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Subject: Re: [PATCH V2 4/5] kvm/stats: Add provisioning for 64-bit vcpu
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To: Paolo Bonzini <pbonzini@redhat.com>,
 Christian Borntraeger <borntraeger@de.ibm.com>,
 David Matlack <dmatlack@google.com>
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From: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
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Date: Tue, 19 Jul 2016 11:31:36 +1000
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On 18/07/16 18:24, Paolo Bonzini wrote:
>
> On 18/07/2016 09:17, Christian Borntraeger wrote:
>> On 07/15/2016 09:52 AM, Suraj Jitindar Singh wrote:
>>>
>>> On 14/07/16 19:42, Paolo Bonzini wrote:
>>>> On 13/07/2016 20:00, Christian Borntraeger wrote:
>>>>>>>>> I thought u64 still existed on 32-bit architectures. unsigned long
>>>>>>>>> would be fine but with the caveat that certain stats would overflow on
>>>>>>>>> 32-bit architectures.
>>>>>>> Yes, but not all 32-bit architectures can do atomic read-modify-write
>>>>>>> (e.g. add) operations on 64-bit values.
>>>>> So what about only doing it for the VCPU events? Those should be only
>>>>> modified by one CPU. We would have some odd values on 32bit overflow, but
>>>>> this will be certainly better than just start with 0
>>>> If that's good enough for PPC, that's fine.
>>>>
>>>> Paolo
>>> I'm don't feel great about having vcpu_stats as u64 and vm_stats still as u32
>>> it's just a bit inconsistent.
>>>
>>> That being said, it's only the vcpu_stats which I require to be u64 at this
>>> stage so it's possible to just upgrade those.
>> Yes, its not nice, but we probably want to avoid the overhead of atomics.
>> What about using u64 for vcpu_stats and unsigned long for vm_stats. This will be
>> correct for anyone and on 64bit systems we get 64 bits for everything?
> That makes sense.
>
> Paolo

Sound good, I am happy with this.