Re: [PATCH v9 2/2] kvm: On Ack, De-assert & Notify KVM_IRQFD extension

Michael S. Tsirkin
2012-08-21T20:36:43Z

On Tue, Aug 21, 2012 at 01:29:14PM -0600, Alex Williamson wrote:
> For VFIO based device assignment we'd like a mechanism to allow level
> triggered interrutps to be directly injected into KVM. KVM_IRQFD
> already allows this for edge triggered interrupts, but for level, we
> need to watch for acknowledgement of the interrupt from the guest to
> provide us a hint when to test the device and allow it to re-assert
> if necessary. To do this, we create a new KVM_IRQFD mode called
> "On Ack, De-assert & Notify", or OADN. In this mode, an interrupt
> injection provides only a gsi assertion. We then hook into the IRQ
> ACK notifier, which when triggered de-asserts the gsi and notifies
> via another eventfd. It's then the responsibility of the user to
> re-assert the interrupt is service is still required.
>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Naming aside, looks good.
I think I see some minor bugs, and I added some improvement
suggestions below.

Thanks!

> ---
>
> Documentation/virtual/kvm/api.txt | 13 ++
> arch/x86/kvm/x86.c | 1
> include/linux/kvm.h | 6 +
> include/linux/kvm_host.h | 1
> virt/kvm/eventfd.c | 193 ++++++++++++++++++++++++++++++++++++-
> 5 files changed, 210 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> index bf33aaa..87d7321 100644
> --- a/Documentation/virtual/kvm/api.txt
> +++ b/Documentation/virtual/kvm/api.txt
> @@ -1946,6 +1946,19 @@ the guest using the specified gsi pin. The irqfd is removed using
> the KVM_IRQFD_FLAG_DEASSIGN flag, specifying both kvm_irqfd.fd
> and kvm_irqfd.gsi.
>
> +With KVM_CAP_IRQFD_OADN, KVM_IRQFD supports an "On Ack, De-assert &
> +Notify" option that allows emulation of level-triggered interrupts.
> +When kvm_irqfd.fd is triggered, the requested gsi is asserted and
> +remains asserted until interaction with the irqchip indicates the
> +VM has acknowledged the interrupt, such as an EOI. On acknoledgement
> +the gsi is automatically de-asserted and the user is notified via
> +kvm_irqfd.notifyfd. The user is then required to re-assert the
> +interrupt if the associated device still requires service. To enable
> +this mode, configure the KVM_IRQFD using the KVM_IRQFD_FLAG_OADN flag
> +and specify kvm_irqfd.notifyfd. Note that closing kvm_irqfd.notifyfd
> +while configured in this mode does not disable the irqfd. The
> +KVM_IRQFD_FLAG_OADN flag is only necessary on assignment.
> +
> 4.76 KVM_PPC_ALLOCATE_HTAB
>
> Capability: KVM_CAP_PPC_ALLOC_HTAB
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index cd98673..fde7b66 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2175,6 +2175,7 @@ int kvm_dev_ioctl_check_extension(long ext)
> case KVM_CAP_PCI_2_3:
> case KVM_CAP_KVMCLOCK_CTRL:
> case KVM_CAP_IRQFD_IRQ_SOURCE_ID:
> + case KVM_CAP_IRQFD_OADN:
> r = 1;
> break;
> case KVM_CAP_COALESCED_MMIO:
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index ae66b9c..ec0f1d8 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -619,6 +619,7 @@ struct kvm_ppc_smmu_info {
> #define KVM_CAP_S390_COW 79
> #define KVM_CAP_PPC_ALLOC_HTAB 80
> #define KVM_CAP_IRQFD_IRQ_SOURCE_ID 81
> +#define KVM_CAP_IRQFD_OADN 82
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> @@ -684,12 +685,15 @@ struct kvm_xen_hvm_config {
> #endif
>
> #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
> +/* Availabie with KVM_CAP_IRQFD_OADN */

Need to also explain what it is.

