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Date: Thu, 18 Nov 2010 11:05:22 +0200
From: Gleb Natapov <gleb@...hat.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
Xiao Guangrong <xiaoguangrong@...fujitsu.com>,
Gregory Haskins <ghaskins@...ell.com>,
Chris Lalancette <clalance@...hat.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: fast-path msi injection with irqfd
On Thu, Nov 18, 2010 at 12:12:54AM +0200, Michael S. Tsirkin wrote:
> Store irq routing table pointer in the irqfd object,
> and use that to inject MSI directly without bouncing out to
> a kernel thread.
>
> While we touch this structure, rearrange irqfd fields to make fastpath
> better packed for better cache utilization.
>
> Some notes on the design:
> - Use pointer into the rt instead of copying an entry,
> to make it possible to use rcu, thus side-stepping
> locking complexities. We also save some memory this way.
What locking complexity is there with copying entry approach?
> - Old workqueue code is still used for level irqs.
> I don't think we DTRT with level anyway, however,
> it seems easier to keep the code around as
> it has been thought through and debugged, and fix level later than
> rip out and re-instate it later.
>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>
> The below is compile tested only. Sending out for early
> flames/feedback. Please review!
>
> include/linux/kvm_host.h | 4 ++
> virt/kvm/eventfd.c | 81 +++++++++++++++++++++++++++++++++++++++------
> virt/kvm/irq_comm.c | 6 ++-
> 3 files changed, 78 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index a055742..b6f7047 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -462,6 +462,8 @@ void kvm_get_intr_delivery_bitmask(struct kvm_ioapic *ioapic,
> unsigned long *deliver_bitmask);
> #endif
> int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level);
> +int kvm_set_msi(struct kvm_kernel_irq_routing_entry *irq_entry, struct kvm *kvm,
> + int irq_source_id, int level);
> void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin);
> void kvm_register_irq_ack_notifier(struct kvm *kvm,
> struct kvm_irq_ack_notifier *kian);
> @@ -603,6 +605,7 @@ static inline void kvm_free_irq_routing(struct kvm *kvm) {}
> void kvm_eventfd_init(struct kvm *kvm);
> int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags);
> void kvm_irqfd_release(struct kvm *kvm);
> +void kvm_irqfd_update(struct kvm *kvm, struct kvm_irq_routing_table *irq_rt);
> int kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args);
>
> #else
> @@ -614,6 +617,7 @@ static inline int kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> }
>
> static inline void kvm_irqfd_release(struct kvm *kvm) {}
> +static inline void kvm_irqfd_update(struct kvm *kvm) {}
> static inline int kvm_ioeventfd(struct kvm *kvm, struct kvm_ioeventfd *args)
> {
> return -ENOSYS;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index c1f1e3c..49c1864 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -44,14 +44,18 @@
> */
>
> struct _irqfd {
> - struct kvm *kvm;
> - struct eventfd_ctx *eventfd;
> - int gsi;
> - struct list_head list;
> - poll_table pt;
> - wait_queue_t wait;
> - struct work_struct inject;
> - struct work_struct shutdown;
> + /* Used for MSI fast-path */
> + struct kvm *kvm;
> + wait_queue_t wait;
> + struct kvm_kernel_irq_routing_entry __rcu *irq_entry;
> + /* Used for level IRQ fast-path */
> + int gsi;
> + struct work_struct inject;
> + /* Used for setup/shutdown */
> + struct eventfd_ctx *eventfd;
> + struct list_head list;
> + poll_table pt;
> + struct work_struct shutdown;
> };
>
> static struct workqueue_struct *irqfd_cleanup_wq;
> @@ -125,10 +129,18 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> {
> struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
> unsigned long flags = (unsigned long)key;
> + struct kvm_kernel_irq_routing_entry *irq;
>
> - if (flags & POLLIN)
> + if (flags & POLLIN) {
> + rcu_read_lock();
> + irq = irqfd->irq_entry;
Why not rcu_dereference()? And why it can't be zero here?
