Re: [PATCH v2] vfio/migration: Refactor and fix print of "Migration disabled"

[Avihai Horon]

Hi Zhenzhong,

On 19/06/2023 11:44, Zhenzhong Duan wrote:
> External email: Use caution opening links or attachments
>
>
> This patch refactors vfio_migration_realize() and its dependend code
> as follows:
>
> 1. Change vfio_block_*_migration() to be only a checker.
> 2. It's redundant in vfio_migration_realize() to registers multiple blockers,
>     refactor it to register one migration blocker per device.
> 3. Remove useless vfio_unblock_*_migration() and *_migration_blocker,
>     remove empty vfio_migration_finalize().
> 4. Change vfio_migration_realize() and dependent vfio_block_*_migration() to
>     return bool type.
> 5. Change to print "Migration disabled" only after adding blocker succeed.
>
> migrate_add_blocker() returns 0 when successfully adding the migration blocker.
> However, the caller of vfio_migration_realize() considers that migration was
> blocked when the latter returned an error. What matters for migration is that
> the blocker is added in core migration, so this cleans up usability such that
> user sees "Migrate disabled" when any of the vfio migration blockers are active.
>
> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
> ---
> v2: Based on suggestions from Joao and Cedric, refactored vfio migration
>      blocker related code.
>      Use Joao's words to update description.
>
> Tested vfio hotplug/unplug with vfio migration supported and unsupported cases.
>
>   hw/vfio/common.c              | 66 +++--------------------------------
>   hw/vfio/migration.c           | 36 +++++++++----------
>   hw/vfio/pci.c                 |  6 +---
>   include/hw/vfio/vfio-common.h |  8 ++---
>   4 files changed, 25 insertions(+), 91 deletions(-)
>
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index fa8fd949b1cf..606e23a66514 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -361,9 +361,6 @@ bool vfio_mig_active(void)
>       return true;
>   }
>
> -static Error *multiple_devices_migration_blocker;
> -static Error *giommu_migration_blocker;
> -
>   static unsigned int vfio_migratable_device_num(void)
>   {
>       VFIOGroup *group;
> @@ -381,37 +378,9 @@ static unsigned int vfio_migratable_device_num(void)
>       return device_num;
>   }
>
> -int vfio_block_multiple_devices_migration(Error **errp)
> -{
> -    int ret;
> -
> -    if (multiple_devices_migration_blocker ||
> -        vfio_migratable_device_num() <= 1) {
> -        return 0;
> -    }
> -
> -    error_setg(&multiple_devices_migration_blocker,
> -               "Migration is currently not supported with multiple "
> -               "VFIO devices");
> -    ret = migrate_add_blocker(multiple_devices_migration_blocker, errp);
> -    if (ret < 0) {
> -        error_free(multiple_devices_migration_blocker);
> -        multiple_devices_migration_blocker = NULL;
> -    }
> -
> -    return ret;
> -}
> -
> -void vfio_unblock_multiple_devices_migration(void)
> +bool vfio_block_multiple_devices_migration(Error **errp)
>   {
> -    if (!multiple_devices_migration_blocker ||
> -        vfio_migratable_device_num() > 1) {
> -        return;
> -    }
> -
> -    migrate_del_blocker(multiple_devices_migration_blocker);
> -    error_free(multiple_devices_migration_blocker);
> -    multiple_devices_migration_blocker = NULL;
> +    return vfio_migratable_device_num() > 1;
>   }
>
>   static bool vfio_viommu_preset(void)
> @@ -427,36 +396,9 @@ static bool vfio_viommu_preset(void)
>       return false;
>   }
>
> -int vfio_block_giommu_migration(Error **errp)
> +bool vfio_block_giommu_migration(Error **errp)
>   {
> -    int ret;
> -
> -    if (giommu_migration_blocker ||
> -        !vfio_viommu_preset()) {
> -        return 0;
> -    }
> -
> -    error_setg(&giommu_migration_blocker,
> -               "Migration is currently not supported with vIOMMU enabled");
> -    ret = migrate_add_blocker(giommu_migration_blocker, errp);
> -    if (ret < 0) {
> -        error_free(giommu_migration_blocker);
> -        giommu_migration_blocker = NULL;
> -    }
> -
> -    return ret;
> -}
> -
> -void vfio_migration_finalize(void)
> -{
> -    if (!giommu_migration_blocker ||
> -        vfio_viommu_preset()) {
> -        return;
> -    }
> -
> -    migrate_del_blocker(giommu_migration_blocker);
> -    error_free(giommu_migration_blocker);
> -    giommu_migration_blocker = NULL;
> +    return vfio_viommu_preset();
>   }
>
>   static void vfio_set_migration_error(int err)
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 6b58dddb8859..bc51aa765cb8 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -632,42 +632,41 @@ int64_t vfio_mig_bytes_transferred(void)
>       return bytes_transferred;
>   }
>
> -int vfio_migration_realize(VFIODevice *vbasedev, Error **errp)
> +bool vfio_migration_realize(VFIODevice *vbasedev, Error **errp)
>   {
> -    int ret = -ENOTSUP;
> +    int ret;
>
> -    if (!vbasedev->enable_migration) {
> +    if (!vbasedev->enable_migration || vfio_migration_init(vbasedev)) {
> +        error_setg(&vbasedev->migration_blocker,
> +                   "VFIO device doesn't support migration");
>           goto add_blocker;
>       }
>
> -    ret = vfio_migration_init(vbasedev);
> -    if (ret) {
> +    if (vfio_block_multiple_devices_migration(errp)) {
> +        error_setg(&vbasedev->migration_blocker,
> +                   "Migration is currently not supported with multiple "
> +                   "VFIO devices");
>           goto add_blocker;
>       }

