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

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Message-ID: <20250317091917.72477-1-liamni-oc@zhaoxin.com>
Date: Mon, 17 Mar 2025 17:19:17 +0800
From: Liam Ni <liamni-oc@...oxin.com>
To: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <tglx@...utronix.de>,
	<mingo@...hat.com>, <bp@...en8.de>, <dave.hansen@...ux.intel.com>,
	<hpa@...or.com>, <LiamNi@...oxin.com>, <CobeChen@...oxin.com>,
	<LouisQi@...oxin.com>, <EwanHai@...oxin.com>, <FrankZhu@...oxin.com>
Subject: [PATCH] KVM: x86:Cancel hrtimer in the process of saving PIT state to reduce the performance overhead caused by hrtimer during guest stop.

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@...oxin.com>
---
 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);
 	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