On Wed, 29 Sep 2021 17:36:59 +0300
Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
> On 9/29/2021 4:50 PM, Alex Williamson wrote:
> > On Wed, 29 Sep 2021 16:26:55 +0300
> > Max Gurtovoy <mgurtovoy@xxxxxxxxxx> wrote:
> >
> >> 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 ?
> > 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?
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
