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    SubjectRe: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity
    On Wed, 29 Sep 2021 13:44:10 +0300
    Max Gurtovoy <mgurtovoy@nvidia.com> wrote:

    > On 9/28/2021 2:12 AM, Jason Gunthorpe wrote:
    > > On Mon, Sep 27, 2021 at 04:46:48PM -0600, Alex Williamson wrote:
    > >>> + enum { MAX_STATE = VFIO_DEVICE_STATE_RESUMING };
    > >>> + static const u8 vfio_from_state_table[MAX_STATE + 1][MAX_STATE + 1] = {
    > >>> + [VFIO_DEVICE_STATE_STOP] = {
    > >>> + [VFIO_DEVICE_STATE_RUNNING] = 1,
    > >>> + [VFIO_DEVICE_STATE_RESUMING] = 1,
    > >>> + },
    > >> Our state transition diagram is pretty weak on reachable transitions
    > >> out of the _STOP state, why do we select only these two as valid?
    > > I have no particular opinion on specific states here, however adding
    > > more states means more stuff for drivers to implement and more risk
    > > driver writers will mess up this uAPI.
    >
    > _STOP == 000b => Device Stopped, not saving or resuming (from UAPI).
    >
    > This is the default initial state and not RUNNING.
    >
    > The user application should move device from STOP => RUNNING or STOP =>
    > RESUMING.
    >
    > Maybe we need to extend the comment in the UAPI file.


    include/uapi/linux/vfio.h:
    ...
    * +------- _RESUMING
    * |+------ _SAVING
    * ||+----- _RUNNING
    * |||
    * 000b => Device Stopped, not saving or resuming
    * 001b => Device running, which is the default state
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...
    * State transitions:
    *
    * _RESUMING _RUNNING Pre-copy Stop-and-copy _STOP
    * (100b) (001b) (011b) (010b) (000b)
    * 0. Running or default state
    * |
    ^^^^^^^^^^^^^
    ...
    * 0. Default state of VFIO device is _RUNNING when the user application starts.
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    The uAPI is pretty clear here. A default state of _STOP is not
    compatible with existing devices and userspace that does not support
    migration. Thanks,

    Alex

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