linux-kernel - [PATCH v1 0/3] kvm:x86: simplify kvmclock update logic

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Message-ID: <20250819152027.1687487-1-lei.chen@smartx.com>
Date: Tue, 19 Aug 2025 23:20:24 +0800
From: Lei Chen <lei.chen@...rtx.com>
To: Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Cc: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH v1 0/3] kvm:x86: simplify kvmclock update logic

This patch series simplifies kvmclock updating logic by reverting
related commits.

Now we have three requests about time updating:

1. KVM_REQ_CLOCK_UPDATE:
The function kvm_guest_time_update gathers info from  master clock
or host.rdtsc() and update vcpu->arch.hvclock, and then kvmclock or hyperv
reference counter.

2. KVM_REQ_MASTERCLOCK_UPDATE: 
The function kvm_update_masterclock updates kvm->arch from
pvclock_gtod_data(a global var updated by timekeeping subsystem), and
then make KVM_REQ_CLOCK_UPDATE request for each vcpu.

3. KVM_REQ_GLOBAL_CLOCK_UPDATE:
The function kvm_gen_kvmclock_update makes KVM_REQ_CLOCK_UPDATE
request for each vcpu.

In the early implementation, functions mentioned above were
synchronous. But things got complicated since the following commits.

1. Commit 7e44e4495a39 ("x86: kvm: rate-limit global clock updates")
intends to use kvmclock_update_work to sync ntp corretion
across all vcpus kvmclock, which is based on commit 0061d53daf26f
("KVM: x86: limit difference between kvmclock updates")

2. Commit 332967a3eac0 ("x86: kvm: introduce periodic global clock
updates") introduced a 300s-interval work to periodically sync
ntp corrections across all vcpus.

I think those commits could be reverted because:
1. Since commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
monotonic raw clock"), kvmclock switched to mono raw clock,
Those two commits could be reverted.

2. the periodic work introduced from commit 332967a3eac0 ("x86:
kvm: introduce periodic global clock updates") always does 
nothing for normal scenarios. If some exceptions happen,
the corresponding logic makes right CLOCK_UPDATE request for right vcpus.
The following shows what exceptions might happen and how they are
handled.
(1). cpu_tsc_khz changed
   __kvmclock_cpufreq_notifier makes KVM_REQ_CLOCK_UPDATE request
(2). use/unuse master clock 
   kvm_track_tsc_matching makes KVM_REQ_MASTERCLOCK_UPDATE, which means
   KVM_REQ_CLOCK_UPDATE for each vcpu.
(3). guest writes MSR_IA32_TSC
   kvm_synchronize_tsc will handle it and finally call
   kvm_track_tsc_matching to make everything well.
(4). enable/disable tsc_catchup
   kvm_arch_vcpu_load and bottom half of vcpu_enter_guest makes
   KVM_REQ_CLOCK_UPDATE request

Really happy for your comments, thanks.

Related links:
https://lkml.indiana.edu/hypermail/linux/kernel/2310.0/04217.html
https://patchew.org/linux/20240522001817.619072-1-dwmw2@infradead.org/20240522001817.619072-20-dwmw2@infradead.org/

Lei Chen (3):
  Revert "x86: kvm: introduce periodic global clock updates"
  Revert "x86: kvm: rate-limit global clock updates"
  KVM: x86: remove comment about ntp correction sync for

 arch/x86/include/asm/kvm_host.h |  2 --
 arch/x86/kvm/x86.c              | 58 +++------------------------------
 2 files changed, 5 insertions(+), 55 deletions(-)

-- 
2.44.0