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

* Alex Williamson (alex.williamson(a)redhat.com) wrote: > On Wed, 3 Jun 2020 01:24:43 -0400 > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > On Tue, Jun 02, 2020 at 09:55:28PM -0600, Alex Williamson wrote: > > > On Tue, 2 Jun 2020 23:19:48 -0400 > > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > > > On Tue, Jun 02, 2020 at 04:55:27PM -0600, Alex Williamson wrote: > > > > > On Wed, 29 Apr 2020 20:39:50 -0400 > > > > > Yan Zhao <yan.y.zhao(a)intel.com&gt; wrote: > > > > > > > > > > > On Wed, Apr 29, 2020 at 05:48:44PM +0800, Dr. David Alan Gilbert wrote: > > > > > > <snip> > > > > > > > > > > > > > > > > > > > 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 > > > > > > > > > > Yes, it's a good idea! > > > > > > > > > > could we add a line in the doc saying that > > > > > > > > > > it is the vendor driver to add a unique string to avoid namespace > > > > > > > > > > collision? > > > > > > > > > > > > > > > > > > So why don't we split the difference; lets say that it should start with > > > > > > > > > the hex PCI Vendor ID. > > > > > > > > > > > > > > > > > The problem is for mdev devices, if the parent devices are not PCI devices, > > > > > > > > they don't have PCI vendor IDs. > > > > > > > > > > > > > > Hmm it would be best not to invent a whole new way of giving unique > > > > > > > idenitifiers for vendors if we can. > > > > > > > > > > > > > what about leveraging the flags in vfio device info ? > > > > > > > > > > > > #define VFIO_DEVICE_FLAGS_RESET (1 << 0) /* Device supports reset */ > > > > > > #define VFIO_DEVICE_FLAGS_PCI (1 << 1) /* vfio-pci device */ > > > > > > #define VFIO_DEVICE_FLAGS_PLATFORM (1 << 2) /* vfio-platform device */ > > > > > > #define VFIO_DEVICE_FLAGS_AMBA (1 << 3) /* vfio-amba device */ > > > > > > #define VFIO_DEVICE_FLAGS_CCW (1 << 4) /* vfio-ccw device */ > > > > > > #define VFIO_DEVICE_FLAGS_AP (1 << 5) /* vfio-ap device */ > > > > > > > > > > > > Then for migration_version string, > > > > > > The first 64 bits are for device type, the second 64 bits are for device id. > > > > > > e.g. > > > > > > for PCI devices, it could be > > > > > > VFIO_DEVICE_FLAGS_PCI + PCI ID. > > > > > > > > > > > > Currently in the doc, we only define PCI devices to use PCI ID as the second > > > > > > 64 bits. In future, if other types of devices want to support migration, > > > > > > they can define their own parts of device id. e.g. use ACPI ID as the > > > > > > second 64-bit... > > > > > > > > > > > > sounds good? > > > > > > > > > > [dead thread resurrection alert] > > > > > > > > > > Not really. We're deep into territory that we were trying to avoid. > > > > > We had previously defined the version string as opaque (not > > > > > transparent) specifically because we did not want userspace to make > > > > > assumptions about compatibility based on the content of the string. It > > > > > was 100% left to the vendor driver to determine compatibility. The > > > > > mdev type was the full extent of the first level filter that userspace > > > > > could use to narrow the set of potentially compatible devices. If we > > > > > remove that due to physical device migration support, I'm not sure how > > > > > we simplify the problem for userspace. > > > > > > > > > > We need to step away from PCI IDs and parent devices. We're not > > > > > designing a solution that only works for PCI, there's no guarantee that > > > > > parent devices are similar or even from the same vendor. > > > > > > > > > > Does the mdev type sufficiently solve the problem for mdev devices? If > > > > > so, then what can we learn from it and how can we apply an equivalence > > > > > to physical devices? For example, should a vfio bus driver (vfio-pci > > > > > or vfio-mdev) expose vfio_migration_type and vfio_migration_version > > > > > attributes under the device in sysfs where the _type provides the first > > > > > level, user transparent, matching string (ex. mdev type for mdev > > > > > devices) while the _version provides the user opaque, vendor known > > > > > compatibility test? > > > > > > > > > > This pushes the problem out to the drivers where we can perhaps > > > > > incorporate the module name to avoid collisions. For example Yan's > > > > > vendor extension proposal makes use of vfio-pci with extension modules > > > > > loaded via an alias incorporating the PCI vendor and device ID. So > > > > > vfio-pci might use a type of "vfio-pci:$ALIAS". > > > > > > > > > > It's still a bit messy that someone needs to go evaluate all these > > > > > types between devices that exist and mdev devices that might exist if > > > > > created, but I don't have any good ideas to resolve that (maybe a new > > > > > class hierarchy?). Thanks, > > > > > > > > hi Alex > > > > > > > > yes, with the same mdev_type, user still has to enumerate all parent > > > > devices and test between the supported mdev_types to know whether two mdev > > > > devices are compatible. > > > > maybe this is not a problem? in reality, it is the administrator that > > > > specifies two devices and the management tool feedbacks compatibility > > > > result. management tool is not required to pre-test and setup the > > > > compatibility map beforehand. > > > > > > That's exactly the purpose of this interface though is to give the > > > management tools some indication that a migration has a chance of > > > working. > > > > > > > If so, then the only problem left is namespace collision. > > > > given that the migration_version nodes is exported by vendor driver, > > > > maybe it can also embed its module name in the migration version string, > > > > like "i915" in "i915-GVTg_V5_8", as you suggested above. > > > > > > No, we've already decided that the version string is opaque, the user > > > is not to attempt to infer anything from it. That's why I've suggested > > > another attribute in sysfs that does present type information that a > > > user can compare. Thanks, > > > > > > Alex > > > > > ok. got it. > > one more thing I want to confirm is that do you think it's a necessary > > restriction that "The mdev devices are of the same type" ? > > could mdev and phys devices both expose "vfio_migration_type" and > > "vfio_migration_version" under device sysfs so that it may not be > > confined in mdev_type? (e.g. when aggregator is enabled, though two > > mdevs are of the same mdev_type, they are not actually compatible; and > > two mdevs are compatible though their mdev_type is not equal.) > > > > for mdev devices, we could still expose vfio_migration_version > > attribute under mdev_type for detection before mdev generated. > > I tried to simplify the problem a bit, but we keep going backwards. If > the requirement is that potentially any source device can migrate to any > target device and we cannot provide any means other than writing an > opaque source string into a version attribute on the target and > evaluating the result to determine compatibility, then we're requiring > userspace to do an exhaustive search to find a potential match. That > sucks. Why is the mechanism a 'write and test' why isn't it a 'write and ask'? i.e. the destination tells the driver what type it's received from the source, and the driver replies with a set of compatible configurations (in some preferred order).

It's also not clear to me why the name has to be that opaque; I agree it's only got to be understood by the driver but that doesn't seem to be a reason for the driver to make it purposely obfuscated. I wouldn't expect a user to be able to parse it necessarily; but would expect something that would be useful for an error message.