
>>      /* At present, channel and lun both are 0 for bootable vhost-scsi disk 
>> */
>> @@ -261,9 +268,14 @@ static void vhost_scsi_realize(DeviceState *dev, Error 
>> **errp)
>>      /* Note: we can also get the minimum tpgt from kernel */
>>      s->target = vs->conf.boot_tpgt;
>>
>> -    error_setg(&s->migration_blocker,
>> -            "vhost-scsi does not support migration");
>> -    migrate_add_blocker(s->migration_blocker);
>> +    return;
>> +
>> + free_vqs:
>> +    migrate_del_blocker(s->migration_blocker);
>> +    g_free(s->dev.vqs);
>> + close_fd:
>> +    close(vhostfd);
>> +    return;
>>  }
>>
>>  static void vhost_scsi_unrealize(DeviceState *dev, Error **errp)
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index f7f7023..6ad0e02 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -1081,6 +1081,7 @@ int vhost_dev_init(struct vhost_dev *hdev, void 
>> *opaque,
>>  {
>>      uint64_t features;
>>      int i, r, n_initialized_vqs = 0;
>> +    Error *local_err = NULL;
>>
>>      hdev->migration_blocker = NULL;
>>
>> @@ -1157,7 +1158,11 @@ int vhost_dev_init(struct vhost_dev *hdev, void 
>> *opaque,
>>      }
>>
>>      if (hdev->migration_blocker != NULL) {
>> -        migrate_add_blocker(hdev->migration_blocker);
>> +        r = migrate_add_blocker(hdev->migration_blocker, &local_err);
>> +        if (local_err) {
>> +            error_report_err(local_err);
>> +            goto fail_busyloop;
>> +        }
>>      }
>>
>>      hdev->mem = g_malloc0(offsetof(struct vhost_memory, regions));
>> diff --git a/include/migration/migration.h b/include/migration/migration.h
>> index 40b3697..bcbdb03 100644
>> --- a/include/migration/migration.h
>> +++ b/include/migration/migration.h
>> @@ -243,6 +243,7 @@ void remove_migration_state_change_notifier(Notifier 
>> *notify);
>>  MigrationState *migrate_init(const MigrationParams *params);
>>  bool migration_is_blocked(Error **errp);
>>  bool migration_in_setup(MigrationState *);
>> +bool migration_is_idle(MigrationState *s);
>>  bool migration_has_finished(MigrationState *);
>>  bool migration_has_failed(MigrationState *);
>>  /* True if outgoing migration has entered postcopy phase */
>> @@ -287,8 +288,12 @@ int ram_postcopy_incoming_init(MigrationIncomingState 
>> *mis);
>>   * @migrate_add_blocker - prevent migration from proceeding
>>   *
>>   * @reason - an error to be returned whenever migration is attempted
>> + *
>> + * @errp - [out] The reason (if any) we cannot block migration right now.
>> + *
>> + * @returns - 0 on success, -EBUSY on failure, with errp set.
>>   */
>> -void migrate_add_blocker(Error *reason);
>> +int migrate_add_blocker(Error *reason, Error **errp);
>>
>>  /**
>>   * @migrate_del_blocker - remove a blocking error from migration
>> diff --git a/migration/migration.c b/migration/migration.c
>> index f498ab8..cca820e 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -1044,6 +1044,31 @@ bool 
>> migration_in_postcopy_after_devices(MigrationState *s)
>>      return migration_in_postcopy(s) && s->postcopy_after_devices;
>>  }
>>
>> +bool migration_is_idle(MigrationState *s)
>> +{
>> +    if (!s) {
>> +        s = migrate_get_current();
>> +    }
>> +
>> +    switch (s->state) {
>> +    case MIGRATION_STATUS_NONE:
>> +    case MIGRATION_STATUS_CANCELLED:
>> +    case MIGRATION_STATUS_COMPLETED:
>> +    case MIGRATION_STATUS_FAILED:
>> +        return true;
>> +    case MIGRATION_STATUS_SETUP:
>> +    case MIGRATION_STATUS_CANCELLING:
>> +    case MIGRATION_STATUS_ACTIVE:
>> +    case MIGRATION_STATUS_POSTCOPY_ACTIVE:
>> +    case MIGRATION_STATUS_COLO:
>> +        return false;
>> +    case MIGRATION_STATUS__MAX:
>> +        g_assert_not_reached();
>> +    }
>> +
>> +    return false;
>> +}
>> +
>>  MigrationState *migrate_init(const MigrationParams *params)
>>  {
>>      MigrationState *s = migrate_get_current();
>> @@ -1086,9 +1111,18 @@ MigrationState *migrate_init(const MigrationParams 
>> *params)
>>
>>  static GSList *migration_blockers;
>>
>> -void migrate_add_blocker(Error *reason)
>> +int migrate_add_blocker(Error *reason, Error **errp)
>>  {
>> -    migration_blockers = g_slist_prepend(migration_blockers, reason);
>> +    if (migration_is_idle(NULL)) {
>> +        migration_blockers = g_slist_prepend(migration_blockers, reason);
>> +        return 0;
>> +    }
>> +
>> +    error_propagate(errp, reason);
>> +    error_prepend(errp, "disallowing migration blocker (migration in "
>> +                      "progress) for: ");
>> +    error_free(reason);
>
> This is wrong: error_propagate() basically does *errp = reason, hence you
> mustn't free reason, otherwise *errp ends up with a dangling pointer.

