On Thu, Nov 23, 2017 at 06:12:41PM +0100, Paolo Bonzini wrote: > On 23/11/2017 18:06, Christoffer Dall wrote: > > On Thu, Nov 23, 2017 at 05:17:00PM +0100, Paolo Bonzini wrote: > >> On 23/11/2017 17:05, Christoffer Dall wrote: > >>> For example, > >>> arm64 is about to do significant work in vcpu load/put when running a > >>> vcpu, but not when doing things like KVM_SET_ONE_REG or > >>> KVM_SET_MP_STATE. > >> > >> Out of curiosity, in what circumstances are these ioctls a hot path? > >> Especially KVM_SET_MP_STATE. > > > > Perhaps my commit message was misleading; we only want to do that for > > KVM_RUN, and not for anything else. We're already doing things like > > potentially jumping to hyp mode and flushing VMIDs which really > > shouldn't be done unless we actually plan on running a VCPU, and we're > > going to do things like setting up the timer to handle timer interrupts > > in an ISR, which doesn't make sense unless the VCPU is running. > > > > Add to that, that loading an entire VM's state onto hardware, only to > > read back a single register from hardware and returning it to user > > space, doesn't really fall within optimization vs. non-optimization in > > the critical path, but is just wrong, IMHO. > > For GET/SET_ONE_REG it certainly makes sense. For everything else, I'm > wondering which ioctls (and how many calls to each of them) exactly you > are seeing, and also on which userspace paths. > Outside of migration, not many. It's not about optimizing certain ioctl's, but rather that I think it's wrong and potentially vulnerable to do significant work on the system which is strictly unnecessary. > > That doesn't solve my need as I want to *only* do the arch vcpu_load for > > KVM_RUN, I should have been more clear in the commit message. > > That's what you want to do, but it might not be what you need to do. > Well, why would we want to do a lot of work when there's absolutely no need to? I see that this patch is invasive, and that's why I originally proposed the other approach of recording the ioctl number. While it may be possible to call kvm_arch_vcpu_load() for a number of non-KVM_RUN ioctls, it makes the KVM/ARM code more difficult to reason about, especially after my optimization series, because a lot of things can now happen, where we have to consider if we're really in the process of running a vcpu or not. Thanks, -Christoffer -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
