[PATCH v3] KVM: x86/xen: improve accuracy of Xen timers

David Woodhouse
2023-12-14T16:54:53Z

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From: David Woodhouse <dwmw@amazon.co.uk>

A test program such as http://david.woodhou.se/timerlat.c confirms user
reports that timers are increasingly inaccurate as the lifetime of a
guest increases. Reporting the actual delay observed when asking for
100µs of sleep, it starts off OK on a newly-launched guest but gets
worse over time, giving incorrect sleep times:

root@ip-10-0-193-21:~# ./timerlat -c -n 5
00000000 latency 103243/100000 (3.2430%)
00000001 latency 103243/100000 (3.2430%)
00000002 latency 103242/100000 (3.2420%)
00000003 latency 103245/100000 (3.2450%)
00000004 latency 103245/100000 (3.2450%)

The biggest problem is that get_kvmclock_ns() returns inaccurate values
when the guest TSC is scaled. The guest sees a TSC value scaled from the
host TSC by a mul/shift conversion (hopefully done in hardware). The
guest then converts that guest TSC value into nanoseconds using the
mul/shift conversion given to it by the KVM pvclock information.

But get_kvmclock_ns() performs only a single conversion directly from
host TSC to nanoseconds, giving a different result. A test program at
http://david.woodhou.se/tsdrift.c demonstrates the cumulative error
over a day.

It's non-trivial to fix get_kvmclock_ns(), although I'll come back to
that. The actual guest hv_clock is per-CPU, and *theoretically* each
vCPU could be running at a *different* frequency. But this patch is
needed anyway because...

The other issue with Xen timers was that the code would snapshot the
host CLOCK_MONOTONIC at some point in time, and then... after a few
interrupts may have occurred, some preemption perhaps... would also read
the guest's kvmclock. Then it would proceed under the false assumption
that those two happened at the *same* time. Any time which *actually*
elapsed between reading the two clocks was introduced as inaccuracies
in the time at which the timer fired.

Fix it to use a variant of kvm_get_time_and_clockread(), which reads the
host TSC just *once*, then use the returned TSC value to calculate the
kvmclock (making sure to do that the way the guest would instead of
making the same mistake get_kvmclock_ns() does).

Sadly, hrtimers based on CLOCK_MONOTONIC_RAW are not supported, so Xen
timers still have to use CLOCK_MONOTONIC. In practice the difference
between the two won't matter over the timescales involved, as the
*absolute* values don't matter; just the delta.

This does mean a new variant of kvm_get_time_and_clockread() is needed;
called kvm_get_monotonic_and_clockread() because that's what it does.

Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
v3:
• Rebase and repost.

v2:
• Fall back to get_kvmclock_ns() if vcpu-arch.hv_clock isn't set up
yet, with a big comment explaining why that's actually OK.
• Fix do_monotonic() *not* to add the boot time offset.
• Rename do_monotonic_raw() → do_kvmclock_base() and add a comment
to make it clear that it *does* add the boot time offset. That
was just left as a bear trap for the unwary developer, wasn't it?

arch/x86/kvm/x86.c |  61 +++++++++++++++++++++--
arch/x86/kvm/x86.h |   1 +
arch/x86/kvm/xen.c | 121 ++++++++++++++++++++++++++++++++++-----------
3 files changed, 149 insertions(+), 34 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6beb6ceb28c1..8faf1bf9e8e3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2852,7 +2852,11 @@ static inline u64 vgettsc(struct pvclock_clock *clock, u64 *tsc_timestamp,
return v * clock->mult;
}

-static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
+/*
+ * As with get_kvmclock_base_ns(), this counts from boot time, at the
+ * frequency of CLOCK_MONOTONIC_RAW (hence adding gtos->offs_boot).
+ */
+static int do_kvmclock_base(s64 *t, u64 *tsc_timestamp)
{
struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
unsigned long seq;
@@ -2871,6 +2875,29 @@ static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
return mode;
}

+/*
+ * This calculates CLOCK_MONOTONIC at the time of the TSC snapshot, with
+ * no boot time offset.
+ */
+static int do_monotonic(s64 *t, u64 *tsc_timestamp)
+{
+       struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
+       unsigned long seq;
+       int mode;
+       u64 ns;
+
+       do {
+               seq = read_seqcount_begin(&gtod->seq);
+               ns = gtod->clock.base_cycles;
+               ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
+               ns >>= gtod->clock.shift;
+               ns += ktime_to_ns(gtod->clock.offset);
+       } while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
+       *t = ns;
+
+       return mode;
+}
+
static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
{
struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
@@ -2892,18 +2919,42 @@ static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
return mode;
}

