[PATCH 2/2] KVM: x86: use raw clock values consistently

Paolo Bonzini
2020-01-22T14:22:45Z

Commit 53fafdbb8b21f ("KVM: x86: switch KVMCLOCK base to monotonic raw
clock") changed kvmclock to use tkr_raw instead of tkr_mono. However,
the default kvmclock_offset for the VM was still based on the monotonic
clock and, if the raw clock drifted enough from the monotonic clock,
this could cause a negative system_time to be written to the guest's
struct pvclock. RHEL5 does not like it and (if it boots fast enough to
observe a negative time value) it hangs.

There is another thing to be careful about: getboottime64 returns the
host boot time in tkr_mono units, and subtracting tkr_raw units will
cause the wallclock to be off if tkr_raw drifts from tkr_mono. To
avoid this, compute the wallclock delta from the current time instead
of being clever and using getboottime64.

Fixes: 53fafdbb8b21f ("KVM: x86: switch KVMCLOCK base to monotonic raw clock")
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
arch/x86/kvm/x86.c | 38 +++++++++++++++++++++++---------------
1 file changed, 23 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1b4273cce63c..b5e0648580e1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1577,6 +1577,18 @@ static void update_pvclock_gtod(struct timekeeper *tk)

write_seqcount_end(&vdata->seq);
}
+
+static s64 get_kvmclock_base_ns(void)
+{
+ /* Count up from boot time, but with the frequency of the raw clock. */
+ return ktime_to_ns(ktime_add(ktime_get_raw(), pvclock_gtod_data.offs_boot));
+}
+#else
+static s64 get_kvmclock_base_ns(void)
+{
+ /* Master clock not used, so we can just use CLOCK_BOOTTIME. */
+ return ktime_get_boottime_ns();
+}
#endif

void kvm_set_pending_timer(struct kvm_vcpu *vcpu)
@@ -1590,7 +1602,7 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
int version;
int r;
struct pvclock_wall_clock wc;
- struct timespec64 boot;
+ u64 wall_nsec;

if (!wall_clock)
return;
@@ -1610,17 +1622,12 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
/*
* The guest calculates current wall clock time by adding
* system time (updated by kvm_guest_time_update below) to the
- * wall clock specified here. guest system time equals host
- * system time for us, thus we must fill in host boot time here.
+ * wall clock specified here. We do the reverse here.
*/
- getboottime64(&boot);
+ wall_nsec = ktime_get_real_ns() - get_kvmclock_ns(kvm);

- if (kvm->arch.kvmclock_offset) {
- struct timespec64 ts = ns_to_timespec64(kvm->arch.kvmclock_offset);
- boot = timespec64_sub(boot, ts);
- }
- wc.sec = (u32)boot.tv_sec; /* overflow in 2106 guest time */
- wc.nsec = boot.tv_nsec;
+ wc.nsec = do_div(wall_nsec, 1000000000);
+ wc.sec = (u32)wall_nsec; /* overflow in 2106 guest time */
wc.version = version;

kvm_write_guest(kvm, wall_clock, &wc, sizeof(wc));
@@ -1868,7 +1875,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)

raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
offset = kvm_compute_tsc_offset(vcpu, data);
- ns = ktime_get_boottime_ns();
+ ns = get_kvmclock_base_ns();
elapsed = ns - kvm->arch.last_tsc_nsec;

if (vcpu->arch.virtual_tsc_khz) {
@@ -2206,7 +2213,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
spin_lock(&ka->pvclock_gtod_sync_lock);
if (!ka->use_master_clock) {
spin_unlock(&ka->pvclock_gtod_sync_lock);
- return ktime_get_boottime_ns() + ka->kvmclock_offset;
+ return get_kvmclock_base_ns() + ka->kvmclock_offset;
}

hv_clock.tsc_timestamp = ka->master_cycle_now;
@@ -2222,7 +2229,7 @@ u64 get_kvmclock_ns(struct kvm *kvm)
&hv_clock.tsc_to_system_mul);
ret = __pvclock_read_cycles(&hv_clock, rdtsc());
} else
- ret = ktime_get_boottime_ns() + ka->kvmclock_offset;
+ ret = get_kvmclock_base_ns() + ka->kvmclock_offset;

put_cpu();

@@ -2321,7 +2328,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
}
if (!use_master_clock) {
host_tsc = rdtsc();
- kernel_ns = ktime_get_boottime_ns();
+ kernel_ns = get_kvmclock_base_ns();
}

tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
@@ -2361,6 +2368,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
vcpu->last_guest_tsc = tsc_timestamp;
+ WARN_ON(vcpu->hv_clock.system_time < 0);

/* If the host uses TSC clocksource, then it is stable */
pvclock_flags = 0;
@@ -9473,7 +9481,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
mutex_init(&kvm->arch.apic_map_lock);
spin_lock_init(&kvm->arch.pvclock_gtod_sync_lock);

- kvm->arch.kvmclock_offset = -ktime_get_boottime_ns();
+ kvm->arch.kvmclock_offset = -get_kvmclock_base_ns();
pvclock_update_vm_gtod_copy(kvm);

kvm->arch.guest_can_read_msr_platform_info = true;
--
1.8.3.1