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

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Message-ID: <ceb1a9b7-4d5c-4873-862f-d1bc63731d27@redhat.com>
Date: Tue, 4 Nov 2025 17:35:46 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Lei Chen <lei.chen@...rtx.com>
Cc: 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 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@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.

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