> /* An event has been signaled, inject an interrupt */
> - schedule_work(&irqfd->inject);
> + if (irq)
> + kvm_set_msi(irq, irqfd->kvm, KVM_USERSPACE_IRQ_SOURCE_ID, 1);
> + else
> + schedule_work(&irqfd->inject);
> + rcu_read_unlock();
> + }
>
> if (flags & POLLHUP) {
> /* The eventfd is closing, detach from KVM */
> @@ -166,6 +178,7 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
> static int
> kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
> {
> + struct kvm_irq_routing_table *irq_rt;
> struct _irqfd *irqfd, *tmp;
> struct file *file = NULL;
> struct eventfd_ctx *eventfd = NULL;
> @@ -215,6 +228,10 @@ kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
> goto fail;
> }
>
> + rcu_read_lock();
> + irqfd_update(kvm, irqfd, rcu_dereference(kvm->irq_routing));
> + rcu_read_unlock();
> +
> events = file->f_op->poll(file, &irqfd->pt);
>
> list_add_tail(&irqfd->list, &kvm->irqfds.items);
> @@ -271,8 +288,15 @@ kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
> spin_lock_irq(&kvm->irqfds.lock);
>
> list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds.items, list) {
> - if (irqfd->eventfd == eventfd && irqfd->gsi == gsi)
> + if (irqfd->eventfd == eventfd && irqfd->gsi == gsi) {
> + /* This rcu_assign_pointer is needed for when
> + * another thread calls kvm_irqfd_update before
> + * we flush workqueue below.
> + * It is paired with synchronize_rcu done by caller
> + * of that function. */
> + rcu_assign_pointer(irqfd->irq_entry, NULL);
> irqfd_deactivate(irqfd);
> + }
> }
>
> spin_unlock_irq(&kvm->irqfds.lock);
> @@ -321,6 +345,41 @@ kvm_irqfd_release(struct kvm *kvm)
>
> }
>
> +/* Must be called under irqfds.lock */
> +static void irqfd_update(struct kvm *kvm, struct _irqfd *irqfd,
> + struct kvm_irq_routing_table *irq_rt)
> +{
> + struct kvm_kernel_irq_routing_entry *e;
> + struct hlist_node *n;
> +
> + if (irqfd->gsi >= irq_rt->nr_rt_entries) {
> + rcu_assign_pointer(irqfd->irq_entry, NULL);
> + return;
> + }
> +
> + hlist_for_each_entry(e, n, &irq_rt->map[irqfd->gsi], link) {
> + /* Only fast-path MSI. */
> + if (e->type == KVM_IRQ_ROUTING_MSI)
> + rcu_assign_pointer(irqfd->irq_entry, e);
> + else
> + rcu_assign_pointer(irqfd->irq_entry, NULL);
> + }
Shouldn't we flush workqueue if routing entry is gone?
> +}
> +
> +/* Update irqfd after a routing change. Caller must invoke synchronize_rcu
> + * afterwards. */
> +void kvm_irqfd_update(struct kvm *kvm, struct kvm_irq_routing_table *irq_rt)
> +{
> + struct _irqfd *irqfd;
> +
> + spin_lock_irq(&kvm->irqfds.lock);
> +
> + list_for_each_entry(irqfd, &kvm->irqfds.items, list)
> + irqfd_update(kvm, irqfd, irq_rt);
> +
> + spin_unlock_irq(&kvm->irqfds.lock);
> +}
> +
> /*
> * create a host-wide workqueue for issuing deferred shutdown requests
> * aggregated from all vm* instances. We need our own isolated single-thread
> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> index 8edca91..265ab72 100644
> --- a/virt/kvm/irq_comm.c
> +++ b/virt/kvm/irq_comm.c
> @@ -114,8 +114,8 @@ int kvm_irq_delivery_to_apic(struct kvm *kvm, struct kvm_lapic *src,
> return r;
> }
>
> -static int kvm_set_msi(struct kvm_kernel_irq_routing_entry *e,
> - struct kvm *kvm, int irq_source_id, int level)
> +int kvm_set_msi(struct kvm_kernel_irq_routing_entry *e,
> + struct kvm *kvm, int irq_source_id, int level)
> {
> struct kvm_lapic_irq irq;
>
> @@ -410,7 +410,9 @@ int kvm_set_irq_routing(struct kvm *kvm,
> mutex_lock(&kvm->irq_lock);
> old = kvm->irq_routing;
> rcu_assign_pointer(kvm->irq_routing, new);
> + kvm_irqfd_update(kvm, new);
> mutex_unlock(&kvm->irq_lock);
> +
> synchronize_rcu();
>
> new = old;
> --
> 1.7.3.2.91.g446ac
--
Gleb.
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