On Thu, Oct 04, 2018 at 03:15:32PM -0700, Andy Lutomirski wrote: > For better or for worse, I'm trying to understand this code. So far, > I've come up with this patch: > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso-tglx&id=14fd71e12b1c4492a06f368f75041f263e6862bf > > Is it correct, or am I missing some subtlety? "In the non-fallback case, a bunch of gnarly arithmetic is done: a hopefully matched pair of (TSC, boot time) is read, then all locks are dropped, then the TSC frequency is read, a branch new multiplier and shift is read, and the result is turned into a time. This seems quite racy to me. Because locks are not held, I don't see what keeps TSC frequency used in sync with the master clock data." If tsc_khz changes, the host TSC clocksource will not be used, which disables masterclock: if ((val == CPUFREQ_PRECHANGE && freq->old < freq->new) || (val == CPUFREQ_POSTCHANGE && freq->old > freq->new)) { *lpj = cpufreq_scale(loops_per_jiffy_ref, ref_freq, freq->new); tsc_khz = cpufreq_scale(tsc_khz_ref, ref_freq, freq->new); if (!(freq->flags & CPUFREQ_CONST_LOOPS)) mark_tsc_unstable("cpufreq changes"); In which case it ends up in: - spin_lock(&ka->pvclock_gtod_sync_lock); - if (!ka->use_master_clock) { - spin_unlock(&ka->pvclock_gtod_sync_lock); - return ktime_get_boot_ns() + ka->kvmclock_offset; - } masterclock -> non masterclock transition sets a REQUEST bit on each vCPU, so as to invalidate any previous clock reads. static void kvm_gen_update_masterclock(struct kvm *kvm) { #ifdef CONFIG_X86_64 int i; struct kvm_vcpu *vcpu; struct kvm_arch *ka = &kvm->arch; spin_lock(&ka->pvclock_gtod_sync_lock); kvm_make_mclock_inprogress_request(kvm); /* no guest entries from this point */ pvclock_update_vm_gtod_copy(kvm); kvm_for_each_vcpu(i, vcpu, kvm) kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); /* guest entries allowed */ kvm_for_each_vcpu(i, vcpu, kvm) kvm_clear_request(KVM_REQ_MCLOCK_INPROGRESS, vcpu); spin_unlock(&ka->pvclock_gtod_sync_lock); #endif /* * If the host uses TSC clock, then passthrough TSC as stable * to the guest. */ host_tsc_clocksource = kvm_get_time_and_clockread( &ka->master_kernel_ns, &ka->master_cycle_now); ka->use_master_clock = host_tsc_clocksource && vcpus_matched && !ka->backwards_tsc_observed && !ka->boot_vcpu_runs_old_kvmclock; _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
