With master clock, a pvclock clock read calculates:
ret = system_timestamp + [ (rdtsc + tsc_offset) - tsc_timestamp ]
Where 'rdtsc' is the host TSC.
system_timestamp and tsc_timestamp are unique, one tuple
per VM: the "master clock".
Given a host with synchronized TSCs, its obvious that
guest TSC must be matched for the above to guarantee monotonicity.
Allow master clock usage only if guest TSCs are synchronized.
Signed-off-by: Marcelo Tosatti <mtosa...@redhat.com>
Index: vsyscall/arch/x86/include/asm/kvm_host.h
===================================================================
--- vsyscall.orig/arch/x86/include/asm/kvm_host.h
+++ vsyscall/arch/x86/include/asm/kvm_host.h
@@ -563,6 +563,7 @@ struct kvm_arch {
u64 cur_tsc_write;
u64 cur_tsc_offset;
u8 cur_tsc_generation;
+ int nr_vcpus_matched_tsc;
spinlock_t pvclock_gtod_sync_lock;
bool use_master_clock;
Index: vsyscall/arch/x86/kvm/x86.c
===================================================================
--- vsyscall.orig/arch/x86/kvm/x86.c
+++ vsyscall/arch/x86/kvm/x86.c
@@ -1047,12 +1047,38 @@ static u64 compute_guest_tsc(struct kvm_
return tsc;
}
+void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
+{
+ bool vcpus_matched;
+ bool do_request = false;
+ struct kvm_arch *ka = &vcpu->kvm->arch;
+ struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
+
+ vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
+ atomic_read(&vcpu->kvm->online_vcpus));
+
+ if (vcpus_matched && gtod->clock.vclock_mode == VCLOCK_TSC)
+ if (!ka->use_master_clock)
+ do_request = 1;
+
+ if (!vcpus_matched && ka->use_master_clock)
+ do_request = 1;
+
+ if (do_request)
+ kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
+
+ trace_kvm_track_tsc(vcpu->vcpu_id, ka->nr_vcpus_matched_tsc,
+ atomic_read(&vcpu->kvm->online_vcpus),
+ ka->use_master_clock, gtod->clock.vclock_mode);
+}
+
void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
{
struct kvm *kvm = vcpu->kvm;
u64 offset, ns, elapsed;
unsigned long flags;
s64 usdiff;
+ bool matched;
raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
offset = kvm_x86_ops->compute_tsc_offset(vcpu, data);
@@ -1095,6 +1121,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu
offset = kvm_x86_ops->compute_tsc_offset(vcpu, data);
pr_debug("kvm: adjusted tsc offset by %llu\n", delta);
}
+ matched = true;
} else {
/*
* We split periods of matched TSC writes into generations.
@@ -1109,6 +1136,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu
kvm->arch.cur_tsc_nsec = ns;
kvm->arch.cur_tsc_write = data;
kvm->arch.cur_tsc_offset = offset;
+ matched = false;
pr_debug("kvm: new tsc generation %u, clock %llu\n",
kvm->arch.cur_tsc_generation, data);
}
@@ -1132,6 +1160,15 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu
kvm_x86_ops->write_tsc_offset(vcpu, offset);
raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
+
+ spin_lock(&kvm->arch.pvclock_gtod_sync_lock);
+ if (matched)
+ kvm->arch.nr_vcpus_matched_tsc++;
+ else
+ kvm->arch.nr_vcpus_matched_tsc = 0;
+
+ kvm_track_tsc_matching(vcpu);
+ spin_unlock(&kvm->arch.pvclock_gtod_sync_lock);
}
EXPORT_SYMBOL_GPL(kvm_write_tsc);
@@ -1222,8 +1259,9 @@ static atomic_t kvm_guest_has_master_clo
/*
*
- * Assuming a stable TSC across physical CPUS, the following condition
- * is possible. Each numbered line represents an event visible to both
+ * Assuming a stable TSC across physical CPUS, and a stable TSC
+ * across virtual CPUs, the following condition is possible.
