Re: [PATCH v6 34/36] vfio/migration: Max in-flight VFIO device state buffer count limit

["Cédric Le Goater" <clg@redhat.com>]

On 3/4/25 23:04, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> 
> Allow capping the maximum count of in-flight VFIO device state buffers
> queued at the destination, otherwise a malicious QEMU source could
> theoretically cause the target QEMU to allocate unlimited amounts of memory
> for buffers-in-flight.
> 
> Since this is not expected to be a realistic threat in most of VFIO live
> migration use cases and the right value depends on the particular setup
> disable the limit by default by setting it to UINT64_MAX.

I agree with Avihai that a limit on bytes would make more sense.
-rc0 is in ~2w. We have time to prepare a patch for this.


Should there be a correlation with :

     /*
      * This is an arbitrary size based on migration of mlx5 devices, where typically
      * total device migration size is on the order of 100s of MB. Testing with
      * larger values, e.g. 128MB and 1GB, did not show a performance improvement.
      */
     #define VFIO_MIG_DEFAULT_DATA_BUFFER_SIZE (1 * MiB)
     
Thanks,

C.





> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>   hw/vfio/migration-multifd.c   | 16 ++++++++++++++++
>   hw/vfio/pci.c                 |  9 +++++++++
>   include/hw/vfio/vfio-common.h |  1 +
>   3 files changed, 26 insertions(+)
> 
> diff --git a/hw/vfio/migration-multifd.c b/hw/vfio/migration-multifd.c
> index 233724710b37..d6dabaf869ca 100644
> --- a/hw/vfio/migration-multifd.c
> +++ b/hw/vfio/migration-multifd.c
> @@ -54,6 +54,7 @@ typedef struct VFIOMultifd {
>       QemuMutex load_bufs_mutex; /* Lock order: this lock -> BQL */
>       uint32_t load_buf_idx;
>       uint32_t load_buf_idx_last;
> +    uint32_t load_buf_queued_pending_buffers;
>   } VFIOMultifd;
>   
>   static void vfio_state_buffer_clear(gpointer data)
> @@ -125,6 +126,17 @@ static bool vfio_load_state_buffer_insert(VFIODevice *vbasedev,
>   
>       assert(packet->idx >= multifd->load_buf_idx);
>   
> +    multifd->load_buf_queued_pending_buffers++;
> +    if (multifd->load_buf_queued_pending_buffers >
> +        vbasedev->migration_max_queued_buffers) {
> +        error_setg(errp,
> +                   "%s: queuing state buffer %" PRIu32
> +                   " would exceed the max of %" PRIu64,
> +                   vbasedev->name, packet->idx,
> +                   vbasedev->migration_max_queued_buffers);
> +        return false;
> +    }
> +
>       lb->data = g_memdup2(&packet->data, packet_total_size - sizeof(*packet));
>       lb->len = packet_total_size - sizeof(*packet);
>       lb->is_present = true;
> @@ -381,6 +393,9 @@ static bool vfio_load_bufs_thread(void *opaque, bool *should_quit, Error **errp)
>               goto thread_exit;
>           }
>   
> +        assert(multifd->load_buf_queued_pending_buffers > 0);
> +        multifd->load_buf_queued_pending_buffers--;
> +
>           if (multifd->load_buf_idx == multifd->load_buf_idx_last - 1) {
>               trace_vfio_load_state_device_buffer_end(vbasedev->name);
>           }
> @@ -417,6 +432,7 @@ static VFIOMultifd *vfio_multifd_new(void)
>   
>       multifd->load_buf_idx = 0;
>       multifd->load_buf_idx_last = UINT32_MAX;
> +    multifd->load_buf_queued_pending_buffers = 0;
>       qemu_cond_init(&multifd->load_bufs_buffer_ready_cond);
>   
>       multifd->load_bufs_thread_running = false;
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index 21605bac2fb0..ce407f971000 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -3383,6 +3383,8 @@ static const Property vfio_pci_dev_properties[] = {
>                   vbasedev.migration_multifd_transfer,
>                   vfio_pci_migration_multifd_transfer_prop, OnOffAuto,
>                   .set_default = true, .defval.i = ON_OFF_AUTO_AUTO),
> +    DEFINE_PROP_UINT64("x-migration-max-queued-buffers", VFIOPCIDevice,
> +                       vbasedev.migration_max_queued_buffers, UINT64_MAX),
>       DEFINE_PROP_BOOL("migration-events", VFIOPCIDevice,
>                        vbasedev.migration_events, false),
>       DEFINE_PROP_BOOL("x-no-mmap", VFIOPCIDevice, vbasedev.no_mmap, false),
> @@ -3444,6 +3446,13 @@ static void vfio_pci_dev_class_init(ObjectClass *klass, void *data)
>                                             "x-migration-multifd-transfer",
>                                             "Transfer this device state via "
>                                             "multifd channels when live migrating it");
> +    object_class_property_set_description(klass, /* 10.0 */
> +                                          "x-migration-max-queued-buffers",
> +                                          "Maximum count of in-flight VFIO "
> +                                          "device state buffers queued at the "
> +                                          "destination when doing live "
> +                                          "migration of device state via "
> +                                          "multifd channels");
>   }
>   
>   static const TypeInfo vfio_pci_dev_info = {
> diff --git a/include/hw/vfio/vfio-common.h b/include/hw/vfio/vfio-common.h
> index 04b123a6c929..c033c3c5134f 100644
> --- a/include/hw/vfio/vfio-common.h
> +++ b/include/hw/vfio/vfio-common.h
> @@ -155,6 +155,7 @@ typedef struct VFIODevice {
>       bool ram_block_discard_allowed;
>       OnOffAuto enable_migration;
>       OnOffAuto migration_multifd_transfer;
> +    uint64_t migration_max_queued_buffers;
>       bool migration_events;
>       VFIODeviceOps *ops;
>       unsigned int num_irqs;
>