Re: [PATCH] KVM: x86:Cancel hrtimer in the process of saving PIT state to reduce the performance overhead caused by hrtimer during guest stop.

[LiamNioc]

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？

> >        mutex_unlock(&kps->lock);
> >        return 0;
> >   }
> > @@ -6428,6 +6430,8 @@ static int kvm_vm_ioctl_get_pit2(struct kvm *kvm, struct kvm_pit_state2 *ps)
> >        memcpy(ps->channels, &kvm->arch.vpit->pit_state.channels,
> >                sizeof(ps->channels));
> >        ps->flags = kvm->arch.vpit->pit_state.flags;
> > +     hrtimer_cancel(&kvm->arch.vpit->pit_state.timer);
> > +     kthread_flush_work(&kvm->arch.vpit->expired);
> >        mutex_unlock(&kvm->arch.vpit->pit_state.lock);
> >        memset(&ps->reserved, 0, sizeof(ps->reserved));
> >        return 0;
> > --
> > 2.25.1