[PATCH v8 5/7] kvm: x86: protect masterclock with a seqcount
Oliver Upton
oupton at google.com
Thu Sep 16 11:15:36 PDT 2021
From: Paolo Bonzini <pbonzini at redhat.com>
Protect the reference point for kvmclock with a seqcount, so that
kvmclock updates for all vCPUs can proceed in parallel. Xen runstate
updates will also run in parallel and not bounce the kvmclock cacheline.
nr_vcpus_matched_tsc is updated outside pvclock_update_vm_gtod_copy
though, so a spinlock must be kept for that one.
Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
[Oliver - drop unused locals, don't double acquire tsc_write_lock]
Signed-off-by: Oliver Upton <oupton at google.com>
---
arch/x86/include/asm/kvm_host.h | 7 ++-
arch/x86/kvm/x86.c | 83 +++++++++++++++++----------------
2 files changed, 49 insertions(+), 41 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 9c34b5b63e39..5accfe7246ce 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1087,6 +1087,11 @@ struct kvm_arch {
unsigned long irq_sources_bitmap;
s64 kvmclock_offset;
+
+ /*
+ * This also protects nr_vcpus_matched_tsc which is read from a
+ * preemption-disabled region, so it must be a raw spinlock.
+ */
raw_spinlock_t tsc_write_lock;
u64 last_tsc_nsec;
u64 last_tsc_write;
@@ -1097,7 +1102,7 @@ struct kvm_arch {
u64 cur_tsc_generation;
int nr_vcpus_matched_tsc;
- spinlock_t pvclock_gtod_sync_lock;
+ seqcount_raw_spinlock_t pvclock_sc;
bool use_master_clock;
u64 master_kernel_ns;
u64 master_cycle_now;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cb5d5cad5124..29156c49cd11 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2533,9 +2533,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
vcpu->arch.this_tsc_write = kvm->arch.cur_tsc_write;
kvm_vcpu_write_tsc_offset(vcpu, offset);
- raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
- spin_lock_irqsave(&kvm->arch.pvclock_gtod_sync_lock, flags);
if (!matched) {
kvm->arch.nr_vcpus_matched_tsc = 0;
} else if (!already_matched) {
@@ -2543,7 +2541,7 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
}
kvm_track_tsc_matching(vcpu);
- spin_unlock_irqrestore(&kvm->arch.pvclock_gtod_sync_lock, flags);
+ raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
}
static inline void adjust_tsc_offset_guest(struct kvm_vcpu *vcpu,
@@ -2731,9 +2729,6 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
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.
@@ -2742,6 +2737,10 @@ static void pvclock_update_vm_gtod_copy(struct kvm *kvm)
&ka->master_kernel_ns,
&ka->master_cycle_now);
+ lockdep_assert_held(&kvm->arch.tsc_write_lock);
+ vcpus_matched = (ka->nr_vcpus_matched_tsc + 1 ==
+ atomic_read(&kvm->online_vcpus));
+
ka->use_master_clock = host_tsc_clocksource && vcpus_matched
&& !ka->backwards_tsc_observed
&& !ka->boot_vcpu_runs_old_kvmclock;
@@ -2760,14 +2759,18 @@ static void kvm_make_mclock_inprogress_request(struct kvm *kvm)
kvm_make_all_cpus_request(kvm, KVM_REQ_MCLOCK_INPROGRESS);
}
-static void kvm_start_pvclock_update(struct kvm *kvm)
+static void __kvm_start_pvclock_update(struct kvm *kvm)
{
- struct kvm_arch *ka = &kvm->arch;
+ raw_spin_lock_irq(&kvm->arch.tsc_write_lock);
+ write_seqcount_begin(&kvm->arch.pvclock_sc);
+}
+static void kvm_start_pvclock_update(struct kvm *kvm)
+{
kvm_make_mclock_inprogress_request(kvm);
/* no guest entries from this point */
- spin_lock_irq(&ka->pvclock_gtod_sync_lock);
+ __kvm_start_pvclock_update(kvm);
}
static void kvm_end_pvclock_update(struct kvm *kvm)
@@ -2776,7 +2779,8 @@ static void kvm_end_pvclock_update(struct kvm *kvm)
struct kvm_vcpu *vcpu;
int i;
- spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
+ write_seqcount_end(&ka->pvclock_sc);
+ raw_spin_unlock_irq(&ka->tsc_write_lock);
kvm_for_each_vcpu(i, vcpu, kvm)
kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
@@ -2797,20 +2801,12 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
{
struct kvm_arch *ka = &kvm->arch;
struct pvclock_vcpu_time_info hv_clock;
- unsigned long flags;
- spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
if (!ka->use_master_clock) {
- spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
data->clock = get_kvmclock_base_ns() + ka->kvmclock_offset;
return;
}
- 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;
- spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
-
/* both __this_cpu_read() and rdtsc() should be on the same cpu */
get_cpu();
@@ -2825,6 +2821,9 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
#endif
data->host_tsc = rdtsc();
+ 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, __this_cpu_read(cpu_tsc_khz) * 1000LL,
&hv_clock.tsc_shift,
&hv_clock.tsc_to_system_mul);
@@ -2839,14 +2838,14 @@ static void get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
u64 get_kvmclock_ns(struct kvm *kvm)
{
struct kvm_clock_data data;
+ struct kvm_arch *ka = &kvm->arch;
+ unsigned seq;
- /*
- * Zero flags as it's accessed RMW, leave everything else uninitialized
- * as clock is always written and no other fields are consumed.
