Re: [PATCH v5 26/36] vfio/migration: Multifd device state transfer support - received buffers queuing

[Avihai Horon <avihaih@nvidia.com>]

On 19/02/2025 22:34, Maciej S. Szmigiero wrote:
> External email: Use caution opening links or attachments
>
>
> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>
> The multifd received data needs to be reassembled since device state
> packets sent via different multifd channels can arrive out-of-order.
>
> Therefore, each VFIO device state packet carries a header indicating its
> position in the stream.
> The raw device state data is saved into a VFIOStateBuffer for later
> in-order loading into the device.
>
> The last such VFIO device state packet should have
> VFIO_DEVICE_STATE_CONFIG_STATE flag set and carry the device config state.
>
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>   hw/vfio/migration-multifd.c | 103 ++++++++++++++++++++++++++++++++++++
>   hw/vfio/migration-multifd.h |   3 ++
>   hw/vfio/migration.c         |   1 +
>   hw/vfio/trace-events        |   1 +
>   4 files changed, 108 insertions(+)
>
> diff --git a/hw/vfio/migration-multifd.c b/hw/vfio/migration-multifd.c
> index c2defc0efef0..5d5ee1393674 100644
> --- a/hw/vfio/migration-multifd.c
> +++ b/hw/vfio/migration-multifd.c
> @@ -42,6 +42,11 @@ typedef struct VFIOStateBuffer {
>   } VFIOStateBuffer;
>
>   typedef struct VFIOMultifd {
> +    VFIOStateBuffers load_bufs;
> +    QemuCond load_bufs_buffer_ready_cond;
> +    QemuMutex load_bufs_mutex; /* Lock order: this lock -> BQL */
> +    uint32_t load_buf_idx;
> +    uint32_t load_buf_idx_last;
>   } VFIOMultifd;
>
>   static void vfio_state_buffer_clear(gpointer data)
> @@ -87,15 +92,113 @@ static VFIOStateBuffer *vfio_state_buffers_at(VFIOStateBuffers *bufs, guint idx)
>       return &g_array_index(bufs->array, VFIOStateBuffer, idx);
>   }
>
> +static bool vfio_load_state_buffer_insert(VFIODevice *vbasedev,
> +                                          VFIODeviceStatePacket *packet,
> +                                          size_t packet_total_size,
> +                                          Error **errp)
> +{
> +    VFIOMigration *migration = vbasedev->migration;
> +    VFIOMultifd *multifd = migration->multifd;
> +    VFIOStateBuffer *lb;
> +
> +    vfio_state_buffers_assert_init(&multifd->load_bufs);
> +    if (packet->idx >= vfio_state_buffers_size_get(&multifd->load_bufs)) {
> +        vfio_state_buffers_size_set(&multifd->load_bufs, packet->idx + 1);
> +    }
> +
> +    lb = vfio_state_buffers_at(&multifd->load_bufs, packet->idx);
> +    if (lb->is_present) {
> +        error_setg(errp, "state buffer %" PRIu32 " already filled",
> +                   packet->idx);

Let's add vbasedev->name to the error message so we know which device 
caused the error.

> +        return false;
> +    }
> +
> +    assert(packet->idx >= multifd->load_buf_idx);
> +
> +    lb->data = g_memdup2(&packet->data, packet_total_size - sizeof(*packet));
> +    lb->len = packet_total_size - sizeof(*packet);
> +    lb->is_present = true;
> +
> +    return true;
> +}
> +
> +bool vfio_load_state_buffer(void *opaque, char *data, size_t data_size,
> +                            Error **errp)
> +{
> +    VFIODevice *vbasedev = opaque;
> +    VFIOMigration *migration = vbasedev->migration;
> +    VFIOMultifd *multifd = migration->multifd;
> +    VFIODeviceStatePacket *packet = (VFIODeviceStatePacket *)data;
> +
> +    /*
> +     * Holding BQL here would violate the lock order and can cause
> +     * a deadlock once we attempt to lock load_bufs_mutex below.
> +     */
> +    assert(!bql_locked());

To be clearer, I'd move the assert down to be just above 
"QEMU_LOCK_GUARD(&multifd->load_bufs_mutex);".

> +
> +    if (!vfio_multifd_transfer_enabled(vbasedev)) {
> +        error_setg(errp,
> +                   "got device state packet but not doing multifd transfer");

Let's add vbasedev->name to the error message so we know which device 
caused the error.

