Re: vtime accounting

[Christoffer Dall <cdall@linaro.org>]

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