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

* Yan Zhao (yan.y.zhao(a)intel.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(a)intel.com&gt; 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(a)intel.com&gt; 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(a)intel.com&gt; 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.

> > 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(a)lists.freedesktop.org > > > https://lists.freedesktop.org/mailman/listinfo/intel-gvt-dev > -- Dr. David Alan Gilbert / dgilbert(a)redhat.com / Manchester, UK