On Wed, Mar 15, 2017 at 09:05:28AM +0100, Paolo Bonzini wrote: > > > 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. > I'll try to prototype it and see how it looks. > > 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 > > Good point, and if it's for statistics, this light change in behavior can hardly constitute any form of user breakage. Thanks, -Christoffer
