>From 09a0183ce2a633a480c465cd23f8f8ba80fbd0c4 Mon Sep 17 00:00:00 2001 From: Serge E. Hallyn Date: Wed, 2 Mar 2011 20:05:34 +0000 Subject: [PATCH 2/3] KVM: x86: Fix a possible backwards warp of kvmclock Backport of commit 1d5f066e0b63271b67eac6d3752f8aa96adcbddb 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 Signed-off-by: Serge E. Hallyn --- arch/x86/include/asm/kvm_host.h | 3 ++ arch/x86/kvm/x86.c | 46 ++++++++++++++++++++++++++++++++++++-- 2 files changed, 46 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 600807b..08bc2ff 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -357,6 +357,9 @@ struct kvm_vcpu_arch { struct page *time_page; bool singlestep; /* guest is single stepped by KVM */ + u64 last_guest_tsc; + u64 last_kernel_ns; + bool nmi_pending; bool nmi_injected; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index b2c02a2..ffd70eb 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -47,6 +47,7 @@ #include #include #include +#include #define MAX_IO_MSRS 256 #define CR0_RESERVED_BITS \ @@ -633,6 +634,8 @@ static void 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, max_kernel_ns; + u64 tsc_timestamp; if ((!vcpu->time_page)) return; @@ -646,15 +649,50 @@ static void kvm_write_guest_time(struct kvm_vcpu *v) /* 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); ktime_get_ts(&ts); monotonic_to_bootbased(&ts); local_irq_restore(flags); + kernel_ns = timespec_to_ns(&ts); + + /* + * 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 (max_kernel_ns > kernel_ns) + kernel_ns = max_kernel_ns; /* 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.system_time = kernel_ns + v->kvm->arch.kvmclock_offset; + vcpu->last_kernel_ns = kernel_ns; /* * The interface expects us to write an even number signaling that the @@ -3695,6 +3733,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run) kvm_x86_ops->prepare_guest_switch(vcpu); kvm_load_guest_fpu(vcpu); + kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc); + local_irq_disable(); clear_bit(KVM_REQ_KICK, &vcpu->requests); -- 1.7.0.4