> +#define KVM_IRQFD_FLAG_OADN (1 << 1)
>
> struct kvm_irqfd {
> __u32 fd;
> __u32 gsi;
> __u32 flags;
> - __u8 pad[20];
> + __u32 notifyfd;

Document that this is only valid with OADN flag. Might be a good idea
to rename this to deassert_on_ack_notifyfd or oadn_notifyfd
to avoid confusion.

> + __u8 pad[16];
> };
>
> struct kvm_clock_data {
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index b763230..d502d08 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -284,6 +284,7 @@ struct kvm {
> struct {
> spinlock_t lock;
> struct list_head items;
> + struct list_head oadns;
> } irqfds;
> struct list_head ioeventfds;
> #endif
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 2245cfa..dfdb5b2 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -43,6 +43,23 @@
> * --------------------------------------------------------------------
> */
>
> +/*
> + * OADN irqfds (On Ack, De-assert & Notify) are a special variety of
> + * irqfds that assert an interrupt to the irqchip on eventfd trigger,
> + * receieve notification when userspace acknowledges the interrupt,
> + * automatically de-asserts the irqchip level, and notifies userspace
> + * via the oadn_eventfd. This object helps to provide one-to-many
> + * deassert-to-notify so we can share a single irq source ID per OADN.
> + */
> +struct _irqfd_oadn {
> + struct kvm *kvm;
> + int irq_source_id; /* IRQ source ID shared by these irqfds */
> + struct list_head irqfds; /* list of irqfds using this object */
> + struct kvm_irq_ack_notifier notifier; /* IRQ ACK notification */
> + struct kref kref; /* Race-free removal */
> + struct list_head list;
> +};
> +
> struct _irqfd {
> /* Used for MSI fast-path */
> struct kvm *kvm;
> @@ -52,6 +69,10 @@ struct _irqfd {
> /* Used for level IRQ fast-path */
> int gsi;
> struct work_struct inject;
> + /* Used for OADN (On Ack, De-assert & Notify) path */
> + struct eventfd_ctx *oadn_eventfd;
> + struct list_head oadn_list;

Could you document RCU and locking rules for this field please?

