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

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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




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