[PATCH v2 01/12] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK

Oliver Upton
2021-07-16T21:26:44Z

Handling the migration of TSCs correctly is difficult, in part because
Linux does not provide userspace with the ability to retrieve a (TSC,
realtime) clock pair for a single instant in time. In lieu of a more
convenient facility, KVM can report similar information in the kvm_clock
structure.

Provide userspace with a host TSC & realtime pair iff the realtime clock
is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
realtime value, advance the KVM clock by the amount of elapsed time. Do
not step the KVM clock backwards, though, as it is a monotonic
oscillator.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Oliver Upton <oupton@google.com>
---
Documentation/virt/kvm/api.rst | 42 +++++++--
arch/x86/include/asm/kvm_host.h | 3 +
arch/x86/kvm/x86.c | 149 ++++++++++++++++++++------------
include/uapi/linux/kvm.h | 7 +-
4 files changed, 137 insertions(+), 64 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index b9ddce5638f5..26bb01a6e82e 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -993,20 +993,34 @@ such as migration.
When KVM_CAP_ADJUST_CLOCK is passed to KVM_CHECK_EXTENSION, it returns the
set of bits that KVM can return in struct kvm_clock_data's flag member.

-The only flag defined now is KVM_CLOCK_TSC_STABLE. If set, the returned
-value is the exact kvmclock value seen by all VCPUs at the instant
-when KVM_GET_CLOCK was called. If clear, the returned value is simply
-CLOCK_MONOTONIC plus a constant offset; the offset can be modified
-with KVM_SET_CLOCK. KVM will try to make all VCPUs follow this clock,
-but the exact value read by each VCPU could differ, because the host
-TSC is not stable.
+FLAGS:
+
+KVM_CLOCK_TSC_STABLE. If set, the returned value is the exact kvmclock
+value seen by all VCPUs at the instant when KVM_GET_CLOCK was called.
+If clear, the returned value is simply CLOCK_MONOTONIC plus a constant
+offset; the offset can be modified with KVM_SET_CLOCK. KVM will try
+to make all VCPUs follow this clock, but the exact value read by each
+VCPU could differ, because the host TSC is not stable.
+
+KVM_CLOCK_REAL_TIME. If set, the `realtime` field in the kvm_clock_data
+structure is populated with the value of the host's real time
+clocksource at the instant when KVM_GET_CLOCK was called. If clear,
+the `realtime` field does not contain a value.
+
+KVM_CLOCK_HOST_TSC. If set, the `host_tsc` field in the kvm_clock_data
+structure is populated with the value of the host's timestamp counter (TSC)
+at the instant when KVM_GET_CLOCK was called. If clear, the `host_tsc` field
+does not contain a value.

::

struct kvm_clock_data {
__u64 clock; /* kvmclock current value */
__u32 flags;
- __u32 pad[9];
+ __u32 pad0;
+ __u64 realtime;
+ __u64 host_tsc;
+ __u32 pad[4];
};

@@ -1023,12 +1037,22 @@ Sets the current timestamp of kvmclock to the value specified in its parameter.
In conjunction with KVM_GET_CLOCK, it is used to ensure monotonicity on scenarios
such as migration.

+FLAGS:
+
+KVM_CLOCK_REAL_TIME. If set, KVM will compare the value of the `realtime` field
+with the value of the host's real time clocksource at the instant when
+KVM_SET_CLOCK was called. The difference in elapsed time is added to the final
+kvmclock value that will be provided to guests.
+
::

struct kvm_clock_data {
__u64 clock; /* kvmclock current value */
__u32 flags;
- __u32 pad[9];
+ __u32 pad0;
+ __u64 realtime;
+ __u64 host_tsc;
+ __u32 pad[4];
};

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 974cbfb1eefe..e527d7259415 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1936,4 +1936,7 @@ int kvm_cpu_dirty_log_size(void);

int alloc_all_memslots_rmaps(struct kvm *kvm);

+#define KVM_CLOCK_VALID_FLAGS \
+ (KVM_CLOCK_TSC_STABLE | KVM_CLOCK_REAL_TIME | KVM_CLOCK_HOST_TSC)
+
#endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d715ae9f9108..4e803632cdca 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2780,17 +2780,24 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
#endif
}

