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 13:44:10 +0300
From:   Max Gurtovoy <mgurtovoy@...dia.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     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 9/28/2021 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.

>
> So only on those grounds I'd suggest to keep this to the minimum
> needed instead of the maximum logically possible..
>
> Also, probably the FSM comment from the uapi header file should be
> moved into a function comment above this function?
>
> Jason