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>

1 files changed, 35 insertions, 11 deletions

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 2c7aafa70702..2bd81e302427 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -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)