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

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Message-ID: <20210927231239.GE3544071@ziepe.ca>
Date:   Mon, 27 Sep 2021 20:12:39 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     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 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.

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