Re: [PATCH v1 0/3] kvm:x86: simplify kvmclock update logic

[Paolo Bonzini]

On 10/15/25 01:59, Sean Christopherson wrote:
> On Tue, Aug 19, 2025, Lei Chen wrote:
> > 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@xxxxxxxxxxxxx/20240522001817.619072-20-dwmw2@xxxxxxxxxxxxx/
>
> I would love, love, *love* to kill of this code, and the justification looks sane
> to me, but I am genuinely not knowledgeable enough in this area to judge whether
> or not this is correct/desirable going forward.

Yes, Lei Chen's reasoning are correct.  In particular for the cases that 
trigger KVM_REQ_CLOCK_UPDATE, they really should do it immediately and 
not rely on periodic updates!

Paolo