KVM: x86: introduce get_kvmclock_ns

Introduce a function that reads the exact nanoseconds value that is
provided to the guest in kvmclock.  This crystallizes the notion of
kvmclock as a thin veneer over a stable TSC, that the guest will
(hopefully) convert with NTP.  In other words, kvmclock is *not* a
paravirtualized host-to-guest NTP.

Drop the get_kernel_ns() function, that was used both to get the base
value of the master clock and to get the current value of kvmclock.
The former use is replaced by ktime_get_boot_ns(), the latter is
the purpose of get_kernel_ns().

This also allows KVM to provide a Hyper-V time reference counter that
is synchronized with the time that is computed from the TSC page.

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/entry/vdso/vclock_gettime.c

@@ -129,7 +129,7 @@ static notrace cycle_t vread_pvclock(int *mode)
 			return 0;
 		}
 
-		ret = __pvclock_read_cycles(pvti);
+		ret = __pvclock_read_cycles(pvti, rdtsc_ordered());
 	} while (pvclock_read_retry(pvti, version));
 
 	/* refer to vread_tsc() comment for rationale */

arch/x86/include/asm/pvclock.h

@@ -87,9 +87,10 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
 }
 
 static __always_inline
-cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src)
+cycle_t __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
+			      u64 tsc)
 {
-	u64 delta = rdtsc_ordered() - src->tsc_timestamp;
+	u64 delta = tsc - src->tsc_timestamp;
 	cycle_t offset = pvclock_scale_delta(delta, src->tsc_to_system_mul,
 					     src->tsc_shift);
 	return src->system_time + offset;

arch/x86/kernel/pvclock.c

@@ -80,7 +80,7 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 
 	do {
 		version = pvclock_read_begin(src);
-		ret = __pvclock_read_cycles(src);
+		ret = __pvclock_read_cycles(src, rdtsc_ordered());
 		flags = src->flags;
 	} while (pvclock_read_retry(src, version));
 

arch/x86/kvm/hyperv.c

@@ -386,7 +386,7 @@ static void synic_init(struct kvm_vcpu_hv_synic *synic)
 
 static u64 get_time_ref_counter(struct kvm *kvm)
 {
-	return div_u64(get_kernel_ns() + kvm->arch.kvmclock_offset, 100);
+	return div_u64(get_kvmclock_ns(kvm), 100);
 }
 
 static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,

arch/x86/kvm/x86.c

@@ -1431,7 +1431,7 @@ 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 = get_kernel_ns();
+	ns = ktime_get_boot_ns();
 	elapsed = ns - kvm->arch.last_tsc_nsec;
 
 	if (vcpu->arch.virtual_tsc_khz) {
@@ -1722,6 +1722,34 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
 #endif
 }
 
+static u64 __get_kvmclock_ns(struct kvm *kvm)
+{
+	struct kvm_vcpu *vcpu = kvm_get_vcpu(kvm, 0);
+	struct kvm_arch *ka = &kvm->arch;
+	s64 ns;
+
+	if (vcpu->arch.hv_clock.flags & PVCLOCK_TSC_STABLE_BIT) {
+		u64 tsc = kvm_read_l1_tsc(vcpu, rdtsc());
+		ns = __pvclock_read_cycles(&vcpu->arch.hv_clock, tsc);
+	} else {
+		ns = ktime_get_boot_ns() + ka->kvmclock_offset;
+	}
+
+	return ns;
+}
+
+u64 get_kvmclock_ns(struct kvm *kvm)
+{
+	unsigned long flags;
+	s64 ns;
+
+	local_irq_save(flags);
+	ns = __get_kvmclock_ns(kvm);
+	local_irq_restore(flags);
+
+	return ns;
+}
+
 static void kvm_setup_pvclock_page(struct kvm_vcpu *v)
 {
 	struct kvm_vcpu_arch *vcpu = &v->arch;
@@ -1811,7 +1839,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 	}
 	if (!use_master_clock) {
 		host_tsc = rdtsc();
-		kernel_ns = get_kernel_ns();
+		kernel_ns = ktime_get_boot_ns();
 	}
 
 	tsc_timestamp = kvm_read_l1_tsc(v, host_tsc);
@@ -4054,7 +4082,6 @@ long kvm_arch_vm_ioctl(struct file *filp,
 	case KVM_SET_CLOCK: {
 		struct kvm_clock_data user_ns;
 		u64 now_ns;
-		s64 delta;
 
 		r = -EFAULT;
 		if (copy_from_user(&user_ns, argp, sizeof(user_ns)))
@@ -4066,21 +4093,18 @@ long kvm_arch_vm_ioctl(struct file *filp,
 
 		r = 0;
 		local_irq_disable();
-		now_ns = get_kernel_ns();
-		delta = user_ns.clock - now_ns;
+		now_ns = __get_kvmclock_ns(kvm);
+		kvm->arch.kvmclock_offset += user_ns.clock - now_ns;
 		local_irq_enable();
-		kvm->arch.kvmclock_offset = delta;
 		kvm_gen_update_masterclock(kvm);
 		break;
 	}
 	case KVM_GET_CLOCK: {
 		struct kvm_clock_data user_ns;
 		u64 now_ns;
 
-		local_irq_disable();
-		now_ns = get_kernel_ns();
-		user_ns.clock = kvm->arch.kvmclock_offset + now_ns;
-		local_irq_enable();
+		now_ns = get_kvmclock_ns(kvm);
+		user_ns.clock = now_ns;
 		user_ns.flags = 0;
 		memset(&user_ns.pad, 0, sizeof(user_ns.pad));
 
@@ -7544,7 +7568,7 @@ int kvm_arch_hardware_enable(void)
 	 * before any KVM threads can be running.  Unfortunately, we can't
 	 * bring the TSCs fully up to date with real time, as we aren't yet far
 	 * enough into CPU bringup that we know how much real time has actually
-	 * elapsed; our helper function, get_kernel_ns() will be using boot
+	 * elapsed; our helper function, ktime_get_boot_ns() will be using boot
 	 * variables that haven't been updated yet.
 	 *
 	 * So we simply find the maximum observed TSC above, then record the
@@ -7779,7 +7803,7 @@ 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 = -get_kernel_ns();
+	kvm->arch.kvmclock_offset = -ktime_get_boot_ns();
 	pvclock_update_vm_gtod_copy(kvm);
 
 	INIT_DELAYED_WORK(&kvm->arch.kvmclock_update_work, kvmclock_update_fn);

arch/x86/kvm/x86.h

@@ -148,11 +148,6 @@ static inline void kvm_register_writel(struct kvm_vcpu *vcpu,
 	return kvm_register_write(vcpu, reg, val);
 }
 
-static inline u64 get_kernel_ns(void)
-{
-	return ktime_get_boot_ns();
-}
-
 static inline bool kvm_check_has_quirk(struct kvm *kvm, u64 quirk)
 {
 	return !(kvm->arch.disabled_quirks & quirk);
@@ -164,6 +159,7 @@ void kvm_set_pending_timer(struct kvm_vcpu *vcpu);
 int kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip);
 
 void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr);
+u64 get_kvmclock_ns(struct kvm *kvm);
 
 int kvm_read_guest_virt(struct x86_emulate_ctxt *ctxt,
 	gva_t addr, void *val, unsigned int bytes,