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

[Sean Christopherson]

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().