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Date: Tue, 12 Dec 2017 16:28:39 +0100
From: Cornelia Huck <cohuck@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v1 for-2-12 06/15] s390x/flic: factor out
 injection of floating interrupts
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To: David Hildenbrand <david@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>, qemu-s390x@nongnu.org, qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>, Alexander Graf <agraf@suse.de>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Thomas Huth <thuth@redhat.com>

On Tue, 12 Dec 2017 16:17:17 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 12.12.2017 15:29, Cornelia Huck wrote:
> > On Tue, 12 Dec 2017 15:13:46 +0100
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >   
> >> On 12/12/2017 02:49 PM, Cornelia Huck wrote:  
> >   
> >>> One thing I noticed: You removed the caching of the flic (in the old
> >>> kvm inject routine), and you generally do more qom invocations (first,
> >>> to find the common flic; then, to translate to the qemu or kvm flic).
> >>> Not sure if this might be a problem (probably not).    
> >>
> >> Is any of these calls on a potential fast path (e.g. guest without adapter
> >> interrupts)? If yes, then QOM is a no-go since it is really slow.  
> > 
> > At least the new airq interface was using QOM without caching before.
> > 
> > It's basically about any interrupt; but otoh we are (for kvm) in
> > userspace already. Caching the flic and just keeping the casting to the
> > specialized flic might be ok (I'd guess that the lookup is the slowest
> > path.)
> >   
> 
> Please note that the lookup is already cached in s390_get_flic(); That
> should be sufficient, as it does the expensive lookup. One cache should
> be enough, no?

Ah, missed that. So the old code actually did double caching...

> 
> The other conversions should be cheap (and already were in place in a
> couple of places before).

Yes, object_resolve_path() is probably the most expensive one.

Did anyone ever check if the (existing) conversions are actually
measurable?