linux-kernel - Re: [PATCH v4 0/2] introduction of migration_version attribute for VFIO live migration

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Message-ID: <20200324092331.GA2645@work-vm>
Date:   Tue, 24 Mar 2020 09:23:31 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "intel-gvt-dev@...ts.freedesktop.org" 
        <intel-gvt-dev@...ts.freedesktop.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        "Zhengxiao.zx@...baba-inc.com" <Zhengxiao.zx@...baba-inc.com>,
        "shuangtai.tst@...baba-inc.com" <shuangtai.tst@...baba-inc.com>,
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        "eauger@...hat.com" <eauger@...hat.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yang, Ziye" <ziye.yang@...el.com>,
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        "Liu, Changpeng" <changpeng.liu@...el.com>,
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Subject: Re: [PATCH v4 0/2] introduction of migration_version attribute for
 VFIO live migration

* Yan Zhao (yan.y.zhao@...el.com) wrote:
> On Tue, Mar 24, 2020 at 05:29:59AM +0800, Alex Williamson wrote:
> > On Mon, 3 Jun 2019 20:34:22 -0400
> > Yan Zhao <yan.y.zhao@...el.com> wrote:
> > 
> > > On Tue, Jun 04, 2019 at 03:29:32AM +0800, Alex Williamson wrote:
> > > > On Thu, 30 May 2019 20:44:38 -0400
> > > > Yan Zhao <yan.y.zhao@...el.com> wrote:
> > > >   
> > > > > This patchset introduces a migration_version attribute under sysfs of VFIO
> > > > > Mediated devices.
> > > > > 
> > > > > This migration_version attribute is used to check migration compatibility
> > > > > between two mdev devices of the same mdev type.
> > > > > 
> > > > > Patch 1 defines migration_version attribute in
> > > > > Documentation/vfio-mediated-device.txt
> > > > > 
> > > > > Patch 2 uses GVT as an example to show how to expose migration_version
> > > > > attribute and check migration compatibility in vendor driver.  
> > > > 
> > > > Thanks for iterating through this, it looks like we've settled on
> > > > something reasonable, but now what?  This is one piece of the puzzle to
> > > > supporting mdev migration, but I don't think it makes sense to commit
> > > > this upstream on its own without also defining the remainder of how we
> > > > actually do migration, preferably with more than one working
> > > > implementation and at least prototyped, if not final, QEMU support.  I
> > > > hope that was the intent, and maybe it's now time to look at the next
> > > > piece of the puzzle.  Thanks,
> > > > 
> > > > Alex  
> > > 
> > > Got it. 
> > > Also thank you and all for discussing and guiding all along:)
> > > We'll move to the next episode now.
> > 
> > Hi Yan,
> > 
> > As we're hopefully moving towards a migration API, would it make sense
> > to refresh this series at the same time?  I think we're still expecting
> > a vendor driver implementing Kirti's migration API to also implement
> > this sysfs interface for compatibility verification.  Thanks,
> >
> Hi Alex
> Got it!
> Thanks for reminding of this. And as now we have vfio-pci implementing
> vendor ops to allow live migration of pass-through devices, is it
> necessary to implement similar sysfs node for those devices?
> or do you think just PCI IDs of those devices are enough for libvirt to
> know device compatibility ?

Wasn't the problem that we'd have to know how to check for things like:
  a) Whether different firmware versions in the device were actually
compatible
  b) Whether minor hardware differences were compatible - e.g. some
hardware might let you migrate to the next version of hardware up.

Dave

> Thanks
> Yan
> 
> 
--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK