Re: exit timing analysis v1 - comments&discussions welcome

[Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>]

Hollis Blanchard wrote:
> Also, it looks like we use the generic find_first_bit(). That may be
> more expensive than we'd like. However, since
> vcpu->arch.pending_exceptions is a single long (not an arbitrary sized
> bitfield), we should be able to use ffs() instead, which has an
> optimized PowerPC implementation. That might help a lot.
>
> We might even be able to replace find_next_bit() too, by shifting a mask
> over each loop, but I don't think we'll have to, since I expect the
> common case to be we can deliver the first pending exception. (Worth
> checking? :)
>   
FFS for first one is working fine.
To check if modifying the following Find next bit to mask&__ffs I did 
some quick accounting and it seems not to be worth.

On the exits casued by external interrupts we hit "find next bit" in 
~50% of the cases (MMIO, DCR, DEC, EXITINT).
But on all the frequent cases like DTLVVIRT, ITLBVIRT and EMULINST we 
are at ~2.75%.
Therefore it should be ok to keep the generic find_next_bit there.

-- 

Grüsse / regards, 
Christian Ehrhardt
IBM Linux Technology Center, Open Virtualization

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