On Wed, 26 Sep 2018, Julien Grall wrote: > 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.gr...@arm.com> > > > > 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).
I like the mechanism and the little ALTERNATIVE trick. I can see it could very useful in some cases. It is just that it is a bit of a waste to use it on SMCCC virtualization -- SMCCC calls cannot be done often enough for this to make a difference. But who am I to complain when you make things faster? :-) _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
