[RFC PATCH 0/3] Target CPU=Host implementation for KVM ARM/ARM64

[Anup Patel]

On Fri, Sep 6, 2013 at 4:04 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 2013-09-06 11:24, Alexander Graf wrote:
>>
>> On 06.09.2013, at 12:05, Anup Patel wrote:
>>
>>> On Fri, Sep 6, 2013 at 2:24 PM, Alexander Graf <agraf at suse.de> wrote:
>>>>
>>>>
>>>> On 06.09.2013, at 09:44, Anup Patel wrote:
>
>
> [...]
>
>
>>>>> Another advantage I saw in extending KVM_ARM_VCPU_INIT ioctl is
>>>>> backward compatibility with current semantics. In other words, this
>>>>> patch
>>>>> does not break current KVMTOOL/QEMU and they can implement
>>>>> "-cpu host" whenever they wish without using any additional ioctl.
>>>>
>>>>
>>>> It's the opposite actually. By making the ioctl parameter in/out
>>>> direction you change the ioctl number, breaking the ABI, no?
>>>
>>>
>>> Originally the ioctl was only "in" and so we are preserving the "in"
>>> semantics. Thats why it is semantically backward compatible.
>>
>>
>> Great. So now we have an ioctl that says it's "in" in its ioctl
>> descriptor, but really it's in/out. This really only works by accident
>> because nobody is filtering the direction today.
>>
>> Nack.
>
>
> Agreed. We don't break the ABI, we don't try to fool the kernel. Please.

We are not breaking the ABI here and also not trying to fool the kernel.

>
> There's been previous suggestions on how to implement this feature, please
> consider them.

I am not convinced about how is this approach not better.

>
>         M.
> --
> Fast, cheap, reliable. Pick two.

--Anup