On Tue, Mar 14, 2017 at 06:09:45PM +0100, Paolo Bonzini wrote: > > > On 14/03/2017 17:58, Radim Krčmář wrote: > >> I assume there's a good reason why we call guest_enter() and > >> guest_exit() in the hot path on every KVM architecture? > > I consider myself biased when it comes to jiffies, so no judgement. :) > > > > From what I see, the mode switch is used only for statistics. > > vtime is only for statistics, but guest_enter/exit are important because > they enter an RCU extended quiescent state. This means that (physical) > CPUs running a guest are effectively "off" from the point of view of the > RCU accounting machinery. Not having to perform any RCU work is very > good for jitter. > 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. For example, when servicing a VM from KVM, should we really be accounting this as kernel time, or as guest time? 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? Thanks, -Christoffer
