netdev - Re: [PATCH V6 mlx5-next 09/15] vfio: Extend the device migration protocol with RUNNING

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Date:   Thu, 3 Feb 2022 10:22:19 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Yishai Hadas <yishaih@...dia.com>, bhelgaas@...gle.com,
        saeedm@...dia.com, linux-pci@...r.kernel.org, kvm@...r.kernel.org,
        netdev@...r.kernel.org, kuba@...nel.org, leonro@...dia.com,
        kwankhede@...dia.com, mgurtovoy@...dia.com, maorg@...dia.com
Subject: Re: [PATCH V6 mlx5-next 09/15] vfio: Extend the device migration
 protocol with RUNNING_P2P

On Wed, Feb 02, 2022 at 04:54:44PM -0700, Alex Williamson wrote:

> I think the argument here is that there's no value to validating or
> bounds checking the end state, which could be done in the core ioctl
> before calling the driver if the first iteration will already fail for
> both the end state and the full path validation.

Yes, I had a version like this in an internal draft, it was something
like this:

if (vfio_mig_get_next_state(vdev, set_state.device_state, 
     set_state.device_state) == VFIO_DEVICE_STATE_ERROR)
    return -EINVAL;

Which is fully redundant with the driver, only does half the check and
looks weird.

> > Perhaps it is confusing using ERROR to indicate that
> > vfio_mig_get_next_state() failed. Would you be happier with a -errno
> > return?
> 
> Yes, it's confusing to me that next_state() returns states that don't
> become the device_state.  Stuffing the next step back into cur_fsm and
> using an errno for a bounds/validity/blocked-arc test would be a better
> API.  Thanks,

OK

Jason