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

From: Cornelia Huck
Subject: Re: [Qemu-devel] [RFC PATCH 1/1] s390x/css: unresrict cssids
Date: Wed, 22 Nov 2017 17:25:22 +0100

On Wed, 22 Nov 2017 15:45:56 +0100
Boris Fiuczynski <address@hidden> wrote:

> On 11/22/2017 01:13 PM, Cornelia Huck wrote:
> >>>>>> +    object_class_property_add_bool(klass, "cssid-unrestricted",
> >>>>>> +                                   prop_get_true, NULL, NULL);  
> >>>>> This looks really, really strange. This is a property that is always
> >>>>> true if it exists.
> >>>>>        
> >>>> Won't argue about that. The libvirt guys are actually not interested
> >>>> int he value at all: only that there is such a property.  
> >>> So what about a machine property? Wouldn't that help as well?
> >>>      
> >> Technically it is doable. The property would be still a weird
> >> one, but from QEMU perspective in a less weird place. I was also
> >> arguing that from OO perspective this kind of a don't care about
> >> it's value property is weird, but AFAIK not having the info if
> >> we can do something with a device at the device is weird from
> >> Libvirt perspective. I'm really uncomforble with speaking for
> >> Libvirt/Boris. Hope he will make his point tomorrow.
> >>  
> >>> Or a css object?
> >>>      
> >> My first internal-only version used this approach, but
> >> I was asked to do it like this.  
> > If we formulate this rather as "we now expose the default css", we (a)
> > have something that really makes the most sense at a channel subsystem
> > or machine level, and (b) can be detected by libvirt as an indicator of
> > "no restriction for virtual vs. non-virtual".  
> I think that there are good reasons for using a device property as well 
> as for using a machine property. Technically both options are possible 
> (even for libvirt :) without too much rope...). At first my personal 
> choice was to express the changed behavior/capability on the device 
> level since that is what a user defines on the command line and also 
> where he gets restricted to use a css matching his device type.
>  From the libvirt perspective we would have supported vfio-ccw devices 
> only if the vfio-ccw would be allowed to be defined with unrestricted 
> cssids.

Thanks, I can now understand where that property came from.

> As I wrote before I also understand the reasoning to express the channel 
> subsystem wide changed behavior as a machine option. In that case 
> libvirt would need to check that both capabilities (vfio-ccw and machine 
> option cssid-unrestricted) are set and not just 
> vfio-cww.cssid-unrestricted. All doable. A qemu command line user would 
> obviously need to know the correlation of the machine option and the ccw 
> devices but that certainly is also nothing new.
> When talking to Christian and Halil I started to favor the idea of a new 
> css object especially when thinking about the future in which the user 
> might want to specify the default css. It for sure would also be able to 
> be set with the use of a machine option but a css object would at that 
> point be much a nicer and more capable approach. What do you think?

I think exposing the css object and making the default_css property
available is the cleanest solution (especially if we want more css
properties in the future). The only drawback I see is that it needs
more coding (and probably care to keep it backwards compatible.)

Halil, do you think you can come up with something?

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