Re: [PATCH 1/4] Move irq routing data structure to rcu locking

Gregory Haskins
2009-07-13T13:16:56Z

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Gleb Natapov wrote:
> On Mon, Jul 13, 2009 at 09:01:33AM -0400, Gregory Haskins wrote:
>
>> Gleb Natapov wrote:
>>
>>> Signed-off-by: Gleb Natapov <gleb@redhat.com>
>>> ---
>>> include/linux/kvm_host.h | 2 +-
>>> virt/kvm/irq_comm.c | 55 +++++++++++++++++++++-------------------------
>>> virt/kvm/kvm_main.c | 1 -
>>> 3 files changed, 26 insertions(+), 32 deletions(-)
>>>
>>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>>> index f54a0d3..6756b3e 100644
>>> --- a/include/linux/kvm_host.h
>>> +++ b/include/linux/kvm_host.h
>>> @@ -161,7 +161,7 @@ struct kvm {
>>>
>>> struct mutex irq_lock;
>>> #ifdef CONFIG_HAVE_KVM_IRQCHIP
>>> - struct list_head irq_routing; /* of kvm_kernel_irq_routing_entry */
>>> + struct kvm_kernel_irq_routing_entry *irq_routing;
>>> struct hlist_head mask_notifier_list;
>>> #endif
>>>
>>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
>>> index 7af18b8..b2fa3f6 100644
>>> --- a/virt/kvm/irq_comm.c
>>> +++ b/virt/kvm/irq_comm.c
>>> @@ -148,7 +148,8 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, int irq, int level)
>>> * IOAPIC. So set the bit in both. The guest will ignore
>>> * writes to the unused one.
>>> */
>>> - list_for_each_entry(e, &kvm->irq_routing, link)
>>> + rcu_read_lock();
>>> + for (e = rcu_dereference(kvm->irq_routing); e && e->set; e++) {
>>>
>>>
>> Hi Gleb,
>> I haven't had a chance to fully digest and review these patches, but
>> one thing I did notice is that you seem to be converting from a list to
>> an open-coded structure. I am just curious why you made this design
>> decision instead of using the RCU variant of list?
>>
>>
> It is not scary "open-coded structure" it's just an array :) As I responded
> to Michael the idea is to move msis out of irq_routing, make the array
> much smaller and either use gsi as an index in the array or use hash table
> instead looping over all entries. For now I can justify array as more
> cache friendly data structure as we scan it linearly.
>

Ok, but that might be a good thing to mention in the patch header ;)

Kind Regards,
-Greg

>
>> Regards,
>> -Greg
>>
>>
>>> if (e->gsi == irq) {
>>> int r = e->set(e, kvm, sig_level);
>>> if (r < 0)
>>> @@ -156,6 +157,8 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, int irq, int level)
>>>
>>> ret = r + ((ret < 0) ? 0 : ret);
>>> }
>>> + }
>>> + rcu_read_unlock();
>>> return ret;
>>> }
>>>
>>> @@ -168,12 +171,15 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
>>>
>>> trace_kvm_ack_irq(irqchip, pin);
>>>
>>> - list_for_each_entry(e, &kvm->irq_routing, link)
>>> + rcu_read_lock();
>>> + for (e = rcu_dereference(kvm->irq_routing); e && e->set; e++) {
>>> if (e->irqchip.irqchip == irqchip &&
>>> e->irqchip.pin == pin) {
>>> gsi = e->gsi;
>>> break;
>>> }
>>> + }
>>> + rcu_read_unlock();
>>>
>>> hlist_for_each_entry(kian, n, &kvm->arch.irq_ack_notifier_list, link)
>>> if (kian->gsi == gsi)
>>> @@ -264,19 +270,11 @@ void kvm_fire_mask_notifiers(struct kvm *kvm, int irq, bool mask)
>>> kimn->func(kimn, mask);
>>> }
>>>
>>> -static void __kvm_free_irq_routing(struct list_head *irq_routing)
>>> -{
>>> - struct kvm_kernel_irq_routing_entry *e, *n;
>>> -
>>> - list_for_each_entry_safe(e, n, irq_routing, link)
>>> - kfree(e);
>>> -}
>>> -
>>> void kvm_free_irq_routing(struct kvm *kvm)
>>> {
>>> - mutex_lock(&kvm->irq_lock);
>>> - __kvm_free_irq_routing(&kvm->irq_routing);
>>> - mutex_unlock(&kvm->irq_lock);
>>> + /* Called only during vm destruction. Nobody can use the pointer
>>> + at this stage */
>>> + kfree(kvm->irq_routing);
>>> }
>>>
>>> static int setup_routing_entry(struct kvm_kernel_irq_routing_entry *e,
>>> @@ -326,43 +324,40 @@ int kvm_set_irq_routing(struct kvm *kvm,
>>> unsigned nr,
>>> unsigned flags)
>>> {
>>> - struct list_head irq_list = LIST_HEAD_INIT(irq_list);
>>> - struct list_head tmp = LIST_HEAD_INIT(tmp);
>>> - struct kvm_kernel_irq_routing_entry *e = NULL;
>>> + struct kvm_kernel_irq_routing_entry *new, *old;
>>> unsigned i;
>>> int r;
>>>
>>> + /* last element is left zeroed and indicates the end of the array */
>>> + new = kzalloc(sizeof(*new) * (nr + 1), GFP_KERNEL);
>>> +
>>> + if (!new)
>>> + return -ENOMEM;
>>> +
>>> for (i = 0; i < nr; ++i) {
>>> r = -EINVAL;
>>> if (ue->gsi >= KVM_MAX_IRQ_ROUTES)
>>> goto out;
>>> if (ue->flags)
>>> goto out;
>>> - r = -ENOMEM;
>>> - e = kzalloc(sizeof(*e), GFP_KERNEL);
>>> - if (!e)
>>> - goto out;
>>> - r = setup_routing_entry(e, ue);
>>> + r = setup_routing_entry(new + i, ue);
>>> if (r)
>>> goto out;
>>> ++ue;
>>> - list_add(&e->link, &irq_list);
>>> - e = NULL;
>>> }
>>>
>>> mutex_lock(&kvm->irq_lock);
>>> - list_splice(&kvm->irq_routing, &tmp);
>>> - INIT_LIST_HEAD(&kvm->irq_routing);
>>> - list_splice(&irq_list, &kvm->irq_routing);
>>> - INIT_LIST_HEAD(&irq_list);
>>> - list_splice(&tmp, &irq_list);
>>> + old = kvm->irq_routing;
>>> + rcu_assign_pointer(kvm->irq_routing, new);
>>> mutex_unlock(&kvm->irq_lock);
>>>
>>> + synchronize_rcu();
>>> +
>>> r = 0;
>>> + new = old;
>>>
>>> out:
>>> - kfree(e);
>>> - __kvm_free_irq_routing(&irq_list);
>>> + kfree(new);
>>> return r;
>>> }
>>>
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index cf20dc1..24013b4 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -945,7 +945,6 @@ static struct kvm *kvm_create_vm(void)
>>> if (IS_ERR(kvm))
>>> goto out;
>>> #ifdef CONFIG_HAVE_KVM_IRQCHIP
>>> - INIT_LIST_HEAD(&kvm->irq_routing);
>>> INIT_HLIST_HEAD(&kvm->mask_notifier_list);
>>> #endif
>>>
>>>
>>>
>>
>
>
>
> --
> Gleb.
> --
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