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@xxxxxxxxxx> --- 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.
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