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

Vitaly Kuznetsov
2020-01-23T13:44:06Z

Paolo Bonzini <pbonzini@redhat.com> writes:

> 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

But we could've still used the RAW+offs_boot version, right? And this is
just to basically preserve the existing behavior on !x86.

>
> 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);

There are not that many hosts with more than 50 years uptime and likely
none running Linux with live kernel patching support so I bet noone will
ever see this overflowing, however, as wall_nsec is u64 and we're
dealing with kvmclock here I'd suggest to add a WARN_ON().

>
> - 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;

This looks correct to me but kvmclock is a glorious beast so take this
with a grain of salt)

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly