Re: [PATCH v5 0/4] introduction of migration_version attribute for VFIO live migration

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 17 Apr 2020 05:52:02 -0400
Yan Zhao <yan.y.zhao@intel.com> wrote:

> On Fri, Apr 17, 2020 at 04:44:50PM +0800, Cornelia Huck wrote:
> > On Mon, 13 Apr 2020 01:52:01 -0400
> > Yan Zhao <yan.y.zhao@intel.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.
> > > 
> > > Currently, it has two locations:
> > > (1) under mdev_type node,
> > >     which can be used even before device creation, but only for mdev
> > >     devices of the same mdev type.
> > > (2) under mdev device node,
> > >     which can only be used after the mdev devices are created, but the src
> > >     and target mdev devices are not necessarily be of the same mdev type
> > > (The second location is newly added in v5, in order to keep consistent
> > > with the migration_version node for migratable pass-though devices)  
> > 
> > What is the relationship between those two attributes?
> >   
> (1) is for mdev devices specifically, and (2) is provided to keep the same
> sysfs interface as with non-mdev cases. so (2) is for both mdev devices and
> non-mdev devices.
> 
> in future, if we enable vfio-pci vendor ops, (i.e. a non-mdev device
> is binding to vfio-pci, but is able to register migration region and do
> migration transactions from a vendor provided affiliate driver),
> the vendor driver would export (2) directly, under device node.
> It is not able to provide (1) as there're no mdev devices involved.

Ok, creating an alternate attribute for non-mdev devices makes sense.
However, wouldn't that rather be a case (3)? The change here only
refers to mdev devices.

> 
> > Is existence (and compatibility) of (1) a pre-req for possible
> > existence (and compatibility) of (2)?
> >  
> no. (2) does not reply on (1).

Hm. Non-existence of (1) seems to imply "this type does not support
migration". If an mdev created for such a type suddenly does support
migration, it feels a bit odd.

(It obviously cannot be a prereq for what I called (3) above.)

> 
> > Does userspace need to check (1) or can it completely rely on (2), if
> > it so chooses?
> >  
> I think it can completely reply on (2) if compatibility check before
> mdev creation is not required.
> 
> > If devices with a different mdev type are indeed compatible, it seems
> > userspace can only find out after the devices have actually been
> > created, as (1) does not apply?  
> yes, I think so. 

How useful would it be for userspace to even look at (1) in that case?
It only knows if things have a chance of working if it actually goes
ahead and creates devices.

> 
> > One of my worries is that the existence of an attribute with the same
> > name in two similar locations might lead to confusion. But maybe it
> > isn't a problem.
> >  
> Yes, I have the same feeling. but as (2) is for sysfs interface
> consistency, to make it transparent to userspace tools like libvirt,
> I guess the same name is necessary?

What do we actually need here, I wonder? (1) and (2) seem to serve
slightly different purposes, while (2) and what I called (3) have the
same purpose. Is it important to userspace that (1) and (2) have the
same name?