Re: [Xen-devel] [RFC 10/16] gic:vgic:gic-vgic: introduce non-atomic bitops

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrii,

On 03/12/2018 12:33, Andrii Anisov wrote:
> On 29.11.18 14:14, Andre Przywara wrote:
> > Nah, please don't do this.
> Sorry for making you crying looking at this code.
> It's terrible, I know. It's rather an idea.
>
> > Can you show that atomic bit ops are a
> > problem? They shouldn't be expensive unless contended, also pretty
> > lightweight on small systems (single cluster).
> Yep, but still it is a call to a function of 10 operations instead of one `orr` 
> for set_bit(). Taking in account a heavy usage of bitops in the old vgic code, 
> this should benefit latency.
That's micro optimizing Xen... there are better (and less risky) place to look 
for optimization.
Knowing how fragile the locking is on the old vGIC, the risk of micro-optimizing 
is not worth it. If you can provide number that shows you can improve 
performance by more than 10% in common case with this patch only, then I will 
reconsider my position.
Cheers,

--
Julien Grall

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