-/* returns true if host is using TSC based clocksource */
+/*
+ * Calculates the kvmclock_base_ns (CLOCK_MONOTONIC_RAW + boot time) and
+ * reports the TSC value from which it do so. Returns true if host is
+ * using TSC based clocksource.
+ */
static bool kvm_get_time_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
{
/* checked again under seqlock below */
if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
return false;

-       return gtod_is_based_on_tsc(do_monotonic_raw(kernel_ns,
-                                                     tsc_timestamp));
+       return gtod_is_based_on_tsc(do_kvmclock_base(kernel_ns,
+                                                    tsc_timestamp));
}

-/* returns true if host is using TSC based clocksource */
+/*
+ * Calculates CLOCK_MONOTONIC and reports the TSC value from which it did
+ * so. Returns true if host is using TSC based clocksource.
+ */
+bool kvm_get_monotonic_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp)
+{
+       /* checked again under seqlock below */
+       if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode))
+               return false;
+
+       return gtod_is_based_on_tsc(do_monotonic(kernel_ns,
+                                                tsc_timestamp));
+}
+
+/*
+ * Calculates CLOCK_REALTIME and reports the TSC value from which it did
+ * so. Returns true if host is using TSC based clocksource.
+ *
+ * DO NOT USE this for anything related to migration. You want CLOCK_TAI
+ * for that.
+ */
static bool kvm_get_walltime_and_clockread(struct timespec64 *ts,
u64 *tsc_timestamp)
{
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 5184fde1dc54..0d6af2a57af7 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -294,6 +294,7 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);

u64 get_kvmclock_ns(struct kvm *kvm);
uint64_t kvm_get_wall_clock_epoch(struct kvm *kvm);
+bool kvm_get_monotonic_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp);

int kvm_read_guest_virt(struct kvm_vcpu *vcpu,
gva_t addr, void *val, unsigned int bytes,
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6667f01170f9..a28f60aa91fb 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -24,6 +24,7 @@
#include <xen/interface/sched.h>

#include <asm/xen/cpuid.h>
+#include <asm/pvclock.h>

#include "cpuid.h"
#include "trace.h"
@@ -149,8 +150,93 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer)
return HRTIMER_NORESTART;
}

-static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, s64 delta_ns)
+static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs,
+                               bool linux_wa)
{
+       uint64_t guest_now;
+       int64_t kernel_now, delta;
+
+       /*
+        * The guest provides the requested timeout in absolute nanoseconds
+        * of the KVM clock — as *it* sees it, based on the scaled TSC and
+        * the pvclock information provided by KVM.
+        *
+        * The kernel doesn't support hrtimers based on CLOCK_MONOTONIC_RAW
+        * so use CLOCK_MONOTONIC. In the timescales covered by timers, the
+        * difference won't matter much as there is no cumulative effect.
+        *
+        * Calculate the time for some arbitrary point in time around "now"
+        * in terms of both kvmclock and CLOCK_MONOTONIC. Calculate the
+        * delta between the kvmclock "now" value and the guest's requested
+        * timeout, apply the "Linux workaround" described below, and add
+        * the resulting delta to the CLOCK_MONOTONIC "now" value, to get
+        * the absolute CLOCK_MONOTONIC time at which the timer should
+        * fire.
+        */
+       if (vcpu->arch.hv_clock.version && vcpu->kvm->arch.use_master_clock &&
+           static_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
+               uint64_t host_tsc, guest_tsc;
+
+               if (!IS_ENABLED(CONFIG_64BIT) ||
+                   !kvm_get_monotonic_and_clockread(&kernel_now, &host_tsc)) {
+                       /*
+                        * Don't fall back to get_kvmclock_ns() because it's
+                        * broken; it has a systemic error in its results
+                        * because it scales directly from host TSC to
+                        * nanoseconds, and doesn't scale first to guest TSC
+                        * and then* to nanoseconds as the guest does.
+                        *
+                        * There is a small error introduced here because time
+                        * continues to elapse between the ktime_get() and the
+                        * subsequent rdtsc(). But not the systemic drift due
+                        * to get_kvmclock_ns().
+                        */
+                       kernel_now = ktime_get(); /* This is CLOCK_MONOTONIC */
+                       host_tsc = rdtsc();
+               }
+
+               /* Calculate the guest kvmclock as the guest would do it. */
+               guest_tsc = kvm_read_l1_tsc(vcpu, host_tsc);
+               guest_now = __pvclock_read_cycles(&vcpu->arch.hv_clock,
+                                                 guest_tsc);
+       } else {
+               /*
+                * Without CONSTANT_TSC, get_kvmclock_ns() is the only option.
+                *
+                * Also if the guest PV clock hasn't been set up yet, as is
+                * likely to be the case during migration when the vCPU has
+                * not been run yet. It would be possible to calculate the
+                * scaling factors properly in that case but there's not much
+                * point in doing so. The get_kvmclock_ns() drift accumulates
+                * over time, so it's OK to use it at startup. Besides, on
+                * migration there's going to be a little bit of skew in the
+                * precise moment at which timers fire anyway. Often they'll
+                * be in the "past" by the time the VM is running again after
+                * migration.
+                */
+               guest_now = get_kvmclock_ns(vcpu->kvm);
+               kernel_now = ktime_get();
+       }
+
+       delta = guest_abs - guest_now;
+
+       /* Xen has a 'Linux workaround' in do_set_timer_op() which
+        * checks for negative absolute timeout values (caused by
+        * integer overflow), and for values about 13 days in the
+        * future (2^50ns) which would be caused by jiffies
+        * overflow. For those cases, it sets the timeout 100ms in
+        * the future (not *too* soon, since if a guest really did
+        * set a long timeout on purpose we don't want to keep
+        * churning CPU time by waking it up).
+        */
+       if (linux_wa) {
+               if ((unlikely((int64_t)guest_abs < 0 ||
+                             (delta > 0 && (uint32_t) (delta >> 50) != 0)))) {
+                       delta = 100 * NSEC_PER_MSEC;
+                       guest_abs = guest_now + delta;
+               }
+       }
+
/*
* Avoid races with the old timer firing. Checking timer_expires
* to avoid calling hrtimer_cancel() will only have false positives
@@ -162,12 +248,11 @@ static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, s64 delta_
atomic_set(&vcpu->arch.xen.timer_pending, 0);
vcpu->arch.xen.timer_expires = guest_abs;

