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* [PATCH v4] KVM: x86: Use current rather than snapshotted TSC frequency if it is constant
@ 2022-05-11  1:42 Anton Romanov
  2022-05-11 19:49 ` Sean Christopherson
  0 siblings, 1 reply; 2+ messages in thread
From: Anton Romanov @ 2022-05-11  1:42 UTC (permalink / raw)
  To: kvm, pbonzini; +Cc: seanjc, vkuznets, mlevitsk, Anton Romanov

Don't snapshot tsc_khz into per-cpu cpu_tsc_khz if the host TSC is
constant, in which case the actual TSC frequency will never change and thus
capturing TSC during initialization is unnecessary, KVM can simply use
tsc_khz.  This value is snapshotted from
kvm_timer_init->kvmclock_cpu_online->tsc_khz_changed(NULL)

On CPUs with constant TSC, but not a hardware-specified TSC frequency,
snapshotting cpu_tsc_khz and using that to set a VM's target TSC frequency
can lead to VM to think its TSC frequency is not what it actually is if
refining the TSC completes after KVM snapshots tsc_khz.  The actual
frequency never changes, only the kernel's calculation of what that
frequency is changes.

Ideally, KVM would not be able to race with TSC refinement, or would have
a hook into tsc_refine_calibration_work() to get an alert when refinement
is complete.  Avoiding the race altogether isn't practical as refinement
takes a relative eternity; it's deliberately put on a work queue outside of
the normal boot sequence to avoid unnecessarily delaying boot.

Adding a hook is doable, but somewhat gross due to KVM's ability to be
built as a module.  And if the TSC is constant, which is likely the case
for every VMX/SVM-capable CPU produced in the last decade, the race can be
hit if and only if userspace is able to create a VM before TSC refinement
completes; refinement is slow, but not that slow.

For now, punt on a proper fix, as not taking a snapshot can help some uses
cases and not taking a snapshot is arguably correct irrespective of the
race with refinement.

Signed-off-by: Anton Romanov <romanton@google.com>
---
v4 :
    * minor feedback changes
    * skip updating per-cpu tsc in kvm_hyperv_tsc_notifier
    
 arch/x86/kvm/x86.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4790f0d7d40b..f0b4d5ae743b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2907,6 +2907,22 @@ static void kvm_update_masterclock(struct kvm *kvm)
 	kvm_end_pvclock_update(kvm);
 }
 
+/*
+ * Use the kernel's tsc_khz directly if the TSC is constant, otherwise use KVM's
+ * per-CPU value (which may be zero if a CPU is going offline).  Note, tsc_khz
+ * can change during boot even if the TSC is constant, as it's possible for KVM
+ * to be loaded before TSC calibration completes.  Ideally, KVM would get a
+ * notification when calibration completes, but practically speaking calibration
+ * will complete before userspace is alive enough to create VMs.
+ */
+static unsigned long get_cpu_tsc_khz(void)
+{
+	if (static_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		return tsc_khz;
+	else
+		return __this_cpu_read(cpu_tsc_khz);
+}
+
 /* Called within read_seqcount_begin/retry for kvm->pvclock_sc.  */
 static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 {
@@ -2917,7 +2933,8 @@ static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 	get_cpu();
 
 	data->flags = 0;
-	if (ka->use_master_clock && __this_cpu_read(cpu_tsc_khz)) {
+	if (ka->use_master_clock &&
+	    (static_cpu_has(X86_FEATURE_CONSTANT_TSC) || __this_cpu_read(cpu_tsc_khz))) {
 #ifdef CONFIG_X86_64
 		struct timespec64 ts;
 
@@ -2931,7 +2948,7 @@ static void __get_kvmclock(struct kvm *kvm, struct kvm_clock_data *data)
 		data->flags |= KVM_CLOCK_TSC_STABLE;
 		hv_clock.tsc_timestamp = ka->master_cycle_now;
 		hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset;
-		kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL,
+		kvm_get_time_scale(NSEC_PER_SEC, get_cpu_tsc_khz() * 1000LL,
 				   &hv_clock.tsc_shift,
 				   &hv_clock.tsc_to_system_mul);
 		data->clock = __pvclock_read_cycles(&hv_clock, data->host_tsc);
@@ -3049,7 +3066,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
 
 	/* Keep irq disabled to prevent changes to the clock */
 	local_irq_save(flags);
-	tgt_tsc_khz = __this_cpu_read(cpu_tsc_khz);
+	tgt_tsc_khz = get_cpu_tsc_khz();
 	if (unlikely(tgt_tsc_khz == 0)) {
 		local_irq_restore(flags);
 		kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
@@ -8646,9 +8663,10 @@ static void tsc_khz_changed(void *data)
 	struct cpufreq_freqs *freq = data;
 	unsigned long khz = 0;
 
+	WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_CONSTANT_TSC));
 	if (data)
 		khz = freq->new;
-	else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+	else
 		khz = cpufreq_quick_get(raw_smp_processor_id());
 	if (!khz)
 		khz = tsc_khz;
@@ -8669,8 +8687,10 @@ static void kvm_hyperv_tsc_notifier(void)
 	hyperv_stop_tsc_emulation();
 
