Re: [Xen-devel] [PATCH v2 4/6] xen/arm: cpufeature: Add helper to check constant caps

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 09/26/2018 05:53 PM, Stefano Stabellini wrote:
> On Tue, 25 Sep 2018, Julien Grall wrote:
> > Some capababilities are set right during boot and will never change
> > afterwards. At the moment, the function cpu_have_caps will check whether
> > the cap is enabled from the memory.
> >
> > It is possible to avoid the load from the memory by using an
> > ALTERNATIVE. With that the check is just reduced to 1 instruction.
> >
> > Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> I enjoyed reading the patch :-)  But I don't think it is worth going
> into this extreme level of optimization. test_bit is efficient enough,
> right? What do you think we need to use alternatives just to check one
> bit?
We already have an helper using test_bit (see cpus_have_cap). However 
test_bit requires to load the word from the memory. This is an overhead 
when the decision never change after boot.
One load may be insignificant by itself (thought may have a cache miss), 
but if you are in hotpath this is a win in long term.
The mechanism is very similar to static key but for the poor (I don't 
have much time to implement better for now). We already use similar 
construction on other places (see CHECK_WORKAROUND_HELPER).
Cheers,

--
Julien Grall

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