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SubjectRe: [PATCH v2] drivers/virt: vmgenid: add vm generation id driver

On 19.11.20 13:51, 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:
>>> 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 :).

Ah right. Found it.
>> 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.

Yes. I think it is probably the best variant to check if we split this into a front end /back end or provide a new driver when we have something.
> The only piece where I'm unsure is how this will interact with CRIU. Can containers emulate ioctls and device nodes?
> Alex
> Amazon Development Center Germany GmbH
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