Messages in this thread | | | Date | Tue, 15 Jun 2010 14:32:19 -1000 | From | Zachary Amsden <> | Subject | Re: [PATCH 16/17] TSC reset compensation |
| |
On 06/15/2010 02:27 PM, Marcelo Tosatti wrote: > On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote: > >> 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> >> --- >> arch/x86/kvm/x86.c | 34 ++++++++++++++++++++++++++++++++-- >> 1 files changed, 32 insertions(+), 2 deletions(-) >> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >> index 8e836e9..cedb71f 100644 >> --- a/arch/x86/kvm/x86.c >> +++ b/arch/x86/kvm/x86.c >> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v) >> set_bit(KVM_REQ_CLOCK_SYNC,&v->requests); >> } >> >> +static inline int kvm_tsc_reliable(void) >> +{ >> + return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC)&& >> + boot_cpu_has(X86_FEATURE_NONSTOP_TSC)&& >> + !check_tsc_unstable()); >> +} >> + >> void guest_write_tsc(struct kvm_vcpu *vcpu, u64 data) >> { >> struct kvm *kvm = vcpu->kvm; >> - u64 offset; >> + u64 offset, ns, elapsed; >> >> spin_lock(&kvm->arch.tsc_write_lock); >> offset = data - native_read_tsc(); >> - kvm->arch.last_tsc_nsec = get_kernel_ns(); >> + ns = get_kernel_ns(); >> + 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< 5 * NSEC_PER_SEC) { >> + if (kvm_tsc_reliable()) { >> + 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); >> -- >> > Could extend this to handle migration. >
Also, this could be extended to cover the kvmclock variables themselves; then, if tsc is reliable, we need not ever recalibrate the kvmclock. In fact, all VMs would have the same parameters for kvmclock in that case, just with a different kvm->arch.kvmclock_offset.
Zach
| |