Re: [PATCH V2 4/5] kvm/stats: Add provisioning for 64-bit vcpu statistics

On 07/15/2016 09:52 AM, Suraj Jitindar Singh wrote:

On 14/07/16 19:42, Paolo Bonzini wrote:

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On 13/07/2016 20:00, Christian Borntraeger wrote:

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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?