linux-kernel - Re: [PATCH 03/17] Unify vendor TSC logic

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Date:	Thu, 17 Jun 2010 16:03:33 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	Zachary Amsden <zamsden@...hat.com>, avi@...hat.com,
	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/17] Unify vendor TSC logic

Glauber Costa wrote:
> On Wed, Jun 16, 2010 at 04:10:10PM +0800, Jason Wang wrote:
>   
>> Zachary Amsden wrote:
>>     
>>>  
>>>  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>  {
>>> +	kvm_x86_ops->vcpu_load(vcpu, cpu);
>>>  	if (unlikely(vcpu->cpu != cpu)) {
>>> +		/* 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 || check_tsc_unstable())
>>>   
>>>       
>> It's better to do the adjustment also when tsc_delta > 0
>>     
> And why do you think so? Doing it on tsc_delta > 0 would force us to adjust
> at every entry but the first. And I guess we want to adjust as few times as 
> we can.
>
>   
This is not strange and is what current SVM code does. If we do not do
this, guest may see a jump in the value of TSC when tsc_delta > 0.
> For example, we would adjust on every cpu bounce even for machines that has
> a perfectly sync tsc. This could introduce an error not present before.
>
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>   

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