Hello, +Cc Eric, Adrian On 11/19/20 6:36 PM, Alexander Graf wrote: > On 19.11.20 18:38, Mike Rapoport wrote: >> 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. [..] >>> 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? > > You can in theory run into the same situation with containers that this > patch is solving for virtual machines. You could for example do a > snapshot of a prewarmed Java runtime with CRIU to get full JIT speeds > starting from the first request. > > That however means you run into the problem of predictable randomness > again. > >> >>> 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. > > Hm. I suppose we could add a CAP_ADMIN ioctl interface to /dev/vmgenid > (when container people get to the point of needing it) that sets the > generation to "at least X". That way on restore, you could just call > that with "generation at snapshot"+1. > > That also means we need to have this interface available without virtual > machines then though, right? Sounds like a good idea. I guess, genvmid can be global on host, rather than per-userns or per-process for simplicity. Later if somebody will have a bottleneck on restore when every process on the machine wakes up from read() it could be virtualized, but doing it now sounds too early. ioctl() probably should go under checkpoint_restore_ns_capable(current_user_ns()), rather than CAP_SYS_ADMIN (I believe it should be safe from DOS as only CRIU should run with this capability, but worth to document this). Thanks, Dmitry