> +        return false;
> +    }
> +
> +    assert(multifd);
> +
> +    if (data_size < sizeof(*packet)) {
> +        error_setg(errp, "packet too short at %zu (min is %zu)",
> +                   data_size, sizeof(*packet));

Ditto.

> +        return false;
> +    }
> +
> +    if (packet->version != VFIO_DEVICE_STATE_PACKET_VER_CURRENT) {
> +        error_setg(errp, "packet has unknown version %" PRIu32,
> +                   packet->version);

Ditto.

> +        return false;
> +    }
> +
> +    if (packet->idx == UINT32_MAX) {
> +        error_setg(errp, "packet has too high idx");

Ditto.

> +        return false;
> +    }
> +
> +    trace_vfio_load_state_device_buffer_incoming(vbasedev->name, packet->idx);
> +
> +    QEMU_LOCK_GUARD(&multifd->load_bufs_mutex);
> +
> +    /* config state packet should be the last one in the stream */
> +    if (packet->flags & VFIO_DEVICE_STATE_CONFIG_STATE) {
> +        multifd->load_buf_idx_last = packet->idx;
> +    }
> +
> +    if (!vfio_load_state_buffer_insert(vbasedev, packet, data_size, errp)) {
> +        return false;
> +    }
> +
> +    qemu_cond_signal(&multifd->load_bufs_buffer_ready_cond);
> +
> +    return true;
> +}
> +
>   VFIOMultifd *vfio_multifd_new(void)
>   {
>       VFIOMultifd *multifd = g_new(VFIOMultifd, 1);
>
> +    vfio_state_buffers_init(&multifd->load_bufs);
> +
> +    qemu_mutex_init(&multifd->load_bufs_mutex);

Nit: move qemu_mutex_init() just above qemu_cond_init()?

Thanks.

> +
> +    multifd->load_buf_idx = 0;
> +    multifd->load_buf_idx_last = UINT32_MAX;
> +    qemu_cond_init(&multifd->load_bufs_buffer_ready_cond);
> +
>       return multifd;
>   }
>
>   void vfio_multifd_free(VFIOMultifd *multifd)
>   {
> +    qemu_cond_destroy(&multifd->load_bufs_buffer_ready_cond);
> +    qemu_mutex_destroy(&multifd->load_bufs_mutex);
> +
>       g_free(multifd);
>   }
>
> diff --git a/hw/vfio/migration-multifd.h b/hw/vfio/migration-multifd.h
> index 1eefba3b2eed..d5ab7d6f85f5 100644
> --- a/hw/vfio/migration-multifd.h
> +++ b/hw/vfio/migration-multifd.h
> @@ -22,4 +22,7 @@ bool vfio_multifd_transfer_enabled(VFIODevice *vbasedev);
>
>   bool vfio_multifd_transfer_setup(VFIODevice *vbasedev, Error **errp);
>
> +bool vfio_load_state_buffer(void *opaque, char *data, size_t data_size,
> +                            Error **errp);
> +
>   #endif
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 4311de763885..abaf4d08d4a9 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -806,6 +806,7 @@ static const SaveVMHandlers savevm_vfio_handlers = {
>       .load_setup = vfio_load_setup,
>       .load_cleanup = vfio_load_cleanup,
>       .load_state = vfio_load_state,
> +    .load_state_buffer = vfio_load_state_buffer,
>       .switchover_ack_needed = vfio_switchover_ack_needed,
>   };
>
> diff --git a/hw/vfio/trace-events b/hw/vfio/trace-events
> index 1bebe9877d88..042a3dc54a33 100644
> --- a/hw/vfio/trace-events
> +++ b/hw/vfio/trace-events
> @@ -153,6 +153,7 @@ vfio_load_device_config_state_start(const char *name) " (%s)"
>   vfio_load_device_config_state_end(const char *name) " (%s)"
>   vfio_load_state(const char *name, uint64_t data) " (%s) data 0x%"PRIx64
>   vfio_load_state_device_data(const char *name, uint64_t data_size, int ret) " (%s) size %"PRIu64" ret %d"
> +vfio_load_state_device_buffer_incoming(const char *name, uint32_t idx) " (%s) idx %"PRIu32
>   vfio_migration_realize(const char *name) " (%s)"
>   vfio_migration_set_device_state(const char *name, const char *state) " (%s) state %s"
>   vfio_migration_set_state(const char *name, const char *new_state, const char *recover_state) " (%s) new state %s, recover state %s"