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

[Alexander Graf <agraf@suse.de>]

On 11/10/2011 05:05 PM, Marcelo Tosatti wrote:
> On Mon, Oct 31, 2011 at 08:53:11AM +0100, 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.
>>
>> Signed-off-by: Alexander Graf<agraf@suse.de>
>> ---
>>   Documentation/virtual/kvm/api.txt |   47 ++++++++++++++++++++++++++++++++++
>>   arch/powerpc/kvm/powerpc.c        |   51 +++++++++++++++++++++++++++++++++++++
>>   include/linux/kvm.h               |   32 +++++++++++++++++++++++
>>   3 files changed, 130 insertions(+), 0 deletions(-)
> I don't see the benefit of this generalization, the current structure where
> context information is hardcoded in the data transmitted works well.

Well, unfortunately it doesn't work quite as well for us because we are 
a much more evolving platform. Also, there are a lot of edges and 
corners of the architecture that simply aren't implemented in KVM as of 
now. I want to have something extensible enough so we don't break the 
ABI along the way.


Alex