Re: [PATCH 09/14] KVM: PPC: Add generic single register ioctls

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-10-31 14:36, Avi Kivity wrote:
> On 10/31/2011 09:53 AM, Alexander Graf wrote:
>> Right now we transfer a static struct every time we want to get or set
>> registers. Unfortunately, over time we realize that there are more of
>> these than we thought of before and the extensibility and flexibility of
>> transferring a full struct every time is limited.
>>
>> So this is a new approach to the problem. With these new ioctls, we can
>> get and set a single register that is identified by an ID. This allows for
>> very precise and limited transmittal of data. When we later realize that
>> it's a better idea to shove over multiple registers at once, we can reuse
>> most of the infrastructure and simply implement a GET_MANY_REGS / SET_MANY_REGS
>> interface.
>>
>> The only downpoint I see to this one is that it needs to pad to 1024 bits
>> (hardware is already on 512 bit registers, so I wanted to leave some room)
>> which is slightly too much for transmitting only 64 bits. But if that's all
>> the tradeoff we have to do for getting an extensible interface, I'd say go
>> for it nevertheless.
> 
> Do we want this for x86 too?  How often do we want just one register?

On x86, a single register is probably only interesting for debugging
purposes. Things that matter (performance wise) are in kvm.run, anything
else does not worry that much about speed. At least for now. I'm still
waiting for Kemari to propose some get/set optimizations, but there is
obviously not that much to gain or still bigger fish to fry.

Also, x86 is less regular than PPC. And where it is fairly regular, we
already have a GET/SET_MANY interface: MSRs.

Jan

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