[PATCH 1/2] KVM: x86: reorganize pvclock_gtod_data members

[PATCH 1/2] KVM: x86: reorganize pvclock_gtod_data members

[Paolo Bonzini]

We will need a copy of tk->offs_boot in the next patch.  Store it and
cleanup the struct: instead of storing tk->tkr_xxx.base with the tk->offs_boot
included, store the raw value in struct pvclock_clock and sum tk->offs_boot
in do_monotonic_raw and do_realtime.   tk->tkr_xxx.xtime_nsec also moves
to struct pvclock_clock.

While at it, fix a (usually harmless) typo in do_monotonic_raw, which
was using gtod->clock.shift instead of gtod->raw_clock.shift.

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 | 29 ++++++++++++-----------------
 1 file changed, 12 insertions(+), 17 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 89621025577a..1b4273cce63c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1532,6 +1532,8 @@ struct pvclock_clock {
 	u64 mask;
 	u32 mult;
 	u32 shift;
+	u64 base_cycles;
+	u64 offset;
 };
 
 struct pvclock_gtod_data {
@@ -1540,11 +1542,8 @@ struct pvclock_gtod_data {
 	struct pvclock_clock clock; /* extract of a clocksource struct */
 	struct pvclock_clock raw_clock; /* extract of a clocksource struct */
 
-	u64		boot_ns_raw;
-	u64		boot_ns;
-	u64		nsec_base;
+	ktime_t		offs_boot;
 	u64		wall_time_sec;
-	u64		monotonic_raw_nsec;
 };
 
 static struct pvclock_gtod_data pvclock_gtod_data;
@@ -1552,10 +1551,6 @@ struct pvclock_gtod_data {
 static void update_pvclock_gtod(struct timekeeper *tk)
 {
 	struct pvclock_gtod_data *vdata = &pvclock_gtod_data;
-	u64 boot_ns, boot_ns_raw;
-
-	boot_ns = ktime_to_ns(ktime_add(tk->tkr_mono.base, tk->offs_boot));
-	boot_ns_raw = ktime_to_ns(ktime_add(tk->tkr_raw.base, tk->offs_boot));
 
 	write_seqcount_begin(&vdata->seq);
 
@@ -1565,20 +1560,20 @@ static void update_pvclock_gtod(struct timekeeper *tk)
 	vdata->clock.mask		= tk->tkr_mono.mask;
 	vdata->clock.mult		= tk->tkr_mono.mult;
 	vdata->clock.shift		= tk->tkr_mono.shift;
+	vdata->clock.base_cycles	= tk->tkr_mono.xtime_nsec;
+	vdata->clock.offset		= tk->tkr_mono.base;
 
 	vdata->raw_clock.vclock_mode	= tk->tkr_raw.clock->archdata.vclock_mode;
 	vdata->raw_clock.cycle_last	= tk->tkr_raw.cycle_last;
 	vdata->raw_clock.mask		= tk->tkr_raw.mask;
 	vdata->raw_clock.mult		= tk->tkr_raw.mult;
 	vdata->raw_clock.shift		= tk->tkr_raw.shift;
-
-	vdata->boot_ns			= boot_ns;
-	vdata->nsec_base		= tk->tkr_mono.xtime_nsec;
+	vdata->raw_clock.base_cycles	= tk->tkr_raw.xtime_nsec;
+	vdata->raw_clock.offset		= tk->tkr_raw.base;
 
 	vdata->wall_time_sec            = tk->xtime_sec;
 
-	vdata->boot_ns_raw		= boot_ns_raw;
-	vdata->monotonic_raw_nsec	= tk->tkr_raw.xtime_nsec;
+	vdata->offs_boot		= tk->offs_boot;
 
 	write_seqcount_end(&vdata->seq);
 }
@@ -2048,10 +2043,10 @@ static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
 
 	do {
 		seq = read_seqcount_begin(&gtod->seq);
-		ns = gtod->monotonic_raw_nsec;
+		ns = gtod->raw_clock.base_cycles;
 		ns += vgettsc(&gtod->raw_clock, tsc_timestamp, &mode);
-		ns >>= gtod->clock.shift;
-		ns += gtod->boot_ns_raw;
+		ns >>= gtod->raw_clock.shift;
+		ns += ktime_to_ns(ktime_add(gtod->raw_clock.offset, gtod->offs_boot));
 	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
 	*t = ns;
 
@@ -2068,7 +2063,7 @@ static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
 	do {
 		seq = read_seqcount_begin(&gtod->seq);
 		ts->tv_sec = gtod->wall_time_sec;
-		ns = gtod->nsec_base;
+		ns = gtod->clock.base_cycles;
 		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
 		ns >>= gtod->clock.shift;
 	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
-- 
1.8.3.1

Re: [PATCH 1/2] KVM: x86: reorganize pvclock_gtod_data members

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@redhat.com> writes:

