Re: [PATCH 05/11] KVM: s390: Support Configuration z/Architecture Mode

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 08/28/2017 04:38 PM, David Hildenbrand wrote:
> On 28.08.2017 16:24, Christian Borntraeger wrote:
>>
>>
>> On 08/28/2017 04:06 PM, David Hildenbrand wrote:
>>> On 28.08.2017 10:07, Christian Borntraeger wrote:
>>>> From: "Jason J. Herne" <jjherne@linux.vnet.ibm.com>
>>>>
>>>> kvm has always supported the concept of starting in z/Arch mode so let's
>>>> reflect the feature bit to the guest.
>>>>
>>>> Also, we change sigp set architecture to reject any request to change
>>>> architecture modes.
>>>
>>> Hm ... this seems to imply that CZAM is always set, but what about
>>> running on old user space (possibly on old hw)? Old QEMU will not enable
>>> CZAM.
>>
>> 3 cases.
>> 1. very old QEMU without user sigp
>> 2. old QEMU with user sigp/without CPU model
>> 3. new QEMU with user sigp/cpu model
>>
>> I think we agree that cases 2 and 3 should not matter at all for this kernel patch
>> as the sigp is handled by QEMU. 
>>
>>
>>
>> This is case 1:
>>> And especially old user space will rely on SET ARCHITECTURE being
>>> handled in the kernel.
>>
>>
>> Yes, and it continues to be handled in the kernel. It is just that the guest
>> will now see a different sigp return code. Before, our sigp implementation lied
>> to the guest in a way that worked for Linux (we lied by saying "yes, we switched"). 
>> We now say "sorry, we are already in zarch mode, sigp ignored" which also works
>> perfectly fine for Linux. And IMHO it is even the better choice even without 
>> STFLE.138 being set as it matches what an old hardware would do when in zarch mode.
> 
> Ok, if it worked for relevant Linux versions, than it should indeed be fine.

I checked back to 2006 and Linux never checked the return value for this sigp.