Commit 72c930dc authored by Radim Krčmář's avatar Radim Krčmář Committed by Paolo Bonzini
Browse files

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: default avatarRadim Krčmář <rkrcmar@redhat.com>
Acked-by: default avatarMarcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 1cea0ce6

arch/x86/kernel/kvmclock.c

+35 −11
Original line number Diff line number Diff line
@@ -32,6 +32,7 @@ 
static int kvmclock = 1;

static int msr_kvm_system_time = MSR_KVM_SYSTEM_TIME;

static int msr_kvm_wall_clock = MSR_KVM_WALL_CLOCK;

static cycle_t kvm_sched_clock_offset;



static int parse_no_kvmclock(char *arg)

{
@@ -92,6 +93,29 @@ static cycle_t kvm_clock_get_cycles(struct clocksource *cs) 
	return kvm_clock_read();

}



static cycle_t kvm_sched_clock_read(void)

{

	return kvm_clock_read() - kvm_sched_clock_offset;

}



static inline void kvm_sched_clock_init(bool stable)

{

	if (!stable) {

		pv_time_ops.sched_clock = kvm_clock_read;

		return;

	}



	kvm_sched_clock_offset = kvm_clock_read();

	pv_time_ops.sched_clock = kvm_sched_clock_read;

	set_sched_clock_stable();



	printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",

			kvm_sched_clock_offset);



	BUILD_BUG_ON(sizeof(kvm_sched_clock_offset) >

	         sizeof(((struct pvclock_vcpu_time_info *)NULL)->system_time));

}



/*

 * If we don't do that, there is the possibility that the guest

 * will calibrate under heavy load - thus, getting a lower lpj -
@@ -248,7 +272,17 @@ void __init kvmclock_init(void) 
		memblock_free(mem, size);

		return;

	}

	pv_time_ops.sched_clock = kvm_clock_read;



	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))

		pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);



	cpu = get_cpu();

	vcpu_time = &hv_clock[cpu].pvti;

	flags = pvclock_read_flags(vcpu_time);



	kvm_sched_clock_init(flags & PVCLOCK_TSC_STABLE_BIT);

	put_cpu();



	x86_platform.calibrate_tsc = kvm_get_tsc_khz;

	x86_platform.get_wallclock = kvm_get_wallclock;

	x86_platform.set_wallclock = kvm_set_wallclock;
@@ -265,16 +299,6 @@ void __init kvmclock_init(void) 
	kvm_get_preset_lpj();

	clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);

	pv_info.name = "KVM";



	if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))

		pvclock_set_flags(~0);



	cpu = get_cpu();

	vcpu_time = &hv_clock[cpu].pvti;

	flags = pvclock_read_flags(vcpu_time);

	if (flags & PVCLOCK_COUNTS_FROM_ZERO)

		set_sched_clock_stable();

	put_cpu();

}



int __init kvm_setup_vsyscall_timeinfo(void)

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