On Tue, Mar 25, 2025, LiamNioc wrote: > On 2025/3/17 21:38, Sean Christopherson wrote: > > On Mon, Mar 17, 2025, Liam Ni wrote: > > > When using the dump-guest-memory command in QEMU to dump > > > the virtual machine's memory,the virtual machine will be > > > paused for a period of time.If the guest (i.e., UEFI) uses > > > the PIT as the system clock,it will be observed that the > > > HRTIMER used by the PIT continues to run during the guest > > > stop process, imposing an additional burden on the system. > > > Moreover, during the guest restart process,the previously > > > established HRTIMER will be canceled,and the accumulated > > > timer events will be flushed.However, before the old > > > HRTIMER is canceled,the accumulated timer events > > > will "surreptitiously" inject interrupts into the guest. > > > > > > SO during the process of saving the KVM PIT state, > > > the HRTIMER need to be canceled to reduce the performance overhead > > > caused by HRTIMER during the guest stop process. > > > > > > i.e. if guest > > > > > > Signed-off-by: Liam Ni <liamni-oc@xxxxxxxxxxx> > > > --- > > > arch/x86/kvm/x86.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > index 045c61cc7e54..75355b315aca 100644 > > > --- a/arch/x86/kvm/x86.c > > > +++ b/arch/x86/kvm/x86.c > > > @@ -6405,6 +6405,8 @@ static int kvm_vm_ioctl_get_pit(struct kvm *kvm, struct kvm_pit_state *ps) > > > > > > mutex_lock(&kps->lock); > > > memcpy(ps, &kps->channels, sizeof(*ps)); > > > + hrtimer_cancel(&kvm->arch.vpit->pit_state.timer); > > > + kthread_flush_work(&kvm->arch.vpit->expired); > > > > KVM cannot assume userspace wants to stop the PIT when grabbing a snapshot. It's > > a significant ABI change, and not desirable in all cases. > > When VM Pause, all devices of the virtual machine are frozen, so the PIT > freeze only saves the PIT device status, but does not cancel HRTIMER, but > chooses to cancel HRTIMER when VM resumes and refresh the pending task. > According to my observation, before refreshing the pending task, these > pending tasks will secretly inject interrupts into the guest. > > So do we need to cancel the HRTIMER when VM pause？ The problem is that KVM has no real concept of pausing a VM. There are a variety of hacky hooks here and there, but no KVM-wide notion of "pause". For this case, after getting PIT state, you should be able to use KVM_SET_PIT2 to set the mode of channel[0] to 0xff, which call destroy_pit_timer() via pit_load_count().
