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Subject: Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state
 transition validity
To:     Alex Williamson <alex.williamson@redhat.com>
CC:     Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
        "Doug Ledford" <dledford@redhat.com>,
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        Saeed Mahameed <saeedm@nvidia.com>,
        Cornelia Huck <cohuck@redhat.com>
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From:   Max Gurtovoy <mgurtovoy@nvidia.com>
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Date:   Wed, 29 Sep 2021 18:28:44 +0300
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On 9/29/2021 6:17 PM, Alex Williamson wrote:
> On Wed, 29 Sep 2021 17:36:59 +0300
> Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>
>> On 9/29/2021 4:50 PM, Alex Williamson wrote:
>>> On Wed, 29 Sep 2021 16:26:55 +0300
>>> Max Gurtovoy <mgurtovoy@nvidia.com> wrote:
>>>   
>>>> On 9/29/2021 3:35 PM, Alex Williamson wrote:
>>>>> 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,
>>>> 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
>