From 741e511b42086a100c05dbe8fd1baeec42e7c584 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Fri, 25 Feb 2022 14:53:03 +0000 Subject: [PATCH] KVM: x86: Don't snapshot "max" TSC if host TSC is constant Don't snapshot tsc_khz into max_tsc_khz during KVM initialization if the host TSC is constant, in which case the actual TSC frequency will never change and thus capturing the "max" TSC during initialization is unnecessary, KVM can simply use tsc_khz during VM creation. On CPUs with constant TSC, but not a hardware-specified TSC frequency, snapshotting max_tsc_khz and using that to set a VM's default TSC frequency can lead to KVM thinking it needs to manually scale the guest's TSC 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. On systems without hardware TSC scaling, this either puts KVM into "always catchup" mode (extremely inefficient), or prevents creating VMs altogether. 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. [ dwmw2: Rebase on top of KVM-wide default_tsc_khz to ensure that all vCPUs get the same frequency even if we hit the race. ] Cc: Suleiman Souhlal Cc: Anton Romanov Signed-off-by: Sean Christopherson Signed-off-by: David Woodhouse Message-Id: <20220225145304.36166-3-dwmw2@infradead.org> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/x86.c | 30 +++++++++++++++--------------- 1 file changed, 15 insertions(+), 15 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index c069b97..dad13fd 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8814,22 +8814,22 @@ static int kvmclock_cpu_online(unsigned int cpu) static void kvm_timer_init(void) { - max_tsc_khz = tsc_khz; - if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) { -#ifdef CONFIG_CPU_FREQ - struct cpufreq_policy *policy; - int cpu; - - cpu = get_cpu(); - policy = cpufreq_cpu_get(cpu); - if (policy) { - if (policy->cpuinfo.max_freq) - max_tsc_khz = policy->cpuinfo.max_freq; - cpufreq_cpu_put(policy); + max_tsc_khz = tsc_khz; + + if (IS_ENABLED(CONFIG_CPU_FREQ)) { + struct cpufreq_policy *policy; + int cpu; + + cpu = get_cpu(); + policy = cpufreq_cpu_get(cpu); + if (policy) { + if (policy->cpuinfo.max_freq) + max_tsc_khz = policy->cpuinfo.max_freq; + cpufreq_cpu_put(policy); + } + put_cpu(); } - put_cpu(); -#endif cpufreq_register_notifier(&kvmclock_cpufreq_notifier_block, CPUFREQ_TRANSITION_NOTIFIER); } @@ -11737,7 +11737,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type) pvclock_update_vm_gtod_copy(kvm); raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags); - kvm->arch.default_tsc_khz = max_tsc_khz; + kvm->arch.default_tsc_khz = max_tsc_khz ? : tsc_khz; kvm->arch.guest_can_read_msr_platform_info = true; kvm->arch.enable_pmu = enable_pmu; -- 2.7.4