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

[Yan Zhao]

On Tue, Mar 24, 2020 at 10:49:54PM +0800, Alex Williamson wrote:
> On Tue, 24 Mar 2020 09:23:31 +0000
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > * Yan Zhao (address@hidden) 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 <address@hidden> 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 <address@hidden> 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.
> 
> Yes, minor changes in hardware or firmware that may not be represented
> in the device ID or hardware revision.  Also the version is as much for
> indicating the compatibility of the vendor defined migration protocol
> as it is for the hardware itself.  I certainly wouldn't be so bold as
> to create a protocol that is guaranteed compatible forever.  We'll need
> to expose the same sysfs attribute in some standard location for
> non-mdev devices.  I assume vfio-pci would provide the vendor ops some
> mechanism to expose these in a standard namespace of sysfs attributes
> under the device itself.  Perhaps that indicates we need to link the
> mdev type version under the mdev device as well to make this
> transparent to userspace tools like libvirt.  Thanks,
>
Got it. will do it.
Thanks!

Yan