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 06:35:51 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Max Gurtovoy <mgurtovoy@...dia.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 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,

Alex