linux-kernel - Re: [PATCH v5 0/8] vfio/hisilicon: add ACC live migration driver

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Date:   Tue, 22 Feb 2022 12:29:39 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, cohuck@...hat.com,
        mgurtovoy@...dia.com, yishaih@...dia.com, linuxarm@...wei.com,
        liulongfang@...wei.com, prime.zeng@...ilicon.com,
        jonathan.cameron@...wei.com, wangzhou1@...ilicon.com
Subject: Re: [PATCH v5 0/8] vfio/hisilicon: add ACC live migration driver

On Mon, 21 Feb 2022 20:49:43 -0400
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Mon, Feb 21, 2022 at 11:40:35AM +0000, Shameer Kolothum wrote:
> > 
> > Hi,
> > 
> > This series attempts to add vfio live migration support for
> > HiSilicon ACC VF devices based on the new v2 migration protocol
> > definition and mlx5 v8 series discussed here[0].
> > 
> > RFCv4 --> v5
> >   - Dropped RFC tag as v2 migration APIs are more stable now.
> >   - Addressed review comments from Jason and Alex (Thanks!).
> > 
> > This is sanity tested on a HiSilicon platform using the Qemu branch
> > provided here[1].
> > 
> > Please take a look and let me know your feedback.
> > 
> > Thanks,
> > Shameer
> > [0] https://lore.kernel.org/kvm/20220220095716.153757-1-yishaih@nvidia.com/
> > [1] https://github.com/jgunthorpe/qemu/commits/vfio_migration_v2
> > 
> > 
> > v3 --> RFCv4
> > -Based on migration v2 protocol and mlx5 v7 series.
> > -Added RFC tag again as migration v2 protocol is still under discussion.
> > -Added new patch #6 to retrieve the PF QM data.
> > -PRE_COPY compatibility check is now done after the migration data
> >  transfer. This is not ideal and needs discussion.  
> 
> Alex, do you want to keep the PRE_COPY in just for acc for now? Or do
> you think this is not a good temporary use for it?
> 
> We have some work toward doing the compatability more generally, but I
> think it will be a while before that is all settled.

In the original migration protocol I recall that we discussed that
using the pre-copy phase for compatibility testing, even without
additional device data, as a valid use case.  The migration driver of
course needs to account for the fact that userspace is not required to
perform a pre-copy, and therefore cannot rely on that exclusively for
compatibility testing, but failing a migration earlier due to detection
of an incompatibility is generally a good thing.

If the ACC driver wants to re-incorporate this behavior into a non-RFC
proposed series and we could align accepting them into the same kernel
release, that sounds ok to me.  Thanks,

Alex