From 1d5f066e0b63271b67eac6d3752f8aa96adcbddb Mon Sep 17 00:00:00 2001 From: Zachary Amsden Date: Thu, 19 Aug 2010 22:07:30 -1000 Subject: KVM: x86: 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 Signed-off-by: Marcelo Tosatti --- arch/x86/kvm/x86.c | 44 ++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 42 insertions(+), 2 deletions(-) (limited to 'arch/x86/kvm/x86.c') diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index d0764a258047..d4d33f943d99 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -55,6 +55,7 @@ #include #include #include +#include #define MAX_IO_MSRS 256 #define CR0_RESERVED_BITS \ @@ -976,14 +977,15 @@ static int kvm_write_guest_time(struct kvm_vcpu *v) struct kvm_vcpu_arch *vcpu = &v->arch; void *shared_kaddr; unsigned long this_tsc_khz; - s64 kernel_ns; + s64 kernel_ns, max_kernel_ns; + u64 tsc_timestamp; if ((!vcpu->time_page)) return 0; /* 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); + kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp); kernel_ns = get_kernel_ns(); this_tsc_khz = __get_cpu_var(cpu_tsc_khz); local_irq_restore(flags); @@ -993,13 +995,49 @@ static int kvm_write_guest_time(struct kvm_vcpu *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. + */ + max_kernel_ns = 0; + if (vcpu->hv_clock.tsc_timestamp && vcpu->last_guest_tsc) { + max_kernel_ns = vcpu->last_guest_tsc - + vcpu->hv_clock.tsc_timestamp; + max_kernel_ns = pvclock_scale_delta(max_kernel_ns, + vcpu->hv_clock.tsc_to_system_mul, + vcpu->hv_clock.tsc_shift); + max_kernel_ns += vcpu->last_kernel_ns; + } + if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) { kvm_set_time_scale(this_tsc_khz, &vcpu->hv_clock); vcpu->hw_tsc_khz = this_tsc_khz; } + if (max_kernel_ns > kernel_ns) + kernel_ns = max_kernel_ns; + /* With all the info we got, fill in the values */ + vcpu->hv_clock.tsc_timestamp = tsc_timestamp; vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset; + vcpu->last_kernel_ns = kernel_ns; vcpu->hv_clock.flags = 0; /* @@ -4931,6 +4969,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) if (hw_breakpoint_active()) hw_breakpoint_restore(); + kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc); + atomic_set(&vcpu->guest_mode, 0); smp_wmb(); local_irq_enable(); -- cgit v1.2.3