BACKPORT: arm/arm64: KVM: Properly account for guest CPU time

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.

BUG=chromium:846515
TEST=build/boot on hana with USE=kvm_host

Change-Id: I6d763052dcce0cfd84f5a1e72b19440489713204
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit 1b3d546daf85ed2bc9966e12cee3e6435fb65eca)
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
[SR: context from not having ccf73aaf5ad]
 Conflicts:
	arch/arm/kvm/arm.c
Reviewed-on: https://chromium-review.googlesource.com/1088115
Reviewed-by: Dylan Reid <dgreid@chromium.org>
1 file changed