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KVM: x86: require matched TSC offsets for master clock

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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 <mtosatti@redhat.com>
This commit is contained in:
Marcelo Tosatti 2012-11-27 23:29:03 -02:00
parent 42897d866b
commit b48aa97e38
3 changed files with 89 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/kvm_host.h
View File

@ -561,6 +561,7 @@ 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;

39
arch/x86/kvm/trace.h
View File

@ -764,21 +764,54 @@ 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))
);

57
arch/x86/kvm/x86.c
View File

@ -1103,12 +1103,40 @@ static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns)
return tsc;
}
void kvm_track_tsc_matching(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
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);
#endif
}
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);
@ -1151,6 +1179,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
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.
@ -1165,6 +1194,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
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);
}
@ -1188,6 +1218,15 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
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);
@ -1279,8 +1318,9 @@ static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
/*
*
* 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
@ -1313,7 +1353,7 @@ static bool kvm_get_time_and_clockread(s64 *kernel_ns, cycle_t *cycle_now)
* 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.
*
*/
@ -1322,20 +1362,27 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
#ifdef CONFIG_X86_64
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);
#endif
}