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KVM: s390: protect VCPU cpu timer with a seqcount

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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>
This commit is contained in:
David Hildenbrand 2016-02-17 21:53:33 +01:00 committed by Christian Borntraeger
parent db0758b297
commit 9c23a1318e
2 changed files with 30 additions and 8 deletions
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8
arch/s390/include/asm/kvm_host.h
View file

@ -20,6 +20,7 @@
#include <linux/kvm_types.h>
#include <linux/kvm_host.h>
#include <linux/kvm.h>
#include <linux/seqlock.h>
#include <asm/debug.h>
#include <asm/cpu.h>
#include <asm/fpu/api.h>
@ -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;
};

30
arch/s390/kvm/kvm-s390.c
View file

@ -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)