linux-kernel - [PATCH 2/3] KVM: x86: conditionally clear KVM_REQ_MASTERCLOCK_UPDATE at the end of KVM_SET

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Message-ID: <20260115202256.119820-3-dongli.zhang@oracle.com>
Date: Thu, 15 Jan 2026 12:22:30 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: kvm@...r.kernel.org
Cc: seanjc@...gle.com, pbonzini@...hat.com, dwmw2@...radead.org,
        dwmw@...zon.co.uk, paul@....org, tglx@...nel.org, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, linux-kernel@...r.kernel.org, joe.jin@...cle.com,
        dongli.zhang@...cle.com
Subject: [PATCH 2/3] KVM: x86: conditionally clear KVM_REQ_MASTERCLOCK_UPDATE at the end of KVM_SET_CLOCK

The KVM_SET_CLOCK command calls pvclock_update_vm_gtod_copy() to update the
masterclock data.

Many vCPUs may already have KVM_REQ_MASTERCLOCK_UPDATE pending before
KVM_SET_CLOCK is invoked. If pvclock_update_vm_gtod_copy() decides to use
the masterclock, there is no need to update the masterclock multiple times
afterward. As noted in commit c52ffadc65e2 ("KVM: x86: Don't unnecessarily
force masterclock update on vCPU hotplug"), each unnecessary
KVM_REQ_MASTERCLOCK_UPDATE can cause the kvm-clock time to jump.

Therefore, clear KVM_REQ_MASTERCLOCK_UPDATE for each vCPU at the end of
KVM_SET_CLOCK when the master clock is active. The 'tsc_write_lock' ensures
that only requests issued before KVM_SET_CLOCK are cleared.

Cc: David Woodhouse <dwmw@...zon.co.uk>
Signed-off-by: Dongli Zhang <dongli.zhang@...cle.com>
---
 arch/x86/kvm/x86.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5e7418cfd0af..0599949a7803 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7173,6 +7173,8 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
 {
 	struct kvm_arch *ka = &kvm->arch;
 	struct kvm_clock_data data;
+	struct kvm_vcpu *vcpu;
+	unsigned long i;
 	u64 now_raw_ns;

 	if (copy_from_user(&data, argp, sizeof(data)))
@@ -7211,6 +7213,12 @@ static int kvm_vm_ioctl_set_clock(struct kvm *kvm, void __user *argp)
 	else
 		now_raw_ns = get_kvmclock_base_ns();
 	ka->kvmclock_offset = data.clock - now_raw_ns;
+
+	if (kvm->arch.use_master_clock) {
+		kvm_for_each_vcpu(i, vcpu, kvm)
+			kvm_clear_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
+	}
+
 	kvm_end_pvclock_update(kvm);
 	return 0;
 }
-- 
2.39.3