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

[Marc Zyngier]

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.

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

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