Re: [Qemu-devel] [RFC PATCH 1/1] s390x/css: unresrict cssids

[Halil Pasic]

On 11/27/2017 05:56 PM, Cornelia Huck wrote:
> On Mon, 27 Nov 2017 16:09:09 +0100
> Boris Fiuczynski <address@hidden> wrote:
> 
>> On 11/27/2017 03:13 PM, Halil Pasic wrote:
>>>
>>>
>>> On 11/27/2017 02:19 PM, Cornelia Huck wrote:  
>>>> On Mon, 27 Nov 2017 14:11:57 +0100
>>>> Halil Pasic <address@hidden> wrote:
>>>>  
>>>>> On 11/27/2017 01:56 PM, Cornelia Huck wrote:  
>>>>>> On Fri, 24 Nov 2017 17:39:04 +0100
>>>>>> Halil Pasic <address@hidden> wrote:
>>>>>>      
>>>>>>> On 11/24/2017 05:15 PM, Cornelia Huck wrote:  
>>>>  
>>>>>>>> (Unless we simply make this a "default cssid" prop after all - then it
>>>>>>>> would be more than just a simple indication for libvirt...)
>>>>>>>>        
>>>>>>>
>>>>>>> We are now talking about the "cssid-unrestricted" property. The default
>>>>>>> cssid is not something I would like to do any time soon.  
>>>>>>
>>>>>> What's so bad about this? As said above, I think it would be much more
>>>>>> useful. If libvirt can detect r/o vs. r/w for properties, we can simply
>>>>>> start out with a r/o variant now...
>>>>>>      
>>>>>
>>>>> I'm not sure I understand you. Are you proposing the following:
>>>>> Drop the restriction, but don't indicate this via a read only
>>>>> "cssid-unrestricted" device property but via a "default-css"
>>>>> read only machine property.
>>>>>
>>>>> Libvirt then should know that if "default-css" is present then
>>>>> we don't have this virtual into 0xfe and non virtual into 0xfd
>>>>> restriction any more.  
>>>>
>>>> Yes.
>>>>  
>>>
>>> @Boris:
>>> Does this work for you? Technically it's equivalent to having
>>> "cssid-unrestricted" at machine level.
>>>
>>> I don't really like the idea of indicating unrestricted via
>>> something mostly unrelated (at least for me it ain't obvious that
>>> having the id of the default css exposed means we got rid of the
>>> limitation.). But I'm tied of discussing this, so I'm willing to
>>> compromise.
>>>
>>> Halil
>>>   
>> I agree with Christian that we should not throw the "setting of a 
>> default cssid" together with "unrestricted cssid" and than use read/only 
>> and read/write to distinguish between the two!
>>
> 
> OK, let's step back and summarize a bit.
> 
> Current situation:
> - virtual devices must go into 0xfe, non-virtual devices must go into
>   non-0xfe
> - 0xfe is the default cssid; non-mcss-e OSs see only devices in there
> - to expose non-virtual devices to today's guests, you need to use the
>   squash-mcss parameter, which tries to put non-virtual devices into
>   the same place in css 0xfe
> 
> This is problematic in various ways (potential for incomprehensible
> errors, amongst others), so we agreed that the way to go is to drop the
> virtual-vs-non-virtual cssid restrictions and allow any device in any
> css image.
> 
> Now, we need a way for libvirt to detect this, so that it knows that it
> can put e.g. vfio-ccw devices in the same css image as virtio devices.
> 
> Proposal 1: Use a device attribute to show that the device can be put
> anywhere. This approach feels wrong to me (the non-restriction is not a
> property of the individual device, but of the whole css resp. the
> machine), so I would continue to veto this.
> 
> Proposal 2: Export the default cssid as a machine property. If this
> property exists, it also implies that devices can be put into any css
> image (although it makes the most sense to put them into the default
> css image as indicated by the property). Can be made r/w later if it is
> too much for 2.12.
> 
> Proposal 3: Add a machine property that indicates cssids are not
> restricted. Later, optionally add a second property that exposes the
> default cssid and makes it configurable.
> 
> Personally, I prefer 2 (especially as this also allows to find out
> where the best place to put devices for non-mcss-e enabled guests is),
> but I could live with 3 as well (if making this r/w later would be
> problematic for libvirt.)
> 
> (In every case, we would deprecate and later remove the squash
> parameter.)
> 
> [As an aside, how hard is it to make the default cssid configurable? At
> a glance, it does not seem too bad to me.]
> 

Will try to spin a patch based on 3 tomorrow as that seems closest
to consensus. One more thing included will be this auto-generated
change.

Halil