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/
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