> + struct _irqfd_oadn *oadn;
> /* Used for setup/shutdown */
> struct eventfd_ctx *eventfd;
> struct list_head list;
> @@ -71,6 +92,51 @@ irqfd_inject(struct work_struct *work)
> kvm_set_irq(kvm, KVM_IRQFD_IRQ_SOURCE_ID, irqfd->gsi, 0);
> }
>
> +static void
> +irqfd_oadn_inject(struct work_struct *work)
> +{
> + struct _irqfd *irqfd = container_of(work, struct _irqfd, inject);
> +
> + kvm_set_irq(irqfd->kvm, irqfd->oadn->irq_source_id, irqfd->gsi, 1);
> +}
> +
> +/*
> + * Since OADN irqfds share an IRQ source ID, we de-assert once then
> + * notify all of the OADN irqfds using this GSI. We can't do multiple
> + * de-asserts or we risk racing with incoming re-asserts.
> + */
> +static void
> +irqfd_oadn_notify(struct kvm_irq_ack_notifier *kian)
> +{
> + struct _irqfd_oadn *oadn;
> + struct _irqfd *irqfd;
> +
> + oadn = container_of(kian, struct _irqfd_oadn, notifier);
> +
> + rcu_read_lock();
> +
> + kvm_set_irq(oadn->kvm, oadn->irq_source_id,
> + oadn->notifier.gsi, 0);
> +
> + list_for_each_entry_rcu(irqfd, &oadn->irqfds, oadn_list)
> + eventfd_signal(irqfd->oadn_eventfd, 1);
> +
> + rcu_read_unlock();
> +}
> +
> +/*
> + * Assumes kvm->irqfds.lock is held. Remove from the list of OADNs, new
> + * users will allocate their own, letting us de-assert this GSI on free,
> + * after the RCU grace period.
> + */
> +static void
> +irqfd_oadn_release(struct kref *kref)
> +{
> + struct _irqfd_oadn *oadn = container_of(kref, struct _irqfd_oadn, kref);
> +
> + list_del(&oadn->list);
> +}
> +
> /*
> * Race-free decouple logic (ordering is critical)
> */
> @@ -93,6 +159,35 @@ irqfd_shutdown(struct work_struct *work)
> flush_work_sync(&irqfd->inject);
>
> /*
> + * OADN irqfds use an RCU list for lockless de-assert and user
> + * notification. Modify the list using RCU under lock and release
> + * the OADN object. Since we later free this irqfd, we must wait
> + * for an RCU grace period. If the OADN was released, we can then
> + * unregister the irq ack notifier, free the irq source ID (assuring
> + * that the GSI is de-asserted), and free the object.
> + */
> + if (irqfd->oadn) {
> + struct _irqfd_oadn *oadn = irqfd->oadn;
> + struct kvm *kvm = oadn->kvm;
> + bool released;
> +
> + spin_lock_irq(&irqfd->kvm->irqfds.lock);
> + list_del_rcu(&irqfd->oadn_list);
> + released = kref_put(&oadn->kref, irqfd_oadn_release);
> + spin_unlock_irq(&irqfd->kvm->irqfds.lock);
> +
> + synchronize_rcu();
> +
> + if (released) {
> + kvm_unregister_irq_ack_notifier(kvm, &oadn->notifier);
> + kvm_free_irq_source_id(kvm, oadn->irq_source_id);
> + kfree(oadn);
> + }
> +
> + eventfd_ctx_put(irqfd->oadn_eventfd);
> + }
> +
> + /*
> * It is now safe to release the object's resources
> */
> eventfd_ctx_put(irqfd->eventfd);
> @@ -202,8 +297,9 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> {
> struct kvm_irq_routing_table *irq_rt;
> struct _irqfd *irqfd, *tmp;
> + struct _irqfd_oadn *oadn = NULL;
> struct file *file = NULL;
> - struct eventfd_ctx *eventfd = NULL;
> + struct eventfd_ctx *eventfd = NULL, *oadn_eventfd = NULL;
> int ret;
> unsigned int events;
>
> @@ -214,7 +310,49 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> irqfd->kvm = kvm;
> irqfd->gsi = args->gsi;
> INIT_LIST_HEAD(&irqfd->list);
> - INIT_WORK(&irqfd->inject, irqfd_inject);
> + INIT_LIST_HEAD(&irqfd->oadn_list);
> +
> + if (args->flags & KVM_IRQFD_FLAG_OADN) {
> + oadn_eventfd = eventfd_ctx_fdget(args->notifyfd);
> + if (IS_ERR(oadn_eventfd)) {
> + ret = PTR_ERR(oadn_eventfd);
> + goto fail;
> + }
> +
> + irqfd->oadn_eventfd = oadn_eventfd;
> +
> + /*
> + * We may be able to share an existing OADN, but we won't
> + * know that until we search under spinlock. We can't get
> + * an irq_source_id under spinlock, and we'd prefer not to
> + * do an atomic allocation, so prep an object here and free
> + * it if we don't need it.
> + */

So if everyone uses same source ID this code will be simpler too?

> + oadn = kzalloc(sizeof(*oadn), GFP_KERNEL);
> + if (!oadn) {
> + ret = -ENOMEM;
> + goto fail;
> + }
> +
> + oadn->kvm = kvm;
> + INIT_LIST_HEAD(&oadn->irqfds);
> + oadn->notifier.gsi = irqfd->gsi;
> + oadn->notifier.irq_acked = irqfd_oadn_notify;
> + kref_init(&oadn->kref);
> + INIT_LIST_HEAD(&oadn->list);
> +
> + oadn->irq_source_id = kvm_request_irq_source_id(kvm);
> + if (oadn->irq_source_id < 0) {
> + ret = oadn->irq_source_id;
> + goto fail;
> + }
> +

This still has the problem that by allocating these
OADN irqfds you can run out of source IDs for other uses.
Why don't OADNs just use KVM_IRQFD_IRQ_SOURCE_ID?
I thought this is why it was introduced in this patchset ...

