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

[Yan Zhao]

On Tue, Apr 21, 2020 at 08:08:49PM +0800, Tian, Kevin wrote:
> > From: Yan Zhao
> > Sent: Tuesday, April 21, 2020 10:37 AM
> > 
> > On Tue, Apr 21, 2020 at 06:56:00AM +0800, Alex Williamson wrote:
> > > On Sun, 19 Apr 2020 21:24:57 -0400
> > > Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> > >
> > > > On Fri, Apr 17, 2020 at 07:24:57PM +0800, Cornelia Huck wrote:
> > > > > On Fri, 17 Apr 2020 05:52:02 -0400
> > > > > Yan Zhao <yan.y.zhao@xxxxxxxxx> 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@xxxxxxxxx> 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.
> > > > >
> > > > as you pointed below, (3) and (2) serve the same purpose.
> > > > and I think a possible usage is to migrate between a non-mdev device and
> > > > an mdev device. so I think it's better for them both to use (2) rather
> > > > than creating (3).
> > >
> > > An mdev type is meant to define a software compatible interface, so in
> > > the case of mdev->mdev migration, doesn't migrating to a different type
> > > fail the most basic of compatibility tests that we expect userspace to
> > > perform?  IOW, if two mdev types are migration compatible, it seems a
> > > prerequisite to that is that they provide the same software interface,
> > > which means they should be the same mdev type.
> > >
> > > In the hybrid cases of mdev->phys or phys->mdev, how does a
> > management
> > > tool begin to even guess what might be compatible?  Are we expecting
> > > libvirt to probe ever device with this attribute in the system?  Is
> > > there going to be a new class hierarchy created to enumerate all
> > > possible migrate-able devices?
> > >
> > yes, management tool needs to guess and test migration compatible
> > between two devices. But I think it's not the problem only for
> > mdev->phys or phys->mdev. even for mdev->mdev, management tool needs
> > to
> > first assume that the two mdevs have the same type of parent devices
> > (e.g.their pciids are equal). otherwise, it's still enumerating
> > possibilities.
> > 
> > on the other hand, for two mdevs,
> > mdev1 from pdev1, its mdev_type is 1/2 of pdev1;
> > mdev2 from pdev2, its mdev_type is 1/4 of pdev2;
> > if pdev2 is exactly 2 times of pdev1, why not allow migration between
> > mdev1 <-> mdev2.
> 
> How could the manage tool figure out that 1/2 of pdev1 is equivalent 
> to 1/4 of pdev2? If we really want to allow such thing happen, the best
> choice is to report the same mdev type on both pdev1 and pdev2.
I think that's exactly the value of this migration_version interface.
the management tool can take advantage of this interface to know if two
devices are migration compatible, no matter they are mdevs, non-mdevs,
or mix.

as I know, (please correct me if not right), current libvirt still
requires manually generating mdev devices, and it just duplicates src vm
configuration to the target vm.
for libvirt, currently it's always phys->phys and mdev->mdev (and of the
same mdev type).
But it does not justify that hybrid cases should not be allowed. otherwise,
why do we need to introduce this migration_version interface and leave
the judgement of migration compatibility to vendor driver? why not simply
set the criteria to something like "pciids of parent devices are equal,
and mdev types are equal" ?


> btw mdev<->phys just brings trouble to upper stack as Alex pointed out. 
could you help me understand why it will bring trouble to upper stack?

I think it just needs to read src migration_version under src dev node,
and test it in target migration version under target dev node. 

after all, through this interface we just help the upper layer
knowing available options through reading and testing, and they decide
to use it or not.

> Can we simplify the requirement by allowing only mdev<->mdev and 
> phys<->phys migration? If an customer does want to migrate between a 
> mdev and phys, he could wrap physical device into a wrapped mdev 
> instance (with the same type as the source mdev) instead of using vendor 
> ops. Doing so does add some burden but if mdev<->phys is not dominant 
> usage then such tradeoff might be worthywhile...
>
If the interfaces for phys<->phys and mdev<->mdev are consistent, it makes no
difference to phys<->mdev, right?
I think the vendor string for a mdev device is something like:
"Parent PCIID + mdev type + software version", and
that for a phys device is something like:
"PCIID + software version".
as long as we don't migrate between devices from different vendors, it's
easy for vendor driver to tell if a phys device is migration compatible
to a mdev device according it supports it or not.


Thanks
Yan
> 
> > 
> > 
> > > I agree that there was a gap in the previous proposal for non-mdev
> > > devices, but I think this bring a lot of questions that we need to
> > > puzzle through and libvirt will need to re-evaluate how they might
> > > decide to pick a migration target device.  For example, I'm sure
> > > libvirt would reject any policy decisions regarding picking a physical
> > > device versus an mdev device.  Had we previously left it that only a
> > > layer above libvirt would select a target device and libvirt only tests
> > > compatibility to that target device?
> > I'm not sure if there's a layer above libvirt would select a target
> > device. but if there is such a layer (even it's human), we need to
> > provide an interface for them to know whether their decision is suitable
> > for migration. The migration_version interface provides a potential to
> > allow mdev->phys migration, even libvirt may currently reject it.
> > 
> > 
> > > We also need to consider that this expands the namespace.  If we no
> > > longer require matching types as the first level of comparison, then
> > > vendor migration strings can theoretically collide.  How do we
> > > coordinate that can't happen?  Thanks,
> > yes, it's indeed a problem.
> > could only allowing migration beteen devices from the same vendor be a
> > good
> > prerequisite?
> > 
> > Thanks
> > Yan
> > >
> > > > > > > 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.
> > > > >
> > > > yes. but I think if the condition happens, it should be reported a bug
> > > > to vendor driver.
> > > > should I add a line in the doc like "vendor driver should ensure that the
> > > > migration compatibility from migration_version under mdev_type should
> > be
> > > > consistent with that from migration_version under device node" ?
> > > >
> > > > > (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.
> > > > >
> > > > hmm, is it useful for userspace to test the migration_version under mdev
> > > > type before it knows what mdev device to generate ?
> > > > like when the userspace wants to migrate an mdev device in src vm,
> > > > but it has not created target vm and the target mdev device.
> > > >
> > > > > >
> > > > > > > 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?
> > > > so change (1) to migration_type_version and (2) to
> > > > migration_instance_version?
> > > > But as they are under different locations, could that location imply
> > > > enough information?
> > > >
> > > >
> > > > Thanks
> > > > Yan
> > > >
> > > >
> > >
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