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x86: kvmclock: abolish PVCLOCK_COUNTS_FROM_ZERO
Newer KVM won't be exposing PVCLOCK_COUNTS_FROM_ZERO anymore. The purpose of that flags was to start counting system time from 0 when the KVM clock has been initialized. We can achieve the same by selecting one read as the initial point. A simple subtraction will work unless the KVM clock count overflows earlier (has smaller width) than scheduler's cycle count. We should be safe till x86_128. Because PVCLOCK_COUNTS_FROM_ZERO was enabled only on new hypervisors, setting sched clock as stable based on PVCLOCK_TSC_STABLE_BIT might regress on older ones. I presume we don't need to change kvm_clock_read instead of introducing kvm_sched_clock_read. A problem could arise in case sched_clock is expected to return the same value as get_cycles, but we should have merged those clocks in that case. Signed-off-by: Radim Krčmář <rkrcmar@redhat.com> Acked-by: Marcelo Tosatti <mtosatti@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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@ -32,6 +32,7 @@
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static int kvmclock = 1;
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static int msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;
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static int msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;
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static cycle_t kvm_sched_clock_offset;
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static int parse_no_kvmclock(char *arg)
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{
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@ -92,6 +93,29 @@ static cycle_t kvm_clock_get_cycles(struct clocksource *cs)
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return kvm_clock_read();
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}
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static cycle_t kvm_sched_clock_read(void)
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{
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return kvm_clock_read() - kvm_sched_clock_offset;
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}
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static inline void kvm_sched_clock_init(bool stable)
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{
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if (!stable) {
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pv_time_ops.sched_clock = kvm_clock_read;
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return;
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}
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kvm_sched_clock_offset = kvm_clock_read();
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pv_time_ops.sched_clock = kvm_sched_clock_read;
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set_sched_clock_stable();
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printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",
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kvm_sched_clock_offset);
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BUILD_BUG_ON(sizeof(kvm_sched_clock_offset) >
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sizeof(((struct pvclock_vcpu_time_info *)NULL)->system_time));
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}
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/*
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* If we don't do that, there is the possibility that the guest
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* will calibrate under heavy load - thus, getting a lower lpj -
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@ -248,7 +272,17 @@ void __init kvmclock_init(void)
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memblock_free(mem, size);
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return;
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}
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pv_time_ops.sched_clock = kvm_clock_read;
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if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
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pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
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cpu = get_cpu();
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vcpu_time = &hv_clock[cpu].pvti;
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flags = pvclock_read_flags(vcpu_time);
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kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT);
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put_cpu();
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x86_platform.calibrate_tsc = kvm_get_tsc_khz;
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x86_platform.get_wallclock = kvm_get_wallclock;
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x86_platform.set_wallclock = kvm_set_wallclock;
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@ -265,16 +299,6 @@ void __init kvmclock_init(void)
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kvm_get_preset_lpj();
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clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
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pv_info.name = "KVM";
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if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
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pvclock_set_flags(~0);
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cpu = get_cpu();
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vcpu_time = &hv_clock[cpu].pvti;
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flags = pvclock_read_flags(vcpu_time);
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if (flags & PVCLOCK_COUNTS_FROM_ZERO)
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set_sched_clock_stable();
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put_cpu();
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}
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int __init kvm_setup_vsyscall_timeinfo(void)
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