Here you are tying the multiple devices blocker to a specific device.
This could be problematic:
If you add vfio device #1 and then device #2 then the blocker will be 
added to device #2. If you then remove device #1, migration will still 
be blocked although it shouldn't.

I think we should keep it as a global blocker and not a per-device blocker.

Thanks.

> -    ret = vfio_block_multiple_devices_migration(errp);
> -    if (ret) {
> -        return ret;
> -    }
> -
> -    ret = vfio_block_giommu_migration(errp);
> -    if (ret) {
> -        return ret;
> +    if (vfio_block_giommu_migration(errp)) {
> +        error_setg(&vbasedev->migration_blocker,
> +                   "Migration is currently not supported with vIOMMU enabled");
> +        goto add_blocker;
>       }
>
>       trace_vfio_migration_probe(vbasedev->name);
> -    return 0;
> +    return true;
>
>   add_blocker:
> -    error_setg(&vbasedev->migration_blocker,
> -               "VFIO device doesn't support migration");
> -
>       ret = migrate_add_blocker(vbasedev->migration_blocker, errp);
>       if (ret < 0) {
>           error_free(vbasedev->migration_blocker);
>           vbasedev->migration_blocker = NULL;
> +    } else {
> +        error_report("%s: Migration disabled", vbasedev->name);
>       }
> -    return ret;
> +    return false;
>   }
>
>   void vfio_migration_exit(VFIODevice *vbasedev)
> @@ -679,7 +678,6 @@ void vfio_migration_exit(VFIODevice *vbasedev)
>           qemu_del_vm_change_state_handler(migration->vm_state);
>           unregister_savevm(VMSTATE_IF(vbasedev->dev), "vfio", vbasedev);
>           vfio_migration_free(vbasedev);
> -        vfio_unblock_multiple_devices_migration();
>       }
>
>       if (vbasedev->migration_blocker) {
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 73874a94de12..12b82b3667ef 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -3207,10 +3207,7 @@ static void vfio_realize(PCIDevice *pdev, Error **errp)
>       }
>
>       if (!pdev->failover_pair_id) {
> -        ret = vfio_migration_realize(vbasedev, errp);
> -        if (ret) {
> -            error_report("%s: Migration disabled", vbasedev->name);
> -        }
> +        vfio_migration_realize(vbasedev, errp);
>       }
>
>       vfio_register_err_notifier(vdev);
> @@ -3247,7 +3244,6 @@ static void vfio_instance_finalize(Object *obj)
>        */
>       vfio_put_device(vdev);
>       vfio_put_group(group);
> -    vfio_migration_finalize();
>   }
>
>   static void vfio_exitfn(PCIDevice *pdev)
> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> index eed244f25f34..23ba3654acda 100644
> --- a/include/hw/vfio/vfio-common.h
> +++ b/include/hw/vfio/vfio-common.h
> @@ -220,9 +220,8 @@ typedef QLIST_HEAD(VFIOGroupList, VFIOGroup) VFIOGroupList;
>   extern VFIOGroupList vfio_group_list;
>
>   bool vfio_mig_active(void);
> -int vfio_block_multiple_devices_migration(Error **errp);
> -void vfio_unblock_multiple_devices_migration(void);
> -int vfio_block_giommu_migration(Error **errp);
> +bool vfio_block_multiple_devices_migration(Error **errp);
> +bool vfio_block_giommu_migration(Error **errp);
>   int64_t vfio_mig_bytes_transferred(void);
>
>   #ifdef CONFIG_LINUX
> @@ -246,8 +245,7 @@ int vfio_spapr_create_window(VFIOContainer *container,
>   int vfio_spapr_remove_window(VFIOContainer *container,
>                                hwaddr offset_within_address_space);
>
> -int vfio_migration_realize(VFIODevice *vbasedev, Error **errp);
> +bool vfio_migration_realize(VFIODevice *vbasedev, Error **errp);
>   void vfio_migration_exit(VFIODevice *vbasedev);
> -void vfio_migration_finalize(void);
>
>   #endif /* HW_VFIO_VFIO_COMMON_H */
> --
> 2.34.1