v2: - Reworked it enough that I brought back the RFC tag and retitled the series. Also had to drop r-b's from a couple of patches, and even drop patches. - Changed approach from writing the QEMU virtual time to the guest vtime counter to saving and restoring the guest vtime counter. - Changed the kvm-adjvtime property, which was off by default, to a kvm-no-adjvtime property, which is also off by default, meaning the effective "adjust vtime" property is now on by default (but only for 5.0 virt machine types and later)
v1: - move from RFC status to v1 - put kvm_arm_vm_state_change() in kvm.c to share among kvm32.c and kvm64.c - add r-b's from Richard This series is inspired by a series[1] posted by Bijan Mottahedeh over a year ago and by the patch[2] posted by Heyi Guo almost a year ago. The problem described in the cover letter of [1] is easily reproducible and some users would like to have the option to avoid it. However the solution, which is to adjust the virtual counter each time the VM transitions to the running state, introduces a different problem, which is that the virtual and physical counters diverge. As described in the cover letter of [1] this divergence is easily observed when comparing the output of `date` and `hwclock` after suspending the guest, waiting a while, and then resuming it. Because this different problem may actually be worse for some users, unlike [1], the series posted here makes the virtual counter adjustment optional. Besides the adjustment being optional, this series approaches the needed changes differently to apply them in more appropriate locations and also integrates some of the approach posted in [2]. Additional notes ---------------- Note 1 ------ As described above, when running a guest with kvm-no-adjtime disabled it will be less likely the guest OS and guest applications get surprise time jumps when they use the virtual counter. However the counter will no longer reflect real time. It will lag behind. If this is a problem then the guest can resynchronize its time from an external source or even from its physical counter. If the suspend/resume is done with libvirt's virsh, and the guest is running the guest agent, then it's also possible to use a sequence like this $ virsh suspend $GUEST $ virsh resume $GUEST $ virsh domtime --sync $GUEST in order to resynchronize a guest right after the resume. Of course there will still be time when the clock is not right, possibly creating confusing timestamps in logs, for example, and the guest must still be tolerant to the time synchronizations. Note 2 ------ Userspace that wants to set KVM_REG_ARM_TIMER_CNT should beware that the KVM register ID is not correct. This cannot be fixed because it's UAPI and if the UAPI headers are used then it can't be a problem. However, if a userspace attempts to create the ID themselves from the register's specification, then they will get KVM_REG_ARM_TIMER_CVAL instead, as the _CNT and _CVAL definitions have their register parameters swapped. Note 3 ------ I didn't test this with a 32-bit KVM host, but the changes to kvm32.c are the same as kvm64.c. So what could go wrong? Test results would be appreciated. [1] https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05713.html [2] https://lists.gnu.org/archive/html/qemu-devel/2019-03/msg03695.html Thanks, drew Andrew Jones (5): hw: add compat machines for 5.0 target/arm/kvm64: kvm64 cpus have timer registers target/arm/kvm: Implement virtual time adjustment tests/arm-cpu-features: Check feature default values target/arm/cpu: Add the kvm-no-adjvtime CPU property docs/arm-cpu-features.rst | 31 +++++++++++++++- hw/arm/virt.c | 17 ++++++++- hw/core/machine.c | 3 ++ hw/i386/pc.c | 3 ++ hw/i386/pc_piix.c | 14 ++++++- hw/i386/pc_q35.c | 13 ++++++- hw/ppc/spapr.c | 15 +++++++- hw/s390x/s390-virtio-ccw.c | 15 +++++++- include/hw/arm/virt.h | 1 + include/hw/boards.h | 3 ++ include/hw/i386/pc.h | 3 ++ target/arm/cpu.c | 2 + target/arm/cpu.h | 9 +++++ target/arm/cpu64.c | 1 + target/arm/kvm.c | 76 ++++++++++++++++++++++++++++++++++++++ target/arm/kvm32.c | 3 ++ target/arm/kvm64.c | 4 ++ target/arm/kvm_arm.h | 34 +++++++++++++++++ target/arm/monitor.c | 1 + tests/arm-cpu-features.c | 48 +++++++++++++++++++----- 20 files changed, 280 insertions(+), 16 deletions(-) -- 2.21.0
