Hi Heyi, On 26/03/2019 13:53, Heyi Guo wrote: > I also tested save/restore operations, and observed that clock in guest > would not jump after restoring either. If we consider guest clock not > being synchronized with real wall clock as an issue, does it mean > save/restore function has the same issue? Basically at the moment when the guest isn't running you have a choice of two behaviours: 1. Stop (i.e. save/restore) CNTVCT - this means that the guest sees no time occur. If the guest needs to have a concept of wall-clock time (e.g. it communicates with other systems over a network) then this can cause problems (e.g. timeouts might be wrong, certificates might start appearing to be in the future etc). 2. Leave CNTVCT running - the guest sees the time pass but interprets the vCPUs as effectively having locked up. Linux will trigger the soft lockup warning. There are two ways of solving this, which match the two behaviours above: 1. Provide the guest with a view of wall-clock time. The obvious way of doing this is with a pvclock implementation like MSR_KVM_WALL_CLOCK_NEW for x86. 2. Inform the guest to ignore the apparent "soft-lockup". There's already an ioctl for x86 for this: KVM_KVMCLOCK_CTRL My preference is for option 1 - as this gives the guest a good view of both the time that it is actually executing (useful for internal watchdog timers like the soft-lockup one in Linux) and maintains a view of wall-clock time (useful when communicating with other external services - e.g. the a server on the internet). Your patch to QEMU provides the first step of that, but as you mention there's much more to do. One thing I haven't investigated in great detail is how KVM handles the timer during various forms of suspend. In particular for suspend types like full hibernation the host's physical counter will jump (quite possibly backwards) - I haven't looked in detail how KVM presents this to the guest. Hopefully not by making it go backwards! I'm not sure how much time I'm going to have to look at this in the near future, but please keep me in the loop if you decide to tackle any of this. Thanks, Steve _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
