From 9c23a1318eb12fcf76d9f663d2c3d88598e62a55 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Wed, 17 Feb 2016 21:53:33 +0100 Subject: KVM: s390: protect VCPU cpu timer with a seqcount For now, only the owning VCPU thread (that has loaded the VCPU) can get a consistent cpu timer value when calculating the delta. However, other threads might also be interested in a more recent, consistent value. Of special interest will be the timer callback of a VCPU that executes without having the VCPU loaded and could run in parallel with the VCPU thread. The cpu timer has a nice property: it is only updated by the owning VCPU thread. And speaking about accounting, a consistent value can only be calculated by looking at cputm_start and the cpu timer itself in one shot, otherwise the result might be wrong. As we only have one writing thread at a time (owning VCPU thread), we can use a seqcount instead of a seqlock and retry if the VCPU refreshed its cpu timer. This avoids any heavy locking and only introduces a counter update/check plus a handful of smp_wmb(). The owning VCPU thread should never have to retry on reads, and also for other threads this might be a very rare scenario. Please note that we have to use the raw_* variants for locking the seqcount as lockdep will produce false warnings otherwise. The rq->lock held during vcpu_load/put is also acquired from hardirq context. Lockdep cannot know that we avoid potential deadlocks by disabling preemption and thereby disable concurrent write locking attempts (via vcpu_put/load). Reviewed-by: Christian Borntraeger Signed-off-by: David Hildenbrand Signed-off-by: Christian Borntraeger --- arch/s390/include/asm/kvm_host.h | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'arch/s390/include/asm/kvm_host.h') diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h index 91796dd2a8ec..d61e64555938 100644 --- a/arch/s390/include/asm/kvm_host.h +++ b/arch/s390/include/asm/kvm_host.h @@ -20,6 +20,7 @@ #include #include #include +#include #include #include #include @@ -553,6 +554,13 @@ struct kvm_vcpu_arch { unsigned long pfault_select; unsigned long pfault_compare; bool cputm_enabled; + /* + * The seqcount protects updates to cputm_start and sie_block.cputm, + * this way we can have non-blocking reads with consistent values. + * Only the owning VCPU thread (vcpu->cpu) is allowed to change these + * values and to start/stop/enable/disable cpu timer accounting. + */ + seqcount_t cputm_seqcount; __u64 cputm_start; }; -- cgit v1.2.3-55-g7522