-u64 get_kvmclock_ns(struct kvm *kvm)
+/**
+ * Returns true if realtime and TSC values were written back to the caller.
+ * Returns false if a clock triplet cannot be obtained, such as if the host's
+ * realtime clock is not based on the TSC.
+ */
+static bool get_kvmclock_and_realtime(struct kvm *kvm, u64 *kvmclock_ns,
+ u64 *realtime_ns, u64 *tsc)
{
struct kvm_arch *ka = &kvm->arch;
struct pvclock_vcpu_time_info hv_clock;
unsigned long flags;
- u64 ret;
+ bool ret = false;

spin_lock_irqsave(&ka->pvclock_gtod_sync_lock, flags);
if (!ka->use_master_clock) {
spin_unlock_irqrestore(&ka->pvclock_gtod_sync_lock, flags);
- return get_kvmclock_base_ns() + ka->kvmclock_offset;
+ *kvmclock_ns = get_kvmclock_base_ns() + ka->kvmclock_offset;
+ return false;
}

hv_clock.tsc_timestamp = ka->master_cycle_now;
@@ -2801,18 +2808,36 @@ u64 get_kvmclock_ns(struct kvm *kvm)
get_cpu();

if (__this_cpu_read(cpu_tsc_khz)) {
+ struct timespec64 ts;
+ u64 tsc_val;
+
kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
&hv_clock.tsc_shift,
&hv_clock.tsc_to_system_mul);
- ret = __pvclock_read_cycles(&hv_clock, rdtsc());
+
+ if (kvm_get_walltime_and_clockread(&ts, &tsc_val)) {
+ *realtime_ns = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec;
+ *tsc = tsc_val;
+ ret = true;
+ }
+
+ *kvmclock_ns = __pvclock_read_cycles(&hv_clock, tsc_val);
} else
- ret = get_kvmclock_base_ns() + ka->kvmclock_offset;
+ *kvmclock_ns = get_kvmclock_base_ns() + ka->kvmclock_offset;

put_cpu();

return ret;
}

+u64 get_kvmclock_ns(struct kvm *kvm)
+{
+ u64 kvmclock_ns, realtime_ns, tsc;
+
+ get_kvmclock_and_realtime(kvm, &kvmclock_ns, &realtime_ns, &tsc);
+ return kvmclock_ns;
+}
+
static void kvm_setup_pvclock_page(struct kvm_vcpu *v,
struct gfn_to_hva_cache *cache,
unsigned int offset)
@@ -4031,7 +4056,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
r = KVM_SYNC_X86_VALID_FIELDS;
break;
case KVM_CAP_ADJUST_CLOCK:
- r = KVM_CLOCK_TSC_STABLE;
+ r = KVM_CLOCK_VALID_FLAGS;
break;
case KVM_CAP_X86_DISABLE_EXITS:
r |= KVM_X86_DISABLE_EXITS_HLT | KVM_X86_DISABLE_EXITS_PAUSE |
@@ -5806,6 +5831,68 @@ int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state)
}
#endif /* CONFIG_HAVE_KVM_PM_NOTIFIER */

