commit 24e1f31a4cdb43a8e5cab6cfb95d710c7c7bf18a Author: Zachary Amsden Date: Fri Feb 26 15:13:31 2010 -1000 Fix a possible backwards warp of kvmclock Kernel time, which advances in discrete steps may progress much slower than TSC. As a result, when kvmclock is adjusted to a new base, the apparent time to the guest, which runs at a much higher, nsec scaled rate based on the current TSC, may have already been observed to have a larger value (kernel_ns + scaled tsc) than the value to which we are setting it (kernel_ns + 0). We must instead compute the clock as potentially observed by the guest for kernel_ns to make sure it does not go backwards. Signed-off-by: Zachary Amsden diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 83df4db..ba765fa 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -453,6 +453,8 @@ struct kvm_vcpu_stat { u32 hypercalls; u32 irq_injections; u32 nmi_injections; + u32 tsc_overshoot; + u32 tsc_ahead; }; struct kvm_x86_ops { diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index bb44f9e..2bf7e86 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -134,6 +134,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { { "insn_emulation_fail", VCPU_STAT(insn_emulation_fail) }, { "irq_injections", VCPU_STAT(irq_injections) }, { "nmi_injections", VCPU_STAT(nmi_injections) }, + { "tsc_overshoot", VCPU_STAT(tsc_overshoot) }, + { "tsc_ahead", VCPU_STAT(tsc_ahead) }, { "mmu_shadow_zapped", VM_STAT(mmu_shadow_zapped) }, { "mmu_pte_write", VM_STAT(mmu_pte_write) }, { "mmu_pte_updated", VM_STAT(mmu_pte_updated) }, @@ -849,35 +851,80 @@ static int kvm_recompute_guest_time(struct kvm_vcpu *v) struct kvm_vcpu_arch *vcpu = &v->arch; void *shared_kaddr; unsigned long this_tsc_khz; + s64 kernel_ns, delta; + u64 tsc_timestamp; + bool upscale; if ((!vcpu->time_page)) return 0; - this_tsc_khz = get_cpu_var(cpu_tsc_khz); - put_cpu_var(cpu_tsc_khz); + /* + * The protection we require is simple: we must not be preempted from + * the CPU between our read of the TSC khz and our read of the TSC. + * Interrupt protection is not strictly required, but it does result in + * greater accuracy for the TSC / kernel_ns measurement. + */ + local_irq_save(flags); + this_tsc_khz = __get_cpu_var(cpu_tsc_khz); + kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp); + ktime_get_ts(&ts); + monotonic_to_bootbased(&ts); + kernel_ns = timespec_to_ns(&ts); + local_irq_restore(flags); + if (unlikely(this_tsc_khz == 0)) { kvm_request_guest_time_update(v); return 1; } + /* + * Time as measured by the TSC may go backwards when resetting the base + * tsc_timestamp. The reason for this is that the TSC resolution is + * higher than the resolution of the other clock scales. Thus, many + * possible measurments of the TSC correspond to one measurement of any + * other clock, and so a spread of values is possible. This is not a + * problem for the computation of the nanosecond clock; with TSC rates + * around 1GHZ, there can only be a few cycles which correspond to one + * nanosecond value, and any path through this code will inevitably + * take longer than that. However, with the kernel_ns value itself, + * the precision may be much lower, down to HZ granularity. If the + * first sampling of TSC against kernel_ns ends in the low part of the + * range, and the second in the high end of the range, we can get: + * + * (TSC - offset_low) * S + kns_old > (TSC - offset_high) * S + kns_new + * + * As the sampling errors potentially range in the thousands of cycles, + * it is possible such a time value has already been observed by the + * guest. To protect against this, we must compute the system time as + * observed by the guest and ensure the new system time is greater. + */ + delta = native_read_tsc() - vcpu->hv_clock.tsc_timestamp; + delta = pvclock_scale_delta(delta, vcpu->hv_clock.tsc_to_system_mul, + vcpu->hv_clock.tsc_shift); + delta += vcpu->hv_clock.system_time; + if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) { + upscale = this_tsc_khz > vcpu->hw_tsc_khz; kvm_get_time_scale(NSEC_PER_SEC / 1000, this_tsc_khz, &vcpu->hv_clock.tsc_shift, &vcpu->hv_clock.tsc_to_system_mul); vcpu->hw_tsc_khz = this_tsc_khz; } - /* Keep irq disabled to prevent changes to the clock */ - local_irq_save(flags); - kvm_get_msr(v, MSR_IA32_TSC, &vcpu->hv_clock.tsc_timestamp); - ktime_get_ts(&ts); - monotonic_to_bootbased(&ts); - local_irq_restore(flags); + if (delta > kernel_ns) { + s64 overshoot = delta - kernel_ns; + ++v->stat.tsc_ahead; + if (upscale) + overshoot = overshoot * 9 / 10; + if (overshoot > 1000ULL * this_tsc_khz / HZ) { + ++v->stat.tsc_overshoot; + } + kernel_ns = delta; + } /* With all the info we got, fill in the values */ - - vcpu->hv_clock.system_time = ts.tv_nsec + - (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset; + vcpu->hv_clock.tsc_timestamp = tsc_timestamp; + vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset; /* * The interface expects us to write an even number signaling that the