netdev - Re: [PATCH V9 mlx5-next 10/15] vfio: Extend the device migration protocol with RUNNING

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Date:   Thu, 24 Feb 2022 13:46:02 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Cornelia Huck <cohuck@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>,
        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,
        ashok.raj@...el.com, kevin.tian@...el.com,
        shameerali.kolothum.thodi@...wei.com
Subject: Re: [PATCH V9 mlx5-next 10/15] vfio: Extend the device migration
 protocol with RUNNING_P2P

On Thu, 24 Feb 2022 17:53:59 +0100
Cornelia Huck <cohuck@...hat.com> wrote:

> On Thu, Feb 24 2022, Alex Williamson <alex.williamson@...hat.com> wrote:
> > Chatting with Connie offline, I think the clarification that might help
> > is something alone the lines that the combination of bits must support
> > migration, which currently requires the STOP_COPY and RESUMING states.
> > The VFIO_MIGRATION_P2P flag alone does not provide these states.  The
> > only flag in the current specification to provide these states is
> > VFIO_MIGRATION_STOP_COPY.  I don't think we want to preclude that some
> > future flag might provide variants of STOP_COPY and RESUMING, so it's
> > not so much that VFIO_MIGRATION_STOP_COPY is mandatory, but it is
> > currently the only flag which provides the base degree of migration
> > support.  
> 
> Indeed.
> 
> >
> > How or if that translates to an actual documentation update, I'm not
> > sure.  As it stands, we're not speculating about future support, we're
> > only stating these two combinations are valid.  Future combinations may
> > or may not include VFIO_MIGRATION_STOP_COPY.  As the existing proposed
> > comment indicates, other combinations are TBD.  Connie?  Thanks,
> >
> > Alex  
> 
> Hm... "a flag indicating support for a migration state machine such as
> VFIO_MIGRATION_STOP_COPY is mandatory"?

TBH, I'm not sure this makes a substantive improvement.  We don't know
what those new flag bits will be used for, including which bit or bits
will combine to indicate a valid state machine.  Userspace written to
this spec needs to support STOP_COPY and optionally P2P as we're
stating.  Nothing really compels us to speculate general rules for
unknown future bit combinations.  Thanks,

Alex