+ * Each numbered line represents an event visible to both
* CPUs at the next numbered event.
*
* "timespecX" represents host monotonic time. "tscX" represents
@@ -1256,7 +1294,7 @@ static atomic_t kvm_guest_has_master_clo
* copy of host monotonic time values. Update that master copy
* in lockstep.
*
- * Rely on synchronization of host TSCs for monotonicity.
+ * Rely on synchronization of host TSCs and guest TSCs for monotonicity.
*
*/
@@ -1264,19 +1302,26 @@ static void pvclock_update_vm_gtod_copy(
{
struct kvm_arch *ka = &kvm->arch;
int vclock_mode;
+ bool host_tsc_clocksource, vcpus_matched;
+ vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
+ atomic_read(&kvm->online_vcpus));
/*
* If the host uses TSC clock, then passthrough TSC as stable
* to the guest.
*/
- ka->use_master_clock = kvm_get_time_and_clockread(
+ 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;
+
if (ka->use_master_clock)
atomic_set(&kvm_guest_has_master_clock, 1);
vclock_mode = pvclock_gtod_data.clock.vclock_mode;
- trace_kvm_update_master_clock(ka->use_master_clock, vclock_mode);
+ trace_kvm_update_master_clock(ka->use_master_clock, vclock_mode,
+ vcpus_matched);
}
static void kvm_write_pvtime(struct kvm_vcpu *v, struct page *page,
Index: vsyscall/arch/x86/kvm/trace.h
===================================================================
--- vsyscall.orig/arch/x86/kvm/trace.h
+++ vsyscall/arch/x86/kvm/trace.h
@@ -762,21 +762,54 @@ TRACE_EVENT(
{VCLOCK_HPET, "hpet"} \
TRACE_EVENT(kvm_update_master_clock,
- TP_PROTO(bool use_master_clock, unsigned int host_clock),
- TP_ARGS(use_master_clock, host_clock),
+ TP_PROTO(bool use_master_clock, unsigned int host_clock, bool
offset_matched),
+ TP_ARGS(use_master_clock, host_clock, offset_matched),
TP_STRUCT__entry(
__field( bool, use_master_clock )
__field( unsigned int, host_clock )
+ __field( bool, offset_matched )
),
TP_fast_assign(
__entry->use_master_clock = use_master_clock;
__entry->host_clock = host_clock;
+ __entry->offset_matched = offset_matched;
),
- TP_printk("masterclock %d hostclock %s",
+ TP_printk("masterclock %d hostclock %s offsetmatched %u",
__entry->use_master_clock,
+ __print_symbolic(__entry->host_clock, host_clocks),
+ __entry->offset_matched)
+);
+
+TRACE_EVENT(kvm_track_tsc,
+ TP_PROTO(unsigned int vcpu_id, unsigned int nr_matched,
+ unsigned int online_vcpus, bool use_master_clock,
+ unsigned int host_clock),
+ TP_ARGS(vcpu_id, nr_matched, online_vcpus, use_master_clock,
+ host_clock),
+
+ TP_STRUCT__entry(
+ __field( unsigned int, vcpu_id )
+ __field( unsigned int, nr_vcpus_matched_tsc )
+ __field( unsigned int, online_vcpus )
+ __field( bool, use_master_clock )
+ __field( unsigned int, host_clock )
+ ),
+
+ TP_fast_assign(
+ __entry->vcpu_id = vcpu_id;
+ __entry->nr_vcpus_matched_tsc = nr_matched;
+ __entry->online_vcpus = online_vcpus;
+ __entry->use_master_clock = use_master_clock;
+ __entry->host_clock = host_clock;
+ ),
+
+ TP_printk("vcpu_id %u masterclock %u offsetmatched %u nr_online %u"
+ " hostclock %s",
+ __entry->vcpu_id, __entry->use_master_clock,
+ __entry->nr_vcpus_matched_tsc, __entry->online_vcpus,
__print_symbolic(__entry->host_clock, host_clocks))
);
--
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