Hi Marc, Thanks for the comment. On Fri, Feb 5, 2021 at 11:24 AM Marc Zyngier <maz@xxxxxxxxxx> wrote: > > Hi Jing, > > On 2021-02-05 04:44, Jing Zhang wrote: > > Remove redundant check for CPU feature S2FWB in dcache flush code > > to save some CPU cycles for every memslot flush and unmapping. > > What CPU cycles? This is only a static branch. Can you actually > measure the overhead? What does it represent in the face of > a full memslot unmapping? For CPU cycles, I mean CPU time spent for S2FWB check. For memslot unmapping, there is actually no improvement, just move the check to the stage2_unmap_walker since we removed the S2FWB check in stage2_flush_dcache. The saving is from the code path of memslot flush. The S2FWB check was in stage2_flush_dcache, in which case, for a memslot flush, the check was done for every page. Now it will save some CPU time if we do the check at a higher level, like in kvm_toggle_cache, access_dcsw, kvm_arch_prepare_memory_region. The redundant check is as follows (Only the first check is necessary): kvm_arch_prepare_memory_region -> S2FWB check -> stage2_flush_memslot -> kvm_pgtable_stage2_flush -> S2FWB check -> stage2_flush_walker -> S2FWB check -> __flush_dcache_area > > Thanks, > > M. > -- > Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