+static int kvm_vm_ioctl_get_clock(struct kvm *kvm,
+ void __user *argp)
+{
+ struct kvm_clock_data data;
+
+ memset(&data, 0, sizeof(data));
+
+ if (get_kvmclock_and_realtime(kvm, &data.clock, &data.realtime,
+ &data.host_tsc))
+ data.flags |= KVM_CLOCK_REAL_TIME | KVM_CLOCK_HOST_TSC;
+
+ if (kvm->arch.use_master_clock)
+ data.flags |= KVM_CLOCK_TSC_STABLE;
+
+ if (copy_to_user(argp, &data, sizeof(data)))
+ return -EFAULT;
+
+ return 0;
+}
+
+static int kvm_vm_ioctl_set_clock(struct kvm *kvm,
+ void __user *argp)
+{
+ struct kvm_arch *ka = &kvm->arch;
+ struct kvm_clock_data data;
+ u64 now_raw_ns;
+
+ if (copy_from_user(&data, argp, sizeof(data)))
+ return -EFAULT;
+
+ if (data.flags & ~KVM_CLOCK_REAL_TIME)
+ return -EINVAL;
+
+ /*
+ * TODO: userspace has to take care of races with VCPU_RUN, so
+ * kvm_gen_update_masterclock() can be cut down to locked
+ * pvclock_update_vm_gtod_copy().
+ */
+ kvm_gen_update_masterclock(kvm);
+
+ spin_lock_irq(&ka->pvclock_gtod_sync_lock);
+ if (data.flags & KVM_CLOCK_REAL_TIME) {
+ u64 now_real_ns = ktime_get_real_ns();
+
+ /*
+ * Avoid stepping the kvmclock backwards.
+ */
+ if (now_real_ns > data.realtime)
+ data.clock += now_real_ns - data.realtime;
+ }
+
+ if (ka->use_master_clock)
+ now_raw_ns = ka->master_kernel_ns;
+ else
+ now_raw_ns = get_kvmclock_base_ns();
+ ka->kvmclock_offset = data.clock - now_raw_ns;
+ spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
+
+ kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
+ return 0;
+}
+
long kvm_arch_vm_ioctl(struct file *filp,
unsigned int ioctl, unsigned long arg)
{
@@ -6050,57 +6137,11 @@ long kvm_arch_vm_ioctl(struct file *filp,
}
#endif
case KVM_SET_CLOCK: {
- struct kvm_arch *ka = &kvm->arch;
- struct kvm_clock_data user_ns;
- u64 now_ns;
-
- r = -EFAULT;
- if (copy_from_user(&user_ns, argp, sizeof(user_ns)))
- goto out;
-
- r = -EINVAL;
- if (user_ns.flags)
- goto out;
-
- r = 0;
- /*
- * TODO: userspace has to take care of races with VCPU_RUN, so
- * kvm_gen_update_masterclock() can be cut down to locked
- * pvclock_update_vm_gtod_copy().
- */
- kvm_gen_update_masterclock(kvm);
-
- /*
- * This pairs with kvm_guest_time_update(): when masterclock is
- * in use, we use master_kernel_ns + kvmclock_offset to set
- * unsigned 'system_time' so if we use get_kvmclock_ns() (which
- * is slightly ahead) here we risk going negative on unsigned
- * 'system_time' when 'user_ns.clock' is very small.
- */
- spin_lock_irq(&ka->pvclock_gtod_sync_lock);
- if (kvm->arch.use_master_clock)
- now_ns = ka->master_kernel_ns;
- else
- now_ns = get_kvmclock_base_ns();
- ka->kvmclock_offset = user_ns.clock - now_ns;
- spin_unlock_irq(&ka->pvclock_gtod_sync_lock);
-
- kvm_make_all_cpus_request(kvm, KVM_REQ_CLOCK_UPDATE);
+ r = kvm_vm_ioctl_set_clock(kvm, argp);
break;
}
case KVM_GET_CLOCK: {
- struct kvm_clock_data user_ns;
- u64 now_ns;
-
- now_ns = get_kvmclock_ns(kvm);
- user_ns.clock = now_ns;
- user_ns.flags = kvm->arch.use_master_clock ? KVM_CLOCK_TSC_STABLE : 0;
- memset(&user_ns.pad, 0, sizeof(user_ns.pad));
-
- r = -EFAULT;
- if (copy_to_user(argp, &user_ns, sizeof(user_ns)))
- goto out;
- r = 0;
+ r = kvm_vm_ioctl_get_clock(kvm, argp);
break;
}
case KVM_MEMORY_ENCRYPT_OP: {
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d9e4aabcb31a..f2da6760b606 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1223,11 +1223,16 @@ struct kvm_irqfd {

/* Do not use 1, KVM_CHECK_EXTENSION returned it before we had flags. */
#define KVM_CLOCK_TSC_STABLE 2
+#define KVM_CLOCK_REAL_TIME (1 << 2)
+#define KVM_CLOCK_HOST_TSC (1 << 3)

struct kvm_clock_data {
__u64 clock;
__u32 flags;
- __u32 pad[9];
+ __u32 pad0;
+ __u64 realtime;
+ __u64 host_tsc;
+ __u32 pad[4];
};

/* For KVM_CAP_SW_TLB */
--
2.32.0.402.g57bb445576-goog