netdev - Re: [PATCH V6 mlx5-next 10/15] vfio: Remove migration protocol v1

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Message-ID: <20220201160106.0760bfea.alex.williamson@redhat.com>
Date:   Tue, 1 Feb 2022 16:01:06 -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
Subject: Re: [PATCH V6 mlx5-next 10/15] vfio: Remove migration protocol v1

On Tue, 01 Feb 2022 13:39:23 +0100
Cornelia Huck <cohuck@...hat.com> wrote:

> On Tue, Feb 01 2022, Jason Gunthorpe <jgg@...dia.com> wrote:
> 
> > On Tue, Feb 01, 2022 at 12:23:05PM +0100, Cornelia Huck wrote:  
> >> On Sun, Jan 30 2022, Yishai Hadas <yishaih@...dia.com> wrote:
> >>   
> >> > From: Jason Gunthorpe <jgg@...dia.com>
> >> >
> >> > v1 was never implemented and is replaced by v2.
> >> >
> >> > The old uAPI definitions are removed from the header file. As per Linus's
> >> > past remarks we do not have a hard requirement to retain compilation
> >> > compatibility in uapi headers and qemu is already following Linus's
> >> > preferred model of copying the kernel headers.  
> >> 
> >> If we are all in agreement that we will replace v1 with v2 (and I think
> >> we are), we probably should remove the x-enable-migration stuff in QEMU
> >> sooner rather than later, to avoid leaving a trap for the next
> >> unsuspecting person trying to update the headers.  
> >
> > Once we have agreement on the kernel patch we plan to send a QEMU
> > patch making it support the v2 interface and the migration
> > non-experimental. We are also working to fixing the error paths, at
> > least least within the limitations of the current qemu design.  
> 
> I'd argue that just ripping out the old interface first would be easier,
> as it does not require us to synchronize with a headers sync (and does
> not require to synchronize a headers sync with ripping it out...)
> 
> > The v1 support should remain in old releases as it is being used in
> > the field "experimentally".  
> 
> Of course; it would be hard to rip it out retroactively :)
> 
> But it should really be gone in QEMU 7.0.
> 
> Considering adding the v2 uapi, we might get unlucky: The Linux 5.18
> merge window will likely be in mid-late March (and we cannot run a
> headers sync before the patches hit Linus' tree), while QEMU 7.0 will
> likely enter freeze in mid-late March as well. So there's a non-zero
> chance that the new uapi will need to be deferred to 7.1.


Agreed that v1 migration TYPE/SUBTYPE should live in infamy as
reserved, but I'm not sure why we need to make the rest of it a big
complicated problem.  On one hand, leaving stubs for the necessary
structure and macros until QEMU gets updated doesn't seem so terrible.
Nor actually does letting the next QEMU header update cause build
breakages, which would probably frustrate the person submitting that
update, but it's not like QEMU hasn't done selective header updates in
the past.  The former is probably the more friendly approach if we
don't outrage someone in the kernel community in the meantime.  Thanks,

Alex