linux-kernel - Re: [KVM timekeeping 12/35] Robust TSC compensation

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Date:	Mon, 23 Aug 2010 15:01:20 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 12/35] Robust TSC compensation

On 08/20/2010 07:40 AM, Glauber Costa wrote:
> On Thu, Aug 19, 2010 at 10:07:26PM -1000, Zachary Amsden wrote:
>    
>> Make the match of TSC find TSC writes that are close to each other
>> instead of perfectly identical; this allows the compensator to also
>> work in migration / suspend scenarios.
>>
>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>> ---
>>   arch/x86/kvm/x86.c |   14 ++++++++++----
>>   1 files changed, 10 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 52680f6..0f3e5fb 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -928,21 +928,27 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
>>   	struct kvm *kvm = vcpu->kvm;
>>   	u64 offset, ns, elapsed;
>>   	unsigned long flags;
>> +	s64 sdiff;
>>
>>   	spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
>>   	offset = data - native_read_tsc();
>>   	ns = get_kernel_ns();
>>   	elapsed = ns - kvm->arch.last_tsc_nsec;
>> +	sdiff = data - kvm->arch.last_tsc_write;
>> +	if (sdiff<  0)
>> +		sdiff = -sdiff;
>>
>>   	/*
>> -	 * Special case: identical write to TSC within 5 seconds of
>> +	 * Special case: close write to TSC within 5 seconds of
>>   	 * another CPU is interpreted as an attempt to synchronize
>> -	 * (the 5 seconds is to accomodate host load / swapping).
>> +	 * The 5 seconds is to accomodate host load / swapping as
>> +	 * well as any reset of TSC during the boot process.
>>   	 *
>>   	 * In that case, for a reliable TSC, we can match TSC offsets,
>> -	 * or make a best guest using kernel_ns value.
>> +	 * or make a best guest using elapsed value.
>>   	 */
>> -	if (data == kvm->arch.last_tsc_write&&  elapsed<  5ULL * NSEC_PER_SEC) {
>> +	if (sdiff<  nsec_to_cycles(5ULL * NSEC_PER_SEC)&&
>> +	    elapsed<  5ULL * NSEC_PER_SEC) {
>>   		if (!check_tsc_unstable()) {
>>      
> Isn't 5 way too long for this case?
>
>
>    

It was actually too short for a while, and I didn't realize why until I 
discovered on SVM, the APs were getting the TSC reset after the startup IPI.

In any case, the value is certainly up for debate.  I chose a large 
number because who knows how badly things can get off in the case of 
host overcommit / swapping.

Zach
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