On 14/03/2017 19:41, Christoffer Dall wrote: > So would it be worth considering factoring out vtime accounting from > guest_enter/exit, such that we could do the vtime accounting from vcpu > load/put and mark the RCU extended quiescent state in the run loop? > > Disclaimer: I haven't completely convinced myself that vtime accounting > from load/put works as it should. Me neither, but if it works, it's worth at least benchmarking it to make an informed decision. > For example, when servicing a VM from > KVM, should we really be accounting this as kernel time, or as guest > time? Servicing the request should be fast enough that it doesn't matter. For the most common x86 vmexits, 50% of the time is spent executing the x86 world switch microcode, and that is already being accounted as guest time already. On microbenchmarks of course it would make a difference: $ time ./x86/run x86/vmexit.flat -append cpuid -smp 4 [...] real 0m1.327s user 0m1.877s << this is really guest time sys 0m1.844s << this is spent mostly in KVM_RUN Paolo > I think we do the former now, but if the latter is the right > thing, would changing the behavior constitute an ABI change to > userspace?
