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 2025/4/4 22:48, Sean Christopherson wrote:
> [这封邮件来自外部发件人 谨防风险]
>
> 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().

Got it.
so i need to modify the behavior of QEMU's code.