Re: [PATCH v2 RFC] KVM: s390/interrupt: do not pin adapter interrupt pages

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On Wed, 12 Feb 2020 13:44:53 +0100
Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:

> On 12.02.20 13:39, Cornelia Huck wrote:
> [...]
> 
> >> +	 */
> >> +	return 0;  
> > 
> > Given that this function now always returns 0, we basically get a
> > completely useless roundtrip into the kernel when userspace is trying
> > to setup the mappings.
> > 
> > Can we define a new IO_ADAPTER_MAPPING_NOT_NEEDED or so capability that
> > userspace can check?  
> 
> Nack. This is one system call per initial indicator ccw. This is so seldom
> and cheap that I do not see a point in optimizing this. 

NB that zpci also calls this. Probably a rare event there as well.

> 
> 
> > This change in behaviour probably wants a change in the documentation
> > as well.  
> 
> Yep. 




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