linux-kernel - Re: [PATCH 16/17] TSC reset compensation

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Date:	Wed, 16 Jun 2010 12:36:17 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	avi@...hat.com, mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] TSC reset compensation

On 06/16/2010 03:52 AM, Glauber Costa wrote:
> On Mon, Jun 14, 2010 at 09:34:18PM -1000, Zachary Amsden wrote:
>    
>> Attempt to synchronize TSCs which are reset to the same value.  In the
>> case of a reliable hardware TSC, we can just re-use the same offset, but
>> on non-reliable hardware, we can get closer by adjusting the offset to
>> match the elapsed time.
>>
>> Signed-off-by: Zachary Amsden<zamsden@...hat.com>
>> ---
>>   arch/x86/kvm/x86.c |   34 ++++++++++++++++++++++++++++++++--
>>   1 files changed, 32 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 8e836e9..cedb71f 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -937,14 +937,44 @@ static inline void kvm_request_guest_time_update(struct kvm_vcpu *v)
>>   	set_bit(KVM_REQ_CLOCK_SYNC,&v->requests);
>>   }
>>
>> +static inline int kvm_tsc_reliable(void)
>> +{
>> +	return (boot_cpu_has(X86_FEATURE_CONSTANT_TSC)&&
>> +		boot_cpu_has(X86_FEATURE_NONSTOP_TSC)&&
>> +		!check_tsc_unstable());
>> +}
>> +
>>      
> why can't we re-use vmware TSC_RELIABLE flag?
>    

It's only set for VMware.  Basically, it means "you are running in a 
VMware hypervisor, TSC is reliable".  Which KVM won't ever be, at least, 
not in production use, so it doesn't make that sort of sense here.  
Besides, a system with a reliable TSC can become a system without  a 
reliable TSC : CPU hotplug will always guarantee this.

We could, however, have the guest set the TSC_RELIABLE flag for itself 
if KVM somehow makes that promise (currently, it does not).

Zach
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