Re: [PATCH RFC] kvm: deliver msix interrupts from irq handler

[Gleb Natapov <gleb@redhat.com>]

On Wed, Jan 18, 2012 at 08:10:24PM +0200, Michael S. Tsirkin wrote:
> We can deliver certain interrupts, notably MSIX,
> from atomic context.  Add a new API kvm_set_irq_inatomic,
> that does exactly that, and use it to implement
> an irq handler for msi.
> 
> This reduces the pressure on scheduler in case
> where host and guest irq share a host cpu.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> Untested.
> Note: this is on top of my host irq patch.
> Probably needs to be rebased to be independent
> and split up to new API + usage.
> 
> ---
>  include/linux/kvm_host.h |    2 +
>  virt/kvm/assigned-dev.c  |   31 +++++++++++++++++++++++++-
>  virt/kvm/irq_comm.c      |   52 ++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 83 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f0361bc..e2b89ea 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -548,6 +548,8 @@ void kvm_get_intr_delivery_bitmask(struct kvm_ioapic *ioapic,
>  #endif
>  int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
>  		int host_irq);
> +int kvm_set_irq_inatomic(struct kvm *kvm, int irq_source_id, u32 irq, int level,
> +			 int host_irq);
>  int kvm_set_msi(struct kvm_kernel_irq_routing_entry *irq_entry, struct kvm *kvm,
>  		int irq_source_id, int level, int host_irq);
>  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin);
> diff --git a/virt/kvm/assigned-dev.c b/virt/kvm/assigned-dev.c
> index cc4bb7a..73bb001 100644
> --- a/virt/kvm/assigned-dev.c
> +++ b/virt/kvm/assigned-dev.c
> @@ -57,6 +57,14 @@ static int find_index_from_host_irq(struct kvm_assigned_dev_kernel
>  	return index;
>  }
>  
> +static irqreturn_t kvm_assigned_dev_msi(int irq, void *dev_id)
> +{
> +	int ret = kvm_set_irq_inatomic(assigned_dev->kvm,
> +				       assigned_dev->irq_source_id,
> +				       assigned_dev->guest_irq, 1, irq);
> +	return unlikely(ret == -EWOULDBLOCK) ? IRQ_WAKE_THREAD : IRQ_HANDLED;
> +}
> +
>  static irqreturn_t kvm_assigned_dev_thread(int irq, void *dev_id)
>  {
>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
> @@ -75,6 +83,23 @@ static irqreturn_t kvm_assigned_dev_thread(int irq, void *dev_id)
>  }
>  
>  #ifdef __KVM_HAVE_MSIX
> +static irqreturn_t kvm_assigned_dev_msix(int irq, void *dev_id)
> +{
> +	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
> +	int index = find_index_from_host_irq(assigned_dev, irq);
> +	u32 vector;
> +	int ret = 0;
> +
> +	if (index >= 0) {
> +		vector = assigned_dev->guest_msix_entries[index].vector;
> +		ret = kvm_set_irq_inatomic(assigned_dev->kvm,
> +					   assigned_dev->irq_source_id,
> +					   vector, 1, irq);
> +	}
> +
> +	return unlikely(ret == -EWOULDBLOCK) ? IRQ_WAKE_THREAD : IRQ_HANDLED;
> +}
> +
>  static irqreturn_t kvm_assigned_dev_thread_msix(int irq, void *dev_id)
>  {
>  	struct kvm_assigned_dev_kernel *assigned_dev = dev_id;
> @@ -266,7 +291,8 @@ static int assigned_device_enable_host_msi(struct kvm *kvm,
>  	}
>  
>  	dev->host_irq = dev->dev->irq;
> -	if (request_threaded_irq(dev->host_irq, NULL, kvm_assigned_dev_thread,
> +	if (request_threaded_irq(dev->host_irq, kvm_assigned_dev_msi,
> +				 kvm_assigned_dev_thread,
>  				 0, dev->irq_name, dev)) {
>  		pci_disable_msi(dev->dev);
>  		return -EIO;
> @@ -293,7 +319,8 @@ static int assigned_device_enable_host_msix(struct kvm *kvm,
>  
>  	for (i = 0; i < dev->entries_nr; i++) {
>  		r = request_threaded_irq(dev->host_msix_entries[i].vector,
> -					 NULL, kvm_assigned_dev_thread_msix,
> +					 kvm_assigned_dev_msix,
> +					 kvm_assigned_dev_thread_msix,
>  					 0, dev->irq_name, dev);
>  		if (r)
>  			goto err;
> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c
> index ba892df..68cd127 100644
> --- a/virt/kvm/irq_comm.c
> +++ b/virt/kvm/irq_comm.c
> @@ -201,6 +201,58 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level,
>  	return ret;
>  }
>  
> +static inline struct kvm_kernel_irq_routing_entry *
> +kvm_get_entry(struct kvm *kvm, struct kvm_irq_routing_table *irq_rq, u32 irq)
> +{
> +	struct kvm_kernel_irq_routing_entry *e;
> +	if (likely(irq < irq_rt->nr_rt_entries))
> +		hlist_for_each_entry(e, n, &irq_rt->map[irq], link)
> +			if (e->type == KVM_IRQ_ROUTING_MSI)
> +				return e;
> +			else
> +				return ERR_PTR(-EWOULDBLOCK);
> +	return ERR_PTR(-EINVAL);
> +}
Unused?

> +
> +/*
> + * Deliver an IRQ in an atomic context if we can, or return a failure,
> + * user can retry in a process context.
> + * Return value:
> + *  -EWOULDBLOCK	Can't deliver in atomic context
> + *  < 0			Interrupt was ignored (masked or not delivered for other reasons)
> + *  = 0			Interrupt was coalesced (previous irq is still pending)
> + *  > 0			Number of CPUs interrupt was delivered to
> + */
> +int kvm_set_irq_inatomic(struct kvm *kvm, int irq_source_id, u32 irq, int level,
> +			 int host_irq)
> +{
> +	struct kvm_kernel_irq_routing_entry *e;
> +	int ret = -EINVAL;
> +	struct kvm_irq_routing_table *irq_rt;
> +	struct hlist_node *n;
> +
> +	trace_kvm_set_irq(irq, level, irq_source_id);
> +
> +	/*
> +	 * We know MSI are safe in interrupt context. They are also
> +	 * easy as there's a single routing entry for these GSIs.
> +	 * So only handle MSI in an atomic context, for now.
> +	 */
> +	rcu_read_lock_bh();
_bh?

/**
 * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section
 *
 * This is equivalent of rcu_read_lock(), but to be used when updates
 * are being done using call_rcu_bh() or synchronize_rcu_bh().
 ....

Since updates to irq routing table are not done using _bh variant I
doubt rcu_read_lock_bh() is justified here.

> +	irq_rt = rcu_dereference(kvm->irq_routing);
> +	if (irq < irq_rt->nr_rt_entries)
> +		hlist_for_each_entry(e, n, &irq_rt->map[irq], link) {
> +			if (ei->type == KVM_IRQ_ROUTING_MSI)
> +				ret = kvm_set_msi(e, kvm, irq_source_id, level,
> +						  host_irq);
> +			else
> +				ret = -EWOULDBLOCK;
> +			break;
> +		}
> +	rcu_read_unlock_bh();
> +	return ret;
> +}
> +
Share implementation with kvm_set_irq().

>  void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
>  {
>  	struct kvm_irq_ack_notifier *kian;
> -- 
> 1.7.8.2.325.g247f9
> --
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--
			Gleb.