Re: [PATCH 0/9] S390x KVM support

[Avi Kivity <avi@redhat.com>]

On 10/22/2009 11:55 AM, Alexander Graf wrote:
>
> On 22.10.2009, at 11:53, Avi Kivity wrote:
>
>> On 10/22/2009 11:11 AM, Alexander Graf wrote:
>>>> Doesn't this break backward compatibility by changing the structure?
>>>>
>>>> Best to put it after the union (and as a copy, so userspace that 
>>>> expects the previous location still works).  If you're reading it 
>>>> from the kernel, also need a way to tell the kernel which copy to 
>>>> read from.
>>>>
>>>> Also advertise with a KVM_CAP.
>>>
>>>
>>> I don't think we need to go through the hassle here. There is 
>>> effectively no user of that code for now and the ABI is considered 
>>> unstable.
>>
>> At the very least we need a KVM_CAP so qemu knows to fail on older 
>> kernels.
>
> Hm. Oh well :-). It can't hurt to have yet another CAP, right?
>
>>>> I'd also appreciate an explanation of what this is all about.
>>>
>>> Explanation in the code or explanation in an email reply?
>>
>>
>> email.  I assume s390 hackers would understand why the psw needs to 
>> be exposed to qemu on every exit.  This is mostly for my personal 
>> interest.
>
> PSW = (eflags << 32) | pc;
>
> :-)
>
> Before that patch it was only synced with the "vmcb" on special 
> userspace handled intercepts, now it's synced on every exit to userspace.

Right, but why?  x86 qemu doesn't care about either pc or eflags (with 
in-kernel irqchip, which s390 essentially is).

-- 
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