linux-kernel - Re: [PATCH 04/17] Fix deep C-state TSC desynchronization

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Date:	Mon, 14 Jun 2010 22:14:02 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	mtosatti@...hat.com, glommer@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/17] Fix deep C-state TSC desynchronization

On 06/14/2010 10:09 PM, Avi Kivity wrote:
> On 06/15/2010 10:34 AM, 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 |   11 +++++++++++
>>   1 files changed, 11 insertions(+), 0 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index c8289d0..618c435 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>   {
>>       kvm_x86_ops->vcpu_put(vcpu);
>>       kvm_put_guest_fpu(vcpu);
>> +
>>       vcpu->arch.last_host_tsc = native_read_tsc();
>> +
>> +    /*
>> +     * When potentially leaving a CPU with unstable TSCs, we risk
>> +     * that the CPU enters deep C-state.  If it does, the TSC may
>> +     * go out of sync but we will not recalibrate because the test
>> +     * vcpu->cpu != cpu can not detect this condition.  So set
>> +     * vcpu->cpu = -1 to force the recalibration above.
>> +     */
>> +    if (check_tsc_unstable())
>> +        vcpu->cpu = -1;
>>   }
>
> That will cause us to miss a vmclear later on.  Also it invalidates an 
> invariant that the per-cpu list vcpus_on_cpu has all the vcpus with 
> vcpu->cpu == cpu on this cpu (try saything that fast).
>

Blasted SVM / VMX differences!  There are other, less elegant ways to 
accomplish this task however.

Good catch, BTW.
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