Hi Paolo, David, Andreas, On 26/04/14 10:37, Paolo Bonzini wrote: > Il 26/04/2014 00:34, James Hogan ha scritto: >> So yes, you could technically manage without (4) by using (2) ((4) was >> implemented first), but I think it probably still has some value since >> you can >> do it with a single ioctl rather than 4 ioctls (freeze timer, read >> resume_time, read or write count, unfreeze timer). >> >> Enough value to be worthwhile? I haven't really made up my mind yet >> but I'm >> leaning towards yes. > > It would be interesting to see how the userspace patches use this > independent of COUNT_RESUME. The implementation in QEMU that I've settled upon makes do with just COUNT_CTL and COUNT_RESUME, but with a slight kernel modification so that COUNT_RESUME is writeable (to any positive monotonic nanosecond value <= now). It works fairly cleanly and correctly even with stopping and starting VM clock (gdb, stop/cont, savevm/loadvm, live migration), to match the behaviour of the existing mips cpu timer emulation, so I plan to drop this bias patch, and will post a v2 patchset soon with just a few modifications. QEMU saves the state of the KVM timer from kvm_arch_get_registers() or when the VM clock is stopped (via a vmstate notifier) - whichever comes first. It then restores the KVM timer from kvm_arch_put_registers() or when the VM clock is started - whichever comes last. Example sequence: stop VM - SAVE get regs - vm clock already stopped, not saved again start VM - regs dirty, not restored put regs - vm clock running, RESTORE Saving involves: COUNT_CTL.DC = 1 (freeze KVM timer) get CP0_Cause, CP0_Count and COUNT_RESUME store a copy of the calculated VM clock @COUNT_RESUME nanoseconds (i.e. the VM clock corresponding to the saved CP0_Count) Restoring involves: put COUNT_RESUME = now - (vm clock @now - saved vm clock) (resume occurs at the same interval into the past that the VM clock has increased since saving) put CP0_Cause, CP0_Count (the stored CP0_Count applies at that resume time) COUNT_CTL.DC = 0 (resume KVM timer from CP0_Count at COUNT_RESUME) I'll post an updated QEMU patchset ASAP after the KVM patchset, but wanted to explain how this API can actually be used. Does it sound reasonable? Thanks James
