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According to the kernel code*1, the virtual time is related to physical timer and cntvoff. When the simulator goes from KVM to gem5, the physical timer is still ticking. After gem5 simulating models and going back to KVM, the virtual time also goes away. We should update cntvoff by ourselve to get correct virtual time. Moreover, according to update_vtimer_cntvoff*2, setting cntvoff affacts all vcpus. Instead of puting individual vtime on BaseArmKvmCPU, we maintain a global vtime, restore it before the first vcpu going into KVM, and save it after the last vcpu back from KVM. 1. https://code.woboq.org/linux/linux/virt/kvm/arm/arch_timer.c.html#826 2. https://code.woboq.org/linux/linux/virt/kvm/arm/arch_timer.c.html#update_vtimer_cntvoff Change-Id: Ie054104642f2a6d5a0740f39b947f5f2c29c36f3 Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42161 Reviewed-by: Earl Ou <shunhsingou@google.com> Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com> Maintainer: Andreas Sandberg <andreas.sandberg@arm.com> Tested-by: kokoro <noreply+kokoro@google.com>