On Tue, Jul 28, 2026, David Woodhouse wrote: > When the clock read fails (e.g. clocksource transitioning away from TSC), > fall back to that path rather than proceeding with uninitialised data or > spinning in the seqcount loop.
This is not restructuring, it is a logical change. It's a good logical change, but it needs to be isolated. This is what I have locally (spoiler alert; I'm working backwards a bit): From: David Woodhouse <[email protected]> Date: Tue, 28 Jul 2026 15:26:35 -0700 Subject: [PATCH] KVM: x86: Fall back to non-master-clock if clockread fails in get_kvmclock() When computing kvmclock and the it's currently in master-clock mode, fall back to the non-master-clock path if the clock read fils, e.g. if the kernel's clocksource transitioning away from TSC but ka->use_master_clock hasn't been udated yet. The rdtsc() fall back was added (well, kept) in commit c68dc1b577ea ("KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK") purely to avoid uninitialized variables and compilation problems on 32-bit kernels (already addressed). In hindsight, keeping the rdtsc() was a hack and a mistake. Link: https://lore.kernel.org/all/caoq_qsgvqs_pujo8f10gg5xw+tkt+5gdx+kjf1j3cipo4ma...@mail.gmail.com Signed-off-by: David Woodhouse <[email protected]> [sean: isolate from refactoring changes, write changelog] Signed-off-by: Sean Christopherson <[email protected]> --- arch/x86/kvm/x86.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a0ed0969fcb3..895d3cf7bfcf 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1657,14 +1657,13 @@ static bool __get_kvmclock_master_clock(struct kvm *kvm, if (!tsc_hz) return false; - if (kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) { - data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec; - data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC; - } else { - data->host_tsc = rdtsc(); - } + if (!kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) + return false; + + data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec; + data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC | + KVM_CLOCK_TSC_STABLE; - data->flags |= KVM_CLOCK_TSC_STABLE; hv_clock.tsc_timestamp = ka->master_cycle_now; hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; kvm_get_time_scale(NSEC_PER_SEC, tsc_hz, base-commit: f8c5e70f5ed58dc9d39db90ecaba0ce76ac52189 --
