Re: RFC: kvmclock / tsc server side fix

[Glauber Costa <glommer@redhat.com>]

On Fri, May 14, 2010 at 04:07:43PM -1000, Zachary Amsden wrote:
> I believe this fixes the root cause of the kvmclock warp.  It's
> quite a plausible phenomenon, and explains why it was so easy to
> produce.
> 
You mean this is the case for both SMP and UP, or just UP as we talked
before?

I don't get the role of upscale in your patch. Frequency changes are
already handled by the cpufreq notifier.

> Currently it depends on some other patches; I can send a whole
> patchset, but with all the patch activity, it isn't clear what has
> been applied and to what trees.  Where have Glauber's recent patches
> been applied?
> 
> I am looking for comments if this is a reasonably good explanation
> and fix for the problem.
> 
> I realize I messed up the overshoot calculation, it is not converted
> to nsec, but the debug stats are just for debugging.
> 
> Thanks,
> 
> Zach

> commit 24e1f31a4cdb43a8e5cab6cfb95d710c7c7bf18a
> Author: Zachary Amsden <zamsden@redhat.com>
> 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 <zamsden@redhat.com>
> 
> 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