[PATCH v2] arm/arm64: KVM: Properly account for guest CPU time

[Christoffer Dall]

On Mon, Jun 08, 2015 at 06:50:08PM +0100, Marc Zyngier wrote:
> Hi Christoffer,
> 
> On 28/05/15 19:49, Christoffer Dall wrote:
> > Until now we have been calling kvm_guest_exit after re-enabling
> > interrupts when we come back from the guest, but this has the
> > unfortunate effect that CPU time accounting done in the context of timer
> > interrupts occurring while the guest is running doesn't properly notice
> > that the time since the last tick was spent in the guest.
> > 
> > Inspired by the comment in the x86 code, move the kvm_guest_exit() call
> > below the local_irq_enable() call and change __kvm_guest_exit() to
> > kvm_guest_exit(), because we are now calling this function with
> > interrupts enabled.  We have to now explicitly disable preemption and
> > not enable preemption before we've called kvm_guest_exit(), since
> > otherwise we could be preempted and everything happening before we
> > eventually get scheduled again would be accounted for as guest time.
> > 
> > At the same time, move the trace_kvm_exit() call outside of the atomic
> > section, since there is no reason for us to do that with interrupts
> > disabled.
> > 
> > Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
> > ---
> > This patch is based on kvm/queue, because it has the kvm_guest_enter/exit
> > rework recently posted by Christian Borntraeger.  I hope I got the logic
> > of this right, there were 2 slightly worrying facts about this:
> > 
> > First, we now enable and disable and enable interrupts on each exit
> > path, but I couldn't see any performance overhead on hackbench - yes the
> > only benchmark we care about.
> > 
> > Second, looking at the ppc and mips code, they seem to also call
> > kvm_guest_exit() before enabling interrupts, so I don't understand how
> > guest CPU time accounting works on those architectures.
> > 
> > Changes since v1:
> >  - Tweak comment and commit text based on Marc's feedback.
> >  - Explicitly disable preemption and enable it only after kvm_guest_exit().
> > 
> >  arch/arm/kvm/arm.c | 21 +++++++++++++++++----
> >  1 file changed, 17 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> > index e41cb11..fe8028d 100644
> > --- a/arch/arm/kvm/arm.c
> > +++ b/arch/arm/kvm/arm.c
> > @@ -532,6 +532,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  		kvm_vgic_flush_hwstate(vcpu);
> >  		kvm_timer_flush_hwstate(vcpu);
> >  
> > +		preempt_disable();
> >  		local_irq_disable();
> >  
> >  		/*
> > @@ -544,6 +545,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  
> >  		if (ret <= 0 || need_new_vmid_gen(vcpu->kvm)) {
> >  			local_irq_enable();
> > +			preempt_enable();
> >  			kvm_timer_sync_hwstate(vcpu);
> >  			kvm_vgic_sync_hwstate(vcpu);
> >  			continue;
> > @@ -559,8 +561,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  		ret = kvm_call_hyp(__kvm_vcpu_run, vcpu);
> >  
> >  		vcpu->mode = OUTSIDE_GUEST_MODE;
> > -		__kvm_guest_exit();
> > -		trace_kvm_exit(kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu));
> > +		/*
> > +		 * Back from guest
> > +		 *************************************************************/
> > +
> >  		/*
> >  		 * We may have taken a host interrupt in HYP mode (ie
> >  		 * while executing the guest). This interrupt is still
> > @@ -574,8 +578,17 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> >  		local_irq_enable();
> >  
> >  		/*
> > -		 * Back from guest
> > -		 *************************************************************/
> > +		 * We do local_irq_enable() before calling kvm_guest_exit() so
> > +		 * that if a timer interrupt hits while running the guest we
> > +		 * account that tick as being spent in the guest.  We enable
> > +		 * preemption after calling kvm_guest_exit() so that if we get
> > +		 * preempted we make sure ticks after that is not counted as
> > +		 * guest time.
> > +		 */
> > +		kvm_guest_exit();
> > +		trace_kvm_exit(kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu));
> > +		preempt_enable();
> > +
> >  
> >  		kvm_timer_sync_hwstate(vcpu);
> >  		kvm_vgic_sync_hwstate(vcpu);
> > 
> 
> I've been thinking about this a bit more, and I wonder if we can
> simplify it a bit. At the moment, we disable the interrupts around the
> HYP entry. But now that you have introduced preempt_disable, it looks
> like we move the local_irq_disable call to be just after
> __kvm_guest_enter, and not bother with having such a long critical section.
> 
> This is possible because entering HYP mode automatically masks
> interrupts, and we restore PSTATE on exception return.
> 
> I think this would slightly reduce the amount of code we run on the host
> that gets accounted to the guest.
> 
> Thoughts?
> 
Isn't there a situation then where the guest can get stuck because we
don't properly check for signal handling?

As I recall, we have the lcoal_irq_disable() call before checking
signal_pending(), so that if, for example, another thread sends a signal
to that VCPU we either:
 (1) handle the IPI and see we have a signal pending, so we abort, or
 (2) don't handle the IPI because IRQs are disabled, enter the guest but
     soon as we run the guest the interrupt hits, we go back, see the
     signal and exit.

There was something like this which caused a guest to get stuck with
userspace timers on v7.

Am I making sense at all?

-Christoffer