- */
- data.flags = 0;
-
- get_kvmclock(kvm, &data);
+ do {
+ seq = read_seqcount_begin(&ka->pvclock_sc);
+ data.flags = 0;
+ get_kvmclock(kvm, &data);
+ } while (read_seqcount_retry(&ka->pvclock_sc, seq));
return data.clock;
}
@@ -2912,6 +2911,7 @@ static void kvm_setup_pvclock_page(struct kvm_vcpu *v,
static int kvm_guest_time_update(struct kvm_vcpu *v)
{
unsigned long flags, tgt_tsc_khz;
+ unsigned seq;
struct kvm_vcpu_arch *vcpu = &v->arch;
struct kvm_arch *ka = &v->kvm->arch;
s64 kernel_ns;
@@ -2926,13 +2926,14 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
* If the host uses TSC clock, then passthrough TSC as stable
* to the guest.
*/
- spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
- use_master_clock = ka->use_master_clock;
- if (use_master_clock) {
- host_tsc = ka->master_cycle_now;
- kernel_ns = ka->master_kernel_ns;
- }
- spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
+ seq = read_seqcount_begin(&ka->pvclock_sc);
+ do {
+ use_master_clock = ka->use_master_clock;
+ if (use_master_clock) {
+ host_tsc = ka->master_cycle_now;
+ kernel_ns = ka->master_kernel_ns;
+ }
+ } while (read_seqcount_retry(&ka->pvclock_sc, seq));
/* Keep irq disabled to prevent changes to the clock */
local_irq_save(flags);
@@ -5855,10 +5856,15 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
static int kvm_vm_ioctl_get_clock(struct kvm *kvm, void __user *argp)
{
- struct kvm_clock_data data;
+ struct kvm_clock_data data = { 0 };
+ unsigned seq;
+
+ do {
+ seq = read_seqcount_begin(&kvm->arch.pvclock_sc);
+ data.flags = 0;
+ get_kvmclock(kvm, &data);
+ } while (read_seqcount_retry(&kvm->arch.pvclock_sc, seq));
- memset(&data, 0, sizeof(data));
- get_kvmclock(kvm, &data);
if (copy_to_user(argp, &data, sizeof(data)))
return -EFAULT;
@@ -8159,9 +8165,7 @@ static void kvm_hyperv_tsc_notifier(void)
kvm_max_guest_tsc_khz = tsc_khz;
list_for_each_entry(kvm, &vm_list, vm_list) {
- struct kvm_arch *ka = &kvm->arch;
-
- spin_lock_irq(&ka->pvclock_gtod_sync_lock);
+ __kvm_start_pvclock_update(kvm);
pvclock_update_vm_gtod_copy(kvm);
kvm_end_pvclock_update(kvm);
}
@@ -11188,8 +11192,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
raw_spin_lock_init(&kvm->arch.tsc_write_lock);
mutex_init(&kvm->arch.apic_map_lock);
- spin_lock_init(&kvm->arch.pvclock_gtod_sync_lock);
-
+ seqcount_raw_spinlock_init(&kvm->arch.pvclock_sc, &kvm->arch.tsc_write_lock);
kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
pvclock_update_vm_gtod_copy(kvm);
--
2.33.0.309.g3052b89438-goog
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