On 09/06/15 15:43, Christoffer Dall wrote: > 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@xxxxxxxxxx> >>> --- >>> 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? Yup, I missed that crucial detail. Applying to queue. Thanks, M. -- Jazz is not dead. It just smells funny... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html