[PATCH v4 08/30] KVM: x86: Avoid NTP frequency skew for KVM clock on 32-bit host

[David Woodhouse <dwmw2@infradead.org>]

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

Commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to monotonic raw
clock") did so only for 64-bit hosts, by capturing the boot offset from
within the existing clocksource notifier update_pvclock_gtod().

That notifier was added in commit 16e8d74d2da9 ("KVM: x86: notifier for
clocksource changes") but only on x86_64, because its original purpose
was just to disable the "master clock" mode which is only supported on
x86_64.

Now that the notifier is used for more than disabling master clock mode,
enable it for the 32-bit build too so that get_kvmclock_base_ns() can be
unaffected by NTP sync on 32-bit too.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
---
 arch/x86/kvm/x86.c | 19 ++++++-------------
 1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 2179ea2da8e0..1e1834533e98 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2342,7 +2342,6 @@ static int do_set_msr(struct kvm_vcpu *vcpu, unsigned index, u64 *data)
 	return kvm_set_msr_ignored_check(vcpu, index, *data, true);
 }
 
-#ifdef CONFIG_X86_64
 struct pvclock_clock {
 	int vclock_mode;
 	u64 cycle_last;
@@ -2400,13 +2399,6 @@ 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
 
 static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock, int sec_hi_ofs)
 {
@@ -10160,6 +10152,7 @@ static void pvclock_irq_work_fn(struct irq_work *w)
 }
 
 static DEFINE_IRQ_WORK(pvclock_irq_work, pvclock_irq_work_fn);
+#endif
 
 /*
  * Notification about pvclock gtod data update.
@@ -10167,26 +10160,26 @@ static DEFINE_IRQ_WORK(pvclock_irq_work, pvclock_irq_work_fn);
 static int pvclock_gtod_notify(struct notifier_block *nb, unsigned long unused,
 			       void *priv)
 {
-	struct pvclock_gtod_data *gtod = &pvclock_gtod_data;
 	struct timekeeper *tk = priv;
 
 	update_pvclock_gtod(tk);
 
+#ifdef CONFIG_X86_64
 	/*
 	 * Disable master clock if host does not trust, or does not use,
 	 * TSC based clocksource. Delegate queue_work() to irq_work as
 	 * this is invoked with tk_core.seq write held.
 	 */
-	if (!gtod_is_based_on_tsc(gtod->clock.vclock_mode) &&
+	if (!gtod_is_based_on_tsc(pvclock_gtod_data.clock.vclock_mode) &&
 	    atomic_read(&kvm_guest_has_master_clock) != 0)
 		irq_work_queue(&pvclock_irq_work);
+#endif
 	return 0;
 }
 
 static struct notifier_block pvclock_gtod_notifier = {
 	.notifier_call = pvclock_gtod_notify,
 };
-#endif
 
 void kvm_setup_xss_caps(void)
 {
@@ -10375,9 +10368,9 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 
 	if (pi_inject_timer == -1)
 		pi_inject_timer = housekeeping_enabled(HK_TYPE_TIMER);
-#ifdef CONFIG_X86_64
 	pvclock_gtod_register_notifier(&pvclock_gtod_notifier);
 
+#ifdef CONFIG_X86_64
 	if (hypervisor_is_type(X86_HYPER_MS_HYPERV))
 		set_hv_tscchange_cb(kvm_hyperv_tsc_notifier);
 #endif
@@ -10434,8 +10427,8 @@ void kvm_x86_vendor_exit(void)
 					    CPUFREQ_TRANSITION_NOTIFIER);
 		cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
 	}
-#ifdef CONFIG_X86_64
 	pvclock_gtod_unregister_notifier(&pvclock_gtod_notifier);
+#ifdef CONFIG_X86_64
 	irq_work_sync(&pvclock_irq_work);
 	cancel_work_sync(&pvclock_gtod_work);
 #endif
-- 
2.51.0