On Thu, Nov 19, 2020 at 01:51:18PM +0100, Alexander Graf wrote: > > > On 19.11.20 13:02, Christian Borntraeger wrote: > > > > On 16.11.20 16:34, Catangiu, Adrian Costin wrote: > > > - Background > > > > > > The VM Generation ID is a feature defined by Microsoft (paper: > > > http://go.microsoft.com/fwlink/?LinkId=260709) and supported by > > > multiple hypervisor vendors. > > > > > > The feature is required in virtualized environments by apps that work > > > with local copies/caches of world-unique data such as random values, > > > uuids, monotonically increasing counters, etc. > > > Such apps can be negatively affected by VM snapshotting when the VM > > > is either cloned or returned to an earlier point in time. > > > > > > The VM Generation ID is a simple concept meant to alleviate the issue > > > by providing a unique ID that changes each time the VM is restored > > > from a snapshot. The hw provided UUID value can be used to > > > differentiate between VMs or different generations of the same VM. > > > > > > - Problem > > > > > > The VM Generation ID is exposed through an ACPI device by multiple > > > hypervisor vendors but neither the vendors or upstream Linux have no > > > default driver for it leaving users to fend for themselves. > > > > I see that the qemu implementation is still under discussion. What is > > Uh, the ACPI Vmgenid device emulation is in QEMU since 2.9.0 :). > > > the status of the other existing implementations. Do they already exist? > > In other words is ACPI a given? > > I think the majority of this driver could be used with just a different > > backend for platforms without ACPI so in any case we could factor out > > the backend (acpi, virtio, whatever) but if we are open we could maybe > > start with something else. > > I agree 100%. I don't think we really need a new framework in the kernel for > that. We can just have for example an s390x specific driver that also > provides the same notification mechanism through a device node that is also > named "/dev/vmgenid", no? > > Or alternatively we can split the generic part of this driver as soon as a > second one comes along and then have both driver include that generic logic. > > The only piece where I'm unsure is how this will interact with CRIU. To C/R applications that use /dev/vmgenid CRIU need to be aware of it. Checkpointing and restoring withing the same "VM generation" shouldn't be a problem, but IMHO, making restore work after genid bump could be challenging. Alex, what scenario involving CRIU did you have in mind? > Can containers emulate ioctls and device nodes? Containers do not emulate ioctls but they can have /dev/vmgenid inside the container, so applications can use it the same way as outside the container. > Alex -- Sincerely yours, Mike.
