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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>
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2 changed files with 30 additions and 8 deletions
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@ -20,6 +20,7 @@
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#include <linux/kvm_types.h>
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#include <linux/kvm_host.h>
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#include <linux/kvm.h>
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#include <linux/seqlock.h>
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#include <asm/debug.h>
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#include <asm/cpu.h>
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#include <asm/fpu/api.h>
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@ -553,6 +554,13 @@ struct kvm_vcpu_arch {
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unsigned long pfault_select;
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unsigned long pfault_compare;
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bool cputm_enabled;
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/*
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* The seqcount protects updates to cputm_start and sie_block.cputm,
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* this way we can have non-blocking reads with consistent values.
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* Only the owning VCPU thread (vcpu->cpu) is allowed to change these
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* values and to start/stop/enable/disable cpu timer accounting.
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*/
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seqcount_t cputm_seqcount;
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__u64 cputm_start;
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};
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@ -1435,15 +1435,19 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
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static void __start_cpu_timer_accounting(struct kvm_vcpu *vcpu)
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{
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WARN_ON_ONCE(vcpu->arch.cputm_start != 0);
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raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
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vcpu->arch.cputm_start = get_tod_clock_fast();
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raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
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}
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/* needs disabled preemption to protect from TOD sync and vcpu_load/put */
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static void __stop_cpu_timer_accounting(struct kvm_vcpu *vcpu)
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{
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WARN_ON_ONCE(vcpu->arch.cputm_start == 0);
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raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
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vcpu->arch.sie_block->cputm -= get_tod_clock_fast() - vcpu->arch.cputm_start;
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vcpu->arch.cputm_start = 0;
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raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
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}
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/* needs disabled preemption to protect from TOD sync and vcpu_load/put */
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@ -1480,28 +1484,37 @@ static void disable_cpu_timer_accounting(struct kvm_vcpu *vcpu)
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void kvm_s390_set_cpu_timer(struct kvm_vcpu *vcpu, __u64 cputm)
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{
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preempt_disable(); /* protect from TOD sync and vcpu_load/put */
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raw_write_seqcount_begin(&vcpu->arch.cputm_seqcount);
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if (vcpu->arch.cputm_enabled)
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vcpu->arch.cputm_start = get_tod_clock_fast();
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vcpu->arch.sie_block->cputm = cputm;
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raw_write_seqcount_end(&vcpu->arch.cputm_seqcount);
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preempt_enable();
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}
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/* update and get the cpu timer - can also be called from other VCPU threads */
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__u64 kvm_s390_get_cpu_timer(struct kvm_vcpu *vcpu)
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{
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unsigned int seq;
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__u64 value;
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int me;
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if (unlikely(!vcpu->arch.cputm_enabled))
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return vcpu->arch.sie_block->cputm;
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me = get_cpu(); /* also protects from TOD sync and vcpu_load/put */
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value = vcpu->arch.sie_block->cputm;
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if (likely(me == vcpu->cpu)) {
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/* the VCPU itself will always read consistent values */
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value -= get_tod_clock_fast() - vcpu->arch.cputm_start;
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}
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put_cpu();
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preempt_disable(); /* protect from TOD sync and vcpu_load/put */
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do {
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seq = raw_read_seqcount(&vcpu->arch.cputm_seqcount);
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/*
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* If the writer would ever execute a read in the critical
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* section, e.g. in irq context, we have a deadlock.
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*/
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WARN_ON_ONCE((seq & 1) && smp_processor_id() == vcpu->cpu);
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value = vcpu->arch.sie_block->cputm;
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/* if cputm_start is 0, accounting is being started/stopped */
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if (likely(vcpu->arch.cputm_start))
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value -= get_tod_clock_fast() - vcpu->arch.cputm_start;
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} while (read_seqcount_retry(&vcpu->arch.cputm_seqcount, seq & ~1));
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preempt_enable();
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return value;
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}
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@ -1704,6 +1717,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
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vcpu->arch.local_int.float_int = &kvm->arch.float_int;
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vcpu->arch.local_int.wq = &vcpu->wq;
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vcpu->arch.local_int.cpuflags = &vcpu->arch.sie_block->cpuflags;
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seqcount_init(&vcpu->arch.cputm_seqcount);
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rc = kvm_vcpu_init(vcpu, kvm, id);
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if (rc)
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