linux-kernel - Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

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Date:	Tue, 14 Sep 2010 09:32:50 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Avi Kivity <avi@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

On 09/14/2010 12:40 AM, Jan Kiszka wrote:
> Am 14.09.2010 11:27, Avi Kivity wrote:
>    
>>    On 09/14/2010 11:10 AM, Jan Kiszka wrote:
>>      
>>> Am 20.08.2010 10:07, Zachary Amsden wrote:
>>>        
>>>> When CPUs with unstable TSCs enter deep C-state, TSC may stop
>>>> running.  This causes us to require resynchronization.  Since
>>>> we can't tell when this may potentially happen, we assume the
>>>> worst by forcing re-compensation for it at every point the VCPU
>>>> task is descheduled.
>>>>
>>>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>>>> ---
>>>>    arch/x86/kvm/x86.c |    2 +-
>>>>    1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>> index 7fc4a55..52b6c21 100644
>>>> --- a/arch/x86/kvm/x86.c
>>>> +++ b/arch/x86/kvm/x86.c
>>>> @@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>>    	}
>>>>
>>>>    	kvm_x86_ops->vcpu_load(vcpu, cpu);
>>>> -	if (unlikely(vcpu->cpu != cpu)) {
>>>> +	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
>>>>    		/* Make sure TSC doesn't go backwards */
>>>>    		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
>>>>    				native_read_tsc() - vcpu->arch.last_host_tsc;
>>>>          
>>> For yet unknown reason, this commit breaks Linux guests here if they are
>>> started with only a single VCPU. They hang during boot, obviously no
>>> longer receiving interrupts.
>>>
>>> I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a side
>>> effect of the wrapping, though I cannot imagine how.
>>>
>>> Anyone any ideas?
>>>
>>>
>>>        
>> Most likely, time went backwards, and some 'future - past' calculation
>> resulted in a negative sleep value which was then interpreted as
>> unsigned and resulted in a 2342525634 year sleep.
>>      
> Looks like that's the case on first glance at the apic state.
>    

This compensation effectively nulls the delta between current and last TSC:

         if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
                 /* Make sure TSC doesn't go backwards */
                 s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
                                 native_read_tsc() - 
vcpu->arch.last_host_tsc;
                 if (tsc_delta < 0)
                         mark_tsc_unstable("KVM discovered backwards TSC");
                 if (check_tsc_unstable())
                         kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
                 kvm_migrate_timers(vcpu);
                 vcpu->cpu = cpu;

If TSC has advanced quite a bit due to a TSC jump during sleep(*), it 
will adjust the offset backwards to compensate; similarly, if it has 
gone backwards, it will advance the offset.

In neither case should the visible TSC go backwards, assuming 
last_host_tsc is recorded properly, and so kvmclock should be similarly 
unaffected.

Perhaps the guest is more intelligent than we hope, and is comparing two 
different clocks: kvmclock or TSC with the rate of PIT interrupts.  This 
could result in negative arithmetic begin interpreted as unsigned.  Are 
you using PIT interrupt reinjection on this guest or passing 
-no-kvm-pit-reinjection?

>    
>> Does your guest use kvmclock, tsc, or some other time source?
>>      
> A kernel that has kvmclock support even hangs in SMP mode. The others
> pick hpet or acpi_pm. TSC is considered unstable.
>    

SMP mode here has always and will always be unreliable.  Are you running 
on an Intel or AMD CPU?  The origin of this code comes from a workaround 
for (*) in vendor-specific code, and perhaps it is inappropriate for both.

Zach
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