> We will need a copy of tk->offs_boot in the next patch.  Store it and
> cleanup the struct: instead of storing tk->tkr_xxx.base with the tk->offs_boot
> included, store the raw value in struct pvclock_clock and sum tk->offs_boot
> in do_monotonic_raw and do_realtime.   tk->tkr_xxx.xtime_nsec also moves
> to struct pvclock_clock.
>
> While at it, fix a (usually harmless) typo in do_monotonic_raw, which
> was using gtod->clock.shift instead of gtod->raw_clock.shift.
>
> 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 | 29 ++++++++++++-----------------
>  1 file changed, 12 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 89621025577a..1b4273cce63c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1532,6 +1532,8 @@ struct pvclock_clock {
>  	u64 mask;
>  	u32 mult;
>  	u32 shift;
> +	u64 base_cycles;
> +	u64 offset;
>  };
>  
>  struct pvclock_gtod_data {
> @@ -1540,11 +1542,8 @@ struct pvclock_gtod_data {
>  	struct pvclock_clock clock; /* extract of a clocksource struct */
>  	struct pvclock_clock raw_clock; /* extract of a clocksource struct */
>  
> -	u64		boot_ns_raw;
> -	u64		boot_ns;
> -	u64		nsec_base;
> +	ktime_t		offs_boot;
>  	u64		wall_time_sec;
> -	u64		monotonic_raw_nsec;
>  };
>  
>  static struct pvclock_gtod_data pvclock_gtod_data;
> @@ -1552,10 +1551,6 @@ struct pvclock_gtod_data {
>  static void update_pvclock_gtod(struct timekeeper *tk)
>  {
>  	struct pvclock_gtod_data *vdata = &pvclock_gtod_data;
> -	u64 boot_ns, boot_ns_raw;
> -
> -	boot_ns = ktime_to_ns(ktime_add(tk->tkr_mono.base, tk->offs_boot));
> -	boot_ns_raw = ktime_to_ns(ktime_add(tk->tkr_raw.base, tk->offs_boot));
>  
>  	write_seqcount_begin(&vdata->seq);
>  
> @@ -1565,20 +1560,20 @@ static void update_pvclock_gtod(struct timekeeper *tk)
>  	vdata->clock.mask		= tk->tkr_mono.mask;
>  	vdata->clock.mult		= tk->tkr_mono.mult;
>  	vdata->clock.shift		= tk->tkr_mono.shift;
> +	vdata->clock.base_cycles	= tk->tkr_mono.xtime_nsec;
> +	vdata->clock.offset		= tk->tkr_mono.base;
>  
>  	vdata->raw_clock.vclock_mode	= tk->tkr_raw.clock->archdata.vclock_mode;
>  	vdata->raw_clock.cycle_last	= tk->tkr_raw.cycle_last;
>  	vdata->raw_clock.mask		= tk->tkr_raw.mask;
>  	vdata->raw_clock.mult		= tk->tkr_raw.mult;
>  	vdata->raw_clock.shift		= tk->tkr_raw.shift;
> -
> -	vdata->boot_ns			= boot_ns;
> -	vdata->nsec_base		= tk->tkr_mono.xtime_nsec;
> +	vdata->raw_clock.base_cycles	= tk->tkr_raw.xtime_nsec;
> +	vdata->raw_clock.offset		= tk->tkr_raw.base;

Likely a personal preference but the suggested naming is a bit
confusing: we use 'base_cycles' to keep 'xtime_nsec' and 'offset' to
keep ... 'base'. Not that I think that 'struct timekeeper' is perfect
but at least it is documented. Should we maybe just stick to it (and
name 'struct pvclock_clock' fields accordingly?)

>  
>  	vdata->wall_time_sec            = tk->xtime_sec;
>  
> -	vdata->boot_ns_raw		= boot_ns_raw;
> -	vdata->monotonic_raw_nsec	= tk->tkr_raw.xtime_nsec;
> +	vdata->offs_boot		= tk->offs_boot;
>  
>  	write_seqcount_end(&vdata->seq);
>  }
> @@ -2048,10 +2043,10 @@ static int do_monotonic_raw(s64 *t, u64 *tsc_timestamp)
>  
>  	do {
>  		seq = read_seqcount_begin(&gtod->seq);
> -		ns = gtod->monotonic_raw_nsec;
> +		ns = gtod->raw_clock.base_cycles;
>  		ns += vgettsc(&gtod->raw_clock, tsc_timestamp, &mode);
> -		ns >>= gtod->clock.shift;
> -		ns += gtod->boot_ns_raw;
> +		ns >>= gtod->raw_clock.shift;
> +		ns += ktime_to_ns(ktime_add(gtod->raw_clock.offset, gtod->offs_boot));
>  	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
>  	*t = ns;
>  
> @@ -2068,7 +2063,7 @@ static int do_realtime(struct timespec64 *ts, u64 *tsc_timestamp)
>  	do {
>  		seq = read_seqcount_begin(&gtod->seq);
>  		ts->tv_sec = gtod->wall_time_sec;
> -		ns = gtod->nsec_base;
> +		ns = gtod->clock.base_cycles;
>  		ns += vgettsc(&gtod->clock, tsc_timestamp, &mode);
>  		ns >>= gtod->clock.shift;
>  	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));

FWIW,

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

-- 
Vitaly

Re: [PATCH 1/2] KVM: x86: reorganize pvclock_gtod_data members

[Paolo Bonzini]

On 23/01/20 12:32, Vitaly Kuznetsov wrote:
> Likely a personal preference but the suggested naming is a bit
> confusing: we use 'base_cycles' to keep 'xtime_nsec' and 'offset' to
> keep ... 'base'. Not that I think that 'struct timekeeper' is perfect
> but at least it is documented. Should we maybe just stick to it (and
> name 'struct pvclock_clock' fields accordingly?)
> 

The problem is that xtime_nsec is not nanoseconds, and I'd really not
want to have a worse name just for consistency. :(  I chose
"base_cycles" as an incremental improvement over nsec_base, even though
that meant also changing struct timekeeper's "base" to "offset".

Paolo

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

[Paolo Bonzini]

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

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

[Vitaly Kuznetsov]

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

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

[Paolo Bonzini]

On 23/01/20 14:43, Vitaly Kuznetsov wrote:
>> +
>> +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.

Yes, there's no reason to restrict the pvclock_gtod notifier to x86_64.
 But this is stable material so I kept it easy.

>>
>> -	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().

You're off by a factor of 10, 2^64 nanoseconds are about 584 years
(584*365*10^9*86400). :)

Paolo