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authorDavid Hildenbrand2016-02-17 21:53:33 +0100
committerChristian Borntraeger2016-03-08 13:57:53 +0100
commit9c23a1318eb12fcf76d9f663d2c3d88598e62a55 (patch)
tree54451983c32ccdc6e28bfa58f69f0d5ebd1438f6 /arch/s390/kvm/kvm-s390.c
parentKVM: s390: step VCPU cpu timer during kvm_run ioctl (diff)
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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 <borntraeger@de.ibm.com> Signed-off-by: David Hildenbrand <dahi@linux.vnet.ibm.com> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Diffstat (limited to 'arch/s390/kvm/kvm-s390.c')
-rw-r--r--arch/s390/kvm/kvm-s390.c30
1 files changed, 22 insertions, 8 deletions
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 76b99149dc65..38223c4603c7 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1435,15 +1435,19 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
static void __start_cpu_timer_accounting(struct kvm_vcpu *vcpu)
{
WARN_ON_ONCE(vcpu->arch.cputm_start != 0);
+ raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
vcpu->arch.cputm_start = get_tod_clock_fast();
+ raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
}
/* needs disabled preemption to protect from TOD sync and vcpu_load/put */
static void __stop_cpu_timer_accounting(struct kvm_vcpu *vcpu)
{
WARN_ON_ONCE(vcpu->arch.cputm_start == 0);
+ raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
vcpu->arch.sie_block->cputm -= get_tod_clock_fast() - vcpu->arch.cputm_start;
vcpu->arch.cputm_start = 0;
+ raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
}
/* needs disabled preemption to protect from TOD sync and vcpu_load/put */
@@ -1480,28 +1484,37 @@ static void disable_cpu_timer_accounting(struct kvm_vcpu *vcpu)
void kvm_s390_set_cpu_timer(struct kvm_vcpu *vcpu, __u64 cputm)
{
preempt_disable(); /* protect from TOD sync and vcpu_load/put */
+ raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
if (vcpu->arch.cputm_enabled)
vcpu->arch.cputm_start = get_tod_clock_fast();
vcpu->arch.sie_block->cputm = cputm;
+ raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
preempt_enable();
}
/* update and get the cpu timer - can also be called from other VCPU threads */
__u64 kvm_s390_get_cpu_timer(struct kvm_vcpu *vcpu)
{
+ unsigned int seq;
__u64 value;
- int me;
if (unlikely(!vcpu->arch.cputm_enabled))
return vcpu->arch.sie_block->cputm;
- me = get_cpu(); /* also protects from TOD sync and vcpu_load/put */
- value = vcpu->arch.sie_block->cputm;
- if (likely(me == vcpu->cpu)) {
- /* the VCPU itself will always read consistent values */
- value -= get_tod_clock_fast() - vcpu->arch.cputm_start;
- }
- put_cpu();
+ preempt_disable(); /* protect from TOD sync and vcpu_load/put */
+ do {
+ seq = raw_read_seqcount(&vcpu->arch.cputm_seqcount);
+ /*
+ * If the writer would ever execute a read in the critical
+ * section, e.g. in irq context, we have a deadlock.
+ */
+ WARN_ON_ONCE((seq & 1) && smp_processor_id() == vcpu->cpu);
+ value = vcpu->arch.sie_block->cputm;
+ /* if cputm_start is 0, accounting is being started/stopped */
+ if (likely(vcpu->arch.cputm_start))
+ value -= get_tod_clock_fast() - vcpu->arch.cputm_start;
+ } while (read_seqcount_retry(&vcpu->arch.cputm_seqcount, seq & ~1));
+ preempt_enable();
return value;
}
@@ -1704,6 +1717,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
vcpu->arch.local_int.float_int = &kvm->arch.float_int;
vcpu->arch.local_int.wq = &vcpu->wq;
vcpu->arch.local_int.cpuflags = &vcpu->arch.sie_block->cpuflags;
+ seqcount_init(&vcpu->arch.cputm_seqcount);
rc = kvm_vcpu_init(vcpu, kvm, id);
if (rc)