Hmm, I checked this with gdb now and I will fix it.

> Also, I think you should propagate a copy like in migration_is_blocked(),
> and let the caller dispose of reason.

Yes, that should do the job.

Ashijeet

>
>> +    return -EBUSY;
>>  }
>>
>>  void migrate_del_blocker(Error *reason)
>> diff --git a/stubs/migr-blocker.c b/stubs/migr-blocker.c
>> index 8ab3604..a5ba18f 100644
>> --- a/stubs/migr-blocker.c
>> +++ b/stubs/migr-blocker.c
>> @@ -2,8 +2,9 @@
>>  #include "qemu-common.h"
>>  #include "migration/migration.h"
>>
>> -void migrate_add_blocker(Error *reason)
>> +int migrate_add_blocker(Error *reason, Error **errp)
>>  {
>> +    return 0;
>>  }
>>
>>  void migrate_del_blocker(Error *reason)
>> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
>> index f62264a..b55092d 100644
>> --- a/target-i386/kvm.c
>> +++ b/target-i386/kvm.c
>
> The per-target folders were changed by this commit:
>
> fcf5ef2ab52c Move target-* CPU file into a target/ folder
>
> Please rebase.
>
>> @@ -702,6 +702,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>      uint32_t signature[3];
>>      int kvm_base = KVM_CPUID_SIGNATURE;
>>      int r;
>> +    Error *local_err;
>
>    Error *local_err = NULL;
>
>>
>>      memset(&cpuid_data, 0, sizeof(cpuid_data));
>>
>> @@ -961,7 +962,11 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>          error_setg(&invtsc_mig_blocker,
>>                     "State blocked by non-migratable CPU device"
>>                     " (invtsc flag)");
>> -        migrate_add_blocker(invtsc_mig_blocker);
>> +        r = migrate_add_blocker(invtsc_mig_blocker, &local_err);
>> +        if (local_err) {
>> +            error_report_err(local_err);
>> +            goto fail;
>> +        }
>>          /* for savevm */
>>          vmstate_x86_cpu.unmigratable = 1;
>>      }
>> @@ -969,12 +974,12 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>      cpuid_data.cpuid.padding = 0;
>>      r = kvm_vcpu_ioctl(cs, KVM_SET_CPUID2, &cpuid_data);
>>      if (r) {
>> -        return r;
>> +        goto fail;
>>      }
>>
>>      r = kvm_arch_set_tsc_khz(cs);
>>      if (r < 0) {
>> -        return r;
>> +        goto fail;
>>      }
>>
>>      /* vcpu's TSC frequency is either specified by user, or following
>> @@ -1001,6 +1006,10 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>      }
>>
>>      return 0;
>> +
>> + fail:
>> +    migrate_del_blocker(invtsc_mig_blocker);
>> +    return r;
>>  }
>>
>>  void kvm_arch_reset_vcpu(X86CPU *cpu)
>