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Re: [qemu-s390x] [PATCH v1 for-2-12 06/15] s390x/flic: factor out inject

From: Christian Borntraeger
Subject: Re: [qemu-s390x] [PATCH v1 for-2-12 06/15] s390x/flic: factor out injection of floating interrupts
Date: Wed, 13 Dec 2017 10:16:34 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 12/12/2017 04:28 PM, Cornelia Huck wrote:
> On Tue, 12 Dec 2017 16:17:17 +0100
> David Hildenbrand <address@hidden> wrote:
>> On 12.12.2017 15:29, Cornelia Huck wrote:
>>> On Tue, 12 Dec 2017 15:13:46 +0100
>>> Christian Borntraeger <address@hidden> 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?

Did some quick measurement.
the initial object resolve takes about 20000ns, the cached ones then is 2-5ns.
(measuring s390_get_flic function).

The other conversions (S390FLICStateClass *fsc = 
takes about 15-25ns (up to 100 cycles). 
For irqfd users this does not matter, but things like plan9fs might benefit
if we speedup this lookup as well?

PS: We can improve the initial object_resolve_path by doing the resolve not for
"/machine/" TYPE_KVM_S390_FLIC
(2500ns instead of 20000)
but its still way too slow.

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