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>
diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index 91796dd..d61e645 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -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 @@
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;
};
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 76b9914..38223c4 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1435,15 +1435,19 @@
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 @@
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 @@
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)
