[PATCH v2] kvm: Fix accounting of interrupts during guest execution on s390

[Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>]

Avi pointed out, that my first patch was broken, here is the 2nd try.
I tested the patch on s390 with CONFIG_VIRT_CPU_ACCOUNTING and on x86_64.
Seems to work.

Currently the scheduler checks for PF_VCPU to decide if this
timeslice has to be accounted as guest time. On s390 host 
interrupts are not disabled during guest execution. This causes
theses interrupts to be accounted as guest time if
CONFIG_VIRT_CPU_ACCOUNTING is set.
Solution is to check if an interrupt triggered account_system_time.
As the tick is timer interrupt based, we have to subtract
hardirq_offset.

Avi, Ingo, Laurent, feedback is welcome.

CC: Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>
CC: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
CC: Laurent Vivier <Laurent.Vivier-6ktuUTfB/bM@public.gmane.org>
Signed-off-by: Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>
---
 kernel/sched.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

Index: kvm/kernel/sched.c
===================================================================
--- kvm.orig/kernel/sched.c
+++ kvm/kernel/sched.c
@@ -3395,10 +3395,8 @@ void account_system_time(struct task_str
 	struct rq *rq = this_rq();
 	cputime64_t tmp;
 
-	if (p->flags & PF_VCPU) {
-		account_guest_time(p, cputime);
-		return;
-	}
+	if ((p->flags & PF_VCPU) && (irq_count() - hardirq_offset == 0))
+		return account_guest_time(p, cputime);
 
 	p->stime = cputime_add(p->stime, cputime);
 

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