> + irqfd->oadn = oadn;
> +
> + INIT_WORK(&irqfd->inject, irqfd_oadn_inject);
> + } else
> + INIT_WORK(&irqfd->inject, irqfd_inject);
> +
> INIT_WORK(&irqfd->shutdown, irqfd_shutdown);
>
> file = eventfd_fget(args->fd);
> @@ -250,6 +388,27 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> goto fail;
> }
>
> + /*
> + * When configuring an OADN irqfd, try to re-use an existing OADN.
> + */
> + if (oadn) {
> + struct _irqfd_oadn *oadn_tmp;
> +
> + list_for_each_entry(oadn_tmp, &kvm->irqfds.oadns, list) {
> + if (oadn_tmp->notifier.gsi == irqfd->gsi) {
> + irqfd->oadn = oadn_tmp;
> + break;
> + }
> + }
> +
> + if (irqfd->oadn != oadn)
> + kref_get(&irqfd->oadn->kref);
> + else
> + list_add(&oadn->list, &kvm->irqfds.oadns);
> +
> + list_add_rcu(&irqfd->oadn_list, &irqfd->oadn->irqfds);

Hang on, don't we need irqfds_lock here?

> + }
> +
> irq_rt = rcu_dereference_protected(kvm->irq_routing,
> lockdep_is_held(&kvm->irqfds.lock));
> irqfd_update(kvm, irqfd, irq_rt);
> @@ -267,6 +426,26 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>
> spin_unlock_irq(&kvm->irqfds.lock);
>
> + if (oadn) {
> + synchronize_rcu();

Seems unexpected on assign.
What does this synchronize_rcu do?

> +
> + /*
> + * If we weren't able to re-use an OADN, setup the irq ack
> + * notifier outside of spinlock. Our interface requires
> + * users to be able to handle spurious de-assert & notifies,
> + * so trigger one here to make sure we didn't miss anything.
> + * Cleanup unused OADN if we share.
> + */
> + if (irqfd->oadn == oadn)
> + kvm_register_irq_ack_notifier(kvm, &oadn->notifier);
> + else {
> + kvm_free_irq_source_id(kvm, oadn->irq_source_id);

This is unfortunate - so we need a spare source ID for internal use.
Thus if 1 source ID is available, then it is possible that

- allocate irqfd for A
- allocate irqfd for B

succeeds because A uses a shared source id so it is freed here.

While

- allocate irqfd for B
- allocate irqfd for A

fails because B used the last ID and now temporary allocation
above fails.

> + kfree(oadn);
> + }
> +
> + irqfd_oadn_notify(&irqfd->oadn->notifier);
> + }
> +
> /*
> * do not drop the file until the irqfd is fully initialized, otherwise
> * we might race against the POLLHUP
> @@ -276,12 +455,19 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
> return 0;
>
> fail:
> + if (oadn_eventfd && !IS_ERR(oadn_eventfd))
> + eventfd_ctx_put(oadn_eventfd);
> +
> + if (oadn && oadn->irq_source_id >= 0)
> + kvm_free_irq_source_id(kvm, oadn->irq_source_id);
> +

A bit cleaner to have distinct labels for failures
at different stages of the setup.
I know it's not written this way ATM so it's not a must
to address this.

> if (eventfd && !IS_ERR(eventfd))
> eventfd_ctx_put(eventfd);
>
> if (!IS_ERR(file))
> fput(file);
>
> + kfree(oadn);
> kfree(irqfd);
> return ret;
> }
> @@ -291,6 +477,7 @@ kvm_eventfd_init(struct kvm *kvm)
> {
> spin_lock_init(&kvm->irqfds.lock);
> INIT_LIST_HEAD(&kvm->irqfds.items);
> + INIT_LIST_HEAD(&kvm->irqfds.oadns);
> INIT_LIST_HEAD(&kvm->ioeventfds);
> }
>
> @@ -340,7 +527,7 @@ kvm_irqfd_deassign(struct kvm *kvm, struct kvm_irqfd *args)
> int
> kvm_irqfd(struct kvm *kvm, struct kvm_irqfd *args)
> {
> - if (args->flags & ~KVM_IRQFD_FLAG_DEASSIGN)
> + if (args->flags & ~(KVM_IRQFD_FLAG_DEASSIGN | KVM_IRQFD_FLAG_OADN))
> return -EINVAL;
>
> if (args->flags & KVM_IRQFD_FLAG_DEASSIGN)