From: David Woodhouse <[email protected]> Using a count and comparing with kvm->online_vcpus was always racy because a new vCPU could be created while kvm_track_tsc_matching() was running and comparing with kvm->online_vcpus. That variable is only atomic with respect to itself; kvm_arch_vcpu_create() runs before kvm->online_vcpus is incremented for the new vCPU.
Replace the count with a boolean that is set in kvm_track_tsc_matching() after comparing the count, and cleared when a new TSC generation starts. The boolean is consumed by pvclock_update_vm_gtod_copy() under the tsc_write_lock, which serializes against __kvm_synchronize_tsc(). Keep the count for now as it's still used in the trace event. Signed-off-by: David Woodhouse <[email protected]> --- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/x86.c | 13 +++++++------ 2 files changed, 8 insertions(+), 6 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 59298a8f78eb..eb81f90284ba 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1492,6 +1492,7 @@ struct kvm_arch { u64 cur_tsc_write; u64 cur_tsc_offset; u64 cur_tsc_generation; + bool all_vcpus_matched_tsc; int nr_vcpus_matched_tsc; u32 default_tsc_khz; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 3c68d2a4c8d0..b74fd8b088ad 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -2648,11 +2648,12 @@ static void kvm_track_tsc_matching(struct kvm_vcpu *vcpu, bool new_generation) /* * To use the masterclock, the host clocksource must be based on TSC - * and all vCPUs must have matching TSCs. Note, the count for matching - * vCPUs doesn't include the reference vCPU, hence "+1". + * and all vCPUs must have matching TSCs. */ - bool use_master_clock = (ka->nr_vcpus_matched_tsc + 1 == - atomic_read(&vcpu->kvm->online_vcpus)) && + ka->all_vcpus_matched_tsc = (ka->nr_vcpus_matched_tsc + 1 == + atomic_read(&vcpu->kvm->online_vcpus)); + + bool use_master_clock = ka->all_vcpus_matched_tsc && gtod_is_based_on_tsc(gtod->clock.vclock_mode); /* @@ -2837,6 +2838,7 @@ static void __kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 offset, u64 tsc, kvm->arch.cur_tsc_write = tsc; kvm->arch.cur_tsc_offset = offset; kvm->arch.nr_vcpus_matched_tsc = 0; + kvm->arch.all_vcpus_matched_tsc = false; } else if (vcpu->arch.this_tsc_generation != kvm->arch.cur_tsc_generation) { kvm->arch.nr_vcpus_matched_tsc++; } @@ -3176,8 +3178,7 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm) bool host_tsc_clocksource, vcpus_matched; lockdep_assert_held(&kvm->arch.tsc_write_lock); - vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 == - atomic_read(&kvm->online_vcpus)); + vcpus_matched = ka->all_vcpus_matched_tsc; /* * If the host uses TSC clock, then passthrough TSC as stable -- 2.51.0
