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

[Mike Rapoport <rppt@kernel.org>]

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.