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KVM: x86: TSC reset compensation

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Attempt to synchronize TSCs which are reset to the same value.  In the
case of a reliable hardware TSC, we can just re-use the same offset, but
on non-reliable hardware, we can get closer by adjusting the offset to
match the elapsed time.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
This commit is contained in:
Zachary Amsden 2010-08-19 22:07:20 -10:00 committed by Avi Kivity
parent 99e3e30aee
commit f38e098ff3
2 changed files with 33 additions and 1 deletions
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3
arch/x86/include/asm/kvm_host.h
View File

@ -396,6 +396,9 @@ struct kvm_arch {
unsigned long irq_sources_bitmap;
s64 kvmclock_offset;
spinlock_t tsc_write_lock;
u64 last_tsc_nsec;
u64 last_tsc_offset;
u64 last_tsc_write;
struct kvm_xen_hvm_config xen_hvm_config;

31
arch/x86/kvm/x86.c
View File

@ -898,11 +898,40 @@ static DEFINE_PER_CPU(unsigned long, cpu_tsc_khz);
void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
{
struct kvm *kvm = vcpu->kvm;
u64 offset;
u64 offset, ns, elapsed;
unsigned long flags;
struct timespec ts;
spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
offset = data - native_read_tsc();
ktime_get_ts(&ts);
monotonic_to_bootbased(&ts);
ns = timespec_to_ns(&ts);
elapsed = ns - kvm->arch.last_tsc_nsec;
/*
* Special case: identical write to TSC within 5 seconds of
* another CPU is interpreted as an attempt to synchronize
* (the 5 seconds is to accomodate host load / swapping).
*
* In that case, for a reliable TSC, we can match TSC offsets,
* or make a best guest using kernel_ns value.
*/
if (data == kvm->arch.last_tsc_write && elapsed < 5ULL * NSEC_PER_SEC) {
if (!check_tsc_unstable()) {
offset = kvm->arch.last_tsc_offset;
pr_debug("kvm: matched tsc offset for %llu\n", data);
} else {
u64 tsc_delta = elapsed * __get_cpu_var(cpu_tsc_khz);
tsc_delta = tsc_delta / USEC_PER_SEC;
offset += tsc_delta;
pr_debug("kvm: adjusted tsc offset by %llu\n", tsc_delta);
}
ns = kvm->arch.last_tsc_nsec;
}
kvm->arch.last_tsc_nsec = ns;
kvm->arch.last_tsc_write = data;
kvm->arch.last_tsc_offset = offset;
kvm_x86_ops->write_tsc_offset(vcpu, offset);
spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);