On Fri, 27 Apr 2012 11:52:13 -0300 Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > Yes but the objective you are aiming for is to read and write sptes > without mmu_lock. That is, i am not talking about this patch. > Please read carefully the two examples i gave (separated by "example)"). The real objective is not still clear. The ~10% improvement reported before was on macro benchmarks during live migration. At least, that optimization was the initial objective. But at some point, the objective suddenly changed to "lock-less" without understanding what introduced the original improvement. Was the problem really mmu_lock contention? If the path being introduced by this patch is really fast, isn't it possible to achieve the same improvement still using mmu_lock? Note: During live migration, the fact that the guest gets faulted is itself a limitation. We could easily see noticeable slowdown of a program even if it runs only between two GET_DIRTY_LOGs. > The rules for code under mmu_lock should be: > > 1) Spte updates under mmu lock must always be atomic and > with locked instructions. > 2) Spte values must be read once, and appropriate action > must be taken when writing them back in case their value > has changed (remote TLB flush might be required). Although I am not certain about what will be really needed in the final form, if this kind of maybe-needed-overhead is going to be added little by little, I worry about possible regression. Thanks, Takuya -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
