From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Mon, 18 Jul 2016 07:17:58 +0000
Subject: Re: [PATCH V2 4/5] kvm/stats: Add provisioning for 64-bit vcpu statistics
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To: Suraj Jitindar Singh <sjitindarsingh@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>, David Matlack <dmatlack@google.com>
Cc: linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org, mpe@ellerman.id.au, paulus@samba.org, benh@kernel.crashing.org, kvm list <kvm@vger.kernel.org>, agraf@suse.com, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>

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?