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

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Message-ID: <1eba059c-4743-4675-9f72-1a26b8f3c0f6@nvidia.com>
Date:   Wed, 29 Sep 2021 16:26:55 +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 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 ?

What is the definition of RUNNING state for a paused VM that is waiting 
for incoming migration blob ?

>
> Alex
>