Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity

[Max Gurtovoy]

On 9/29/2021 3:35 PM, Alex Williamson wrote:
> On Wed, 29 Sep 2021 13:44:10 +0300
> Max Gurtovoy <mgurtovoy@xxxxxxxxxx> 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,

Why do you need this state machine for userspace that doesn't support 
migration ?

What is the definition of RUNNING state for a paused VM that is waiting 
for incoming migration blob ?

> Alex