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

["Maciej S. Szmigiero" <mail@maciej.szmigiero.name>]

On 26.02.2025 11:43, Cédric Le Goater wrote:
> On 2/19/25 21:34, Maciej S. Szmigiero wrote:
>> 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);
>>   }
> 
> this routine expects load_bufs_mutex to be locked ? May be say so.

I guess the comment above pertains to the vfio_load_state_buffer_insert()
below.

Do you mean it should have a comment that it expects to be called
under load_bufs_mutex?

>> +static bool vfio_load_state_buffer_insert(VFIODevice *vbasedev,
> 
> could you pass VFIOMultifd* instead  ?

No, it needs vbasedev->migration_max_queued_buffers too (introduced
in later patch).

Also, most of VFIO routines (besides very small helpers/wrappers)
take VFIODevice *.

>> +                                          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);
>> +        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)
> 
> 
> AFAICS, the only users of the .load_state_buffer() handlers is
> multifd_device_state_recv().
> 
> Please rename to vfio_multifd_load_state_buffer().

Renamed it accordingly.

>> +{
>> +    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());
>> +
>> +    if (!vfio_multifd_transfer_enabled(vbasedev)) {
>> +        error_setg(errp,
>> +                   "got device state packet but not doing multifd transfer");
>> +        return false;
>> +    }
>> +
>> +    assert(multifd);
>> +
>> +    if (data_size < sizeof(*packet)) {
>> +        error_setg(errp, "packet too short at %zu (min is %zu)",
>> +                   data_size, sizeof(*packet));
>> +        return false;
>> +    }
>> +
>> +    if (packet->version != VFIO_DEVICE_STATE_PACKET_VER_CURRENT) {
>> +        error_setg(errp, "packet has unknown version %" PRIu32,
>> +                   packet->version);
>> +        return false;
>> +    }
>> +
>> +    if (packet->idx == UINT32_MAX) {
>> +        error_setg(errp, "packet has too high idx");
> 
> or "packet index is invalid" ?

Changed the error message.

>> +        return false;
>> +    }
>> +
>> +    trace_vfio_load_state_device_buffer_incoming(vbasedev->name, packet->idx);
>> +
>> +    QEMU_LOCK_GUARD(&multifd->load_bufs_mutex);
> 
> Using WITH_QEMU_LOCK_GUARD() would be cleaner I think.

Changed into a WITH_QEMU_LOCK_GUARD() block.
  
> 
> 
> Thanks,
> 
> C.

Thanks,
Maciej