-       if (delta_ns <= 0) {
+       if (delta <= 0) {
xen_timer_callback(&vcpu->arch.xen.timer);
} else {
-               ktime_t ktime_now = ktime_get();
hrtimer_start(&vcpu->arch.xen.timer,
-                             ktime_add_ns(ktime_now, delta_ns),
+                             ktime_add_ns(kernel_now, delta),
HRTIMER_MODE_ABS_HARD);
}
}
@@ -991,8 +1076,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
/* Start the timer if the new value has a valid vector+expiry. */
if (data->u.timer.port && data->u.timer.expires_ns)
kvm_xen_start_timer(vcpu, data->u.timer.expires_ns,
-                                           data->u.timer.expires_ns -
-                                           get_kvmclock_ns(vcpu->kvm));
+                                           false);

r = 0;
break;
@@ -1448,7 +1532,6 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
{
struct vcpu_set_singleshot_timer oneshot;
struct x86_exception e;
-       s64 delta;

if (!kvm_xen_timer_enabled(vcpu))
return false;
@@ -1482,9 +1565,7 @@ static bool kvm_xen_hcall_vcpu_op(struct kvm_vcpu *vcpu, bool longmode, int cmd,
return true;
}

-               /* A delta <= 0 results in an immediate callback, which is what we want */
-               delta = oneshot.timeout_abs_ns - get_kvmclock_ns(vcpu->kvm);
-               kvm_xen_start_timer(vcpu, oneshot.timeout_abs_ns, delta);
+               kvm_xen_start_timer(vcpu, oneshot.timeout_abs_ns, false);
*r = 0;
return true;

@@ -1508,25 +1589,7 @@ static bool kvm_xen_hcall_set_timer_op(struct kvm_vcpu *vcpu, uint64_t timeout,
return false;

if (timeout) {
-               uint64_t guest_now = get_kvmclock_ns(vcpu->kvm);
-               int64_t delta = timeout - guest_now;
-
-               /* Xen has a 'Linux workaround' in do_set_timer_op() which
-                * checks for negative absolute timeout values (caused by
-                * integer overflow), and for values about 13 days in the
-                * future (2^50ns) which would be caused by jiffies
-                * overflow. For those cases, it sets the timeout 100ms in
-                * the future (not *too* soon, since if a guest really did
-                * set a long timeout on purpose we don't want to keep
-                * churning CPU time by waking it up).
-                */
-               if (unlikely((int64_t)timeout < 0 ||
-                            (delta > 0 && (uint32_t) (delta >> 50) != 0))) {
-                       delta = 100 * NSEC_PER_MSEC;
-                       timeout = guest_now + delta;
-               }
-
-               kvm_xen_start_timer(vcpu, timeout, delta);
+               kvm_xen_start_timer(vcpu, timeout, true);
} else {
kvm_xen_stop_timer(vcpu);
}
--
2.41.0

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