On 9/29/2021 6:17 PM, Alex Williamson wrote:

On Wed, 29 Sep 2021 17:36:59 +0300
Max Gurtovoy [off-list ref] wrote:

quoted

On 9/29/2021 4:50 PM, Alex Williamson wrote:

quoted

On Wed, 29 Sep 2021 16:26:55 +0300
Max Gurtovoy [off-list ref] wrote:
  

quoted

On 9/29/2021 3:35 PM, Alex Williamson wrote:

quoted

On Wed, 29 Sep 2021 13:44:10 +0300
Max Gurtovoy [off-list ref] wrote:
     

quoted

On 9/28/2021 2:12 AM, Jason Gunthorpe wrote:

quoted

On Mon, Sep 27, 2021 at 04:46:48PM -0600, Alex Williamson wrote:

quoted

quoted

+	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 ?

For userspace that doesn't support migration, there's one state,
_RUNNING.  That's what we're trying to be compatible and consistent
with.  Migration is an extension, not a base requirement.

Userspace without migration doesn't care about this state.

We left with kernel now. vfio-pci today doesn't support migration, right
? state is in theory is 0 (STOP).

This state machine is controlled by the migration SW. The drivers don't
move state implicitly.

mlx5-vfio-pci support migration and will work fine with non-migration SW
(it will stay with state = 0 unless someone will move it. but nobody
will) exactly like vfio-pci does today.

So where is the problem ?

So you have a device that's actively modifying its internal state,
performing I/O, including DMA (thereby dirtying VM memory), all while
in the _STOP state?  And you don't see this as a problem?

I don't see how is it different from vfio-pci situation.

And you said you're worried from compatibility. I can't see a 
compatibility issue here.

Maybe we need to rename STOP state. We can call it READY or LIVE or 
NON_MIGRATION_STATE.

There's a major inconsistency if the migration interface is telling us
something different than we can actually observe through the behavior of
the device.  Thanks,

Alex