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

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Message-ID: <aO7j7lcqmL-n599m@google.com>
Date: Tue, 14 Oct 2025 16:59:42 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Lei Chen <lei.chen@...rtx.com>
Cc: 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>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/3] kvm:x86: simplify kvmclock update logic

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@infradead.org/20240522001817.619072-20-dwmw2@infradead.org/

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.

Paolo?