 	/* TSC frequency always matches when on Hyper-V */
-	for_each_present_cpu(cpu)
-		per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
+	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
+		for_each_present_cpu(cpu)
+			per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
+	}
 	kvm_max_guest_tsc_khz = tsc_khz;
 
 	list_for_each_entry(kvm, &vm_list, vm_list) {
@@ -8783,7 +8803,8 @@ static struct notifier_block kvmclock_cpufreq_notifier_block = {
 
 static int kvmclock_cpu_online(unsigned int cpu)
 {
-	tsc_khz_changed(NULL);
+	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+		tsc_khz_changed(NULL);
 	return 0;
 }
 
-- 
2.36.0.550.gb090851708-goog


^ permalink raw reply related	[flat|nested] 2+ messages in thread

* Re: [PATCH v4] KVM: x86: Use current rather than snapshotted TSC frequency if it is constant
  2022-05-11  1:42 [PATCH v4] KVM: x86: Use current rather than snapshotted TSC frequency if it is constant Anton Romanov
@ 2022-05-11 19:49 ` Sean Christopherson
  0 siblings, 0 replies; 2+ messages in thread
From: Sean Christopherson @ 2022-05-11 19:49 UTC (permalink / raw)
  To: Anton Romanov; +Cc: kvm, pbonzini, vkuznets, mlevitsk

On Wed, May 11, 2022, Anton Romanov wrote:
> @@ -8646,9 +8663,10 @@ static void tsc_khz_changed(void *data)
>  	struct cpufreq_freqs *freq = data;
>  	unsigned long khz = 0;
>  
> +	WARN_ON_ONCE(boot_cpu_has(X86_FEATURE_CONSTANT_TSC));

Nit, please add a newline to isolate the WARN.

>  	if (data)
>  		khz = freq->new;
> -	else if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
> +	else
>  		khz = cpufreq_quick_get(raw_smp_processor_id());
>  	if (!khz)
>  		khz = tsc_khz;
> @@ -8669,8 +8687,10 @@ static void kvm_hyperv_tsc_notifier(void)
>  	hyperv_stop_tsc_emulation();
>  
>  	/* TSC frequency always matches when on Hyper-V */
> -	for_each_present_cpu(cpu)
> -		per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
> +	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
> +		for_each_present_cpu(cpu)
> +			per_cpu(cpu_tsc_khz, cpu) = tsc_khz;
> +	}
>  	kvm_max_guest_tsc_khz = tsc_khz;
>  
>  	list_for_each_entry(kvm, &vm_list, vm_list) {
> @@ -8783,7 +8803,8 @@ static struct notifier_block kvmclock_cpufreq_notifier_block = {
>  
>  static int kvmclock_cpu_online(unsigned int cpu)
>  {
> -	tsc_khz_changed(NULL);
> +	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
> +		tsc_khz_changed(NULL);


Ah rats, I missed something in v3.  Rather than handle this in the notifier, KVM
can simply not register the notifier in the first place.  The CPUHP_AP_X86_KVM_CLK_ONLINE
hook exists purely to muck with cpu_tsc_khz.  And that makes the WARN_ON_ONCE in
tsc_khz_changed() much less rendunat (having a caller and its callee check the
same thing in quick succession felt silly).

I.e. instead of modifying kvmclock_cpu_online(), do:

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6567aec84223..e9efb8d00673 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8882,10 +8882,10 @@ static void kvm_timer_init(void)
                }
                cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block,
                                          CPUFREQ_TRANSITION_NOTIFIER);
-       }
 
-       cpuhp_setup_state(CPUHP_AP_X86_KVM_CLK_ONLINE, "x86/kvm/clk:online",
-                         kvmclock_cpu_online, kvmclock_cpu_down_prep);
+               cpuhp_setup_state(CPUHP_AP_X86_KVM_CLK_ONLINE, "x86/kvm/clk:online",
+                                 kvmclock_cpu_online, kvmclock_cpu_down_prep);
+       }
 }
 
 #ifdef CONFIG_X86_64
@@ -9038,10 +9038,11 @@ void kvm_arch_exit(void)
 #endif
        kvm_lapic_exit();
 
-       if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
+       if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {
                cpufreq_unregister_notifier(&kvmclock_cpufreq_notifier_block,
                                            CPUFREQ_TRANSITION_NOTIFIER);
-       cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
+               cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
+       }
 #ifdef CONFIG_X86_64
        pvclock_gtod_unregister_notifier(&pvclock_gtod_notifier);
        irq_work_sync(&pvclock_irq_work);



>  	return 0;
>  }
>  
> -- 
> 2.36.0.550.gb090851708-goog
> 

^ permalink raw reply related	[flat|nested] 2+ messages in thread

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2022-05-11  1:42 [PATCH v4] KVM: x86: Use current rather than snapshotted TSC frequency if it is constant Anton Romanov
2022-05-11 19:49 ` Sean Christopherson

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