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

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Date:   Wed, 29 Sep 2021 17:36:59 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
CC:     Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>,
        "Doug Ledford" <dledford@...hat.com>,
        Yishai Hadas <yishaih@...dia.com>,
        "Bjorn Helgaas" <bhelgaas@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Kirti Wankhede <kwankhede@...dia.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <linux-rdma@...r.kernel.org>, <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Cornelia Huck <cohuck@...hat.com>
Subject: Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state
 transition validity


On 9/29/2021 4:50 PM, Alex Williamson wrote:
> On Wed, 29 Sep 2021 16:26:55 +0300
> Max Gurtovoy <mgurtovoy@...dia.com> wrote:
>
>> On 9/29/2021 3:35 PM, Alex Williamson wrote:
>>> On Wed, 29 Sep 2021 13:44:10 +0300
>>> Max Gurtovoy <mgurtovoy@...dia.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 ?

>> What is the definition of RUNNING state for a paused VM that is waiting
>> for incoming migration blob ?
> A VM supporting migration of the device would move the device to
> _RESUMING to load the incoming data.  If the VM leaves the device in
> _RUNNING, then it doesn't support migration of the device and it's out
> of scope how it handles that device state.  Existing devices continue
> running regardless of whether the VM state is paused, it's only devices
> supporting migration where userspace could optionally have the device
> run state follow the VM run state.  Thanks,
>
> Alex
>