Date:	Wed, 16 Jun 2010 09:36:25 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
CC:	avi@...hat.com, mtosatti@...hat.com, glommer@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock

On 06/15/2010 10:11 PM, Jason Wang wrote:
> Zachary Amsden wrote:
>    
>> 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).
>>
>>      
> This is one issue of kvmclock which tries to supply a clocksource whose
> precision may even higher than host.
>    
>> 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@...hat.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h |    4 ++
>>   arch/x86/kvm/x86.c              |   79 +++++++++++++++++++++++++++++++++------
>>   2 files changed, 71 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 1afecd7..7ec2472 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -338,6 +338,8 @@ struct kvm_vcpu_arch {
>>   	struct page *time_page;
>>   	u64 last_host_tsc;
>>   	u64 last_host_ns;
>> +	u64 last_guest_tsc;
>> +	u64 last_kernel_ns;
>>
>>   	bool nmi_pending;
>>   	bool nmi_injected;
>> @@ -455,6 +457,8 @@ 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 52d7d34..703ea43 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -138,6 +138,8 @@ 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) },
>> @@ -927,33 +929,84 @@ 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, max_kernel_ns;
>> +	u64 tsc_timestamp;
>>
>>   	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.
>> + 	 */
>> +	max_kernel_ns = 0;
>> +	if (vcpu->hv_clock.tsc_timestamp) {
>> +		max_kernel_ns = vcpu->last_guest_tsc -
>>
>>      
> Since you do the comparison with kernel_ns, so what you need here is
> tsc_timestamp which looks more like the 'last' tsc seen by guest.
>    

What this is computing is the highest bootbased nanosecond time value 
potentially seen by the guest:

last_guest_tsc - hv_clock.tsc_timestamp  is the maximum cycle offset the 
guest has seen against the last version of kvmclock.

Then it is scaled and added to the last_kernel_ns value used for 
hv_clock.  I chose to cache vcpu->last_kernel_ns separately from 
hv_clock so that kvmclock_offset can not change in the meantime, so the 
value deliberately discounts kvmclock_offset.

>> +				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);
>> +		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 (max_kernel_ns>  kernel_ns) {
>>
>>      
> Both max_kernel_ns and kernel_ns are not adjusted by kvmclock_offset, so
> this comparing is not safe after migration.
>    

They are deliberately not adjusted by kvmclock_offset, so they are 
simply scalar bootbased nanosecond values, not kvmclock_offset values.  
Thus, we can add, subtract and take maximums of them without worrying 
about kvmclock_offset at all.

>> +		s64 overshoot = max_kernel_ns - kernel_ns;
>> +		++v->stat.tsc_ahead;
>> +		if (overshoot>  NSEC_PER_SEC / HZ) {
>> +			++v->stat.tsc_overshoot;
>> +			if (printk_ratelimit())
>> +				pr_debug("ns overshoot: %lld\n", overshoot);
>> +		}
>> +		kernel_ns = max_kernel_ns;
>> +	}
>>
>>      
> A tsc_behind or something like this would make the problem more clear,
> and tsc_ahead should be zero when host using tsc as its clocksource.
>    
>>
>>   	/* 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;
>> +	vcpu->last_kernel_ns = kernel_ns;
>>
>>   	vcpu->hv_clock.flags = 0;
>>
>> @@ -4836,6 +4889,8 @@ 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);
>>
>>      
> This could be dropped since it does not take the time of guest execution
> into account.
>    

This is required is for the maximum cycle offset above.
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