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

From: Alex Williamson
Date: Tue Aug 21 2012 - 15:29:13 EST

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@xxxxxxxxxx>
---

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 */
+#define KVM_IRQFD_FLAG_OADN (1 << 1)

struct kvm_irqfd {
__u32 fd;
__u32 gsi;
__u32 flags;
- __u8 pad[20];
+ __u32 notifyfd;
+ __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;
+ 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.
+ */
+ 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;
+ }
+
+ 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);
+ }
+
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();
+
+ /*
+ * 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);
+ 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);
+
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)

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