Message-ID: <20200428141437.GG2794@work-vm>
Date:   Tue, 28 Apr 2020 15:14:37 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
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Subject: Re: [PATCH v5 0/4] introduction of migration_version attribute for
 VFIO live migration

* Yan Zhao (yan.y.zhao@...el.com) wrote:
> On Mon, Apr 27, 2020 at 11:37:43PM +0800, Dr. David Alan Gilbert wrote:
> > * Yan Zhao (yan.y.zhao@...el.com) wrote:
> > > On Sat, Apr 25, 2020 at 03:10:49AM +0800, Dr. David Alan Gilbert wrote:
> > > > * Yan Zhao (yan.y.zhao@...el.com) wrote:
> > > > > 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@...el.com> 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@...el.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@...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.
> > > > > > > > > > > > >
> > > > > > > > > > > > > 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.
> > > > 
> > > > It surprises me that the PCIID matching is a requirement; I'd assumed
> > > > with this clever mdev name setup that you could migrate between two
> > > > different models in a series, or to a newer model, as long as they
> > > > both supported the same mdev view.
> > > > 
> > > hi Dave
> > > the migration_version string is transparent to userspace, and is
> > > completely defined by vendor driver.
> > > I put it there just as an example of how vendor driver may implement it.
> > > e.g.
> > > the src migration_version string is "src PCIID + src software version", 
> > > then when this string is write to target migration_version node,
> > > the vendor driver in the target device will compare it with its own
> > > device info and software version.
> > > If different models are allowed, the write just succeeds even
> > > PCIIDs in src and target are different.
> > > 
> > > so, it is the vendor driver to define whether two devices are able to
> > > migrate, no matter their PCIIDs, mdev types, software versions..., which
> > > provides vendor driver full flexibility.
> > > 
> > > do you think it's good?
> > 
> > Yeh that's OK; I guess it's going to need to have a big table in their
> > with all the PCIIDs in.
> > The alternative would be to abstract it a little; e.g. to say it's
> > an Intel-gpu-core-v4  and then it would be less worried about the exact
> > clock speed etc - but yes you might be right htat PCIIDs might be best
> > for checking for quirks.
> >
> glad that you are agreed with it:)
> I think the vendor driver still can choose a way to abstract a little
> (e.g. Intel-gpu-core-v4...) if they think it's better. In that case, the
> migration_string would be something like "Intel-gpu-core-v4 + instance
> number + software version".
> IOW, they can choose anything they think appropriate to identify migration
> compatibility of a device.
> But Alex is right, we have to prevent namespace overlapping. So I think
> we need to ensure src and target devices are from the same vendors.
> or, any other ideas?

That's why I kept the 'Intel' in that example; or PCI vendor ID; I was
only really trying to say that within one vendors range there are often
a lot of PCI-IDs that have really minor variations.

Dave

> 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
> > > > > > > > >
> > > > > > > > >
> > > > > > > >
> > > > > > > _______________________________________________
> > > > > > > intel-gvt-dev mailing list
> > > > > > > intel-gvt-dev@...ts.freedesktop.org
> > > > > > > https://lists.freedesktop.org/mailman/listinfo/intel-gvt-dev
> > > > > 
> > > > --
> > > > Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
> > > > 
> > > 
> > --
> > Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK

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