Re: [KVM PATCH v6 4/4] KVM: add irqfd DEASSIGN feature

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 29, 2009 at 11:44:20AM -0400, Gregory Haskins wrote:
> DEASSIGN allows us to optionally disassociate an IRQFD from its underlying
> eventfd without destroying the eventfd in the process.  This is useful
> for conditions like live-migration which may have an eventfd associated
> with a device and an IRQFD.  We need to be able to decouple the guest
> from the event source while not perturbing the event source itself.
> 
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> CC: Michael S. Tsirkin <mst@redhat.com>

Very happy to see this feature back. ACK.

> ---
> 
>  include/linux/kvm.h |    2 ++
>  virt/kvm/eventfd.c  |   46 ++++++++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 46 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kvm.h b/include/linux/kvm.h
> index 38ff31e..6710518 100644
> --- a/include/linux/kvm.h
> +++ b/include/linux/kvm.h
> @@ -490,6 +490,8 @@ struct kvm_x86_mce {
>  };
>  #endif
>  
> +#define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
> +
>  struct kvm_irqfd {
>  	__u32 fd;
>  	__u32 gsi;
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index 76ad125..409d9e1 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -152,8 +152,8 @@ irqfd_ptable_queue_proc(struct file *file, wait_queue_head_t *wqh,
>  	add_wait_queue(wqh, &irqfd->wait);
>  }
>  
> -int
> -kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> +static int
> +kvm_irqfd_assign(struct kvm *kvm, int fd, int gsi)
>  {
>  	struct _irqfd *irqfd;
>  	struct file *file = NULL;
> @@ -233,6 +233,48 @@ kvm_irqfd_init(struct kvm *kvm)
>  }
>  
>  /*
> + * shutdown any irqfd's that match fd+gsi
> + */
> +static int
> +kvm_irqfd_deassign(struct kvm *kvm, int fd, int gsi)
> +{
> +	struct _irqfd *irqfd, *tmp;
> +	struct eventfd_ctx *eventfd;
> +
> +	eventfd = eventfd_ctx_fdget(fd);
> +	if (IS_ERR(eventfd))
> +		return PTR_ERR(eventfd);
> +
> +	spin_lock_irq(&kvm->irqfds.lock);
> +
> +	list_for_each_entry_safe(irqfd, tmp, &kvm->irqfds.items, list) {
> +		if (irqfd->eventfd == eventfd && irqfd->gsi == gsi)
> +			irqfd_deactivate(irqfd);
> +	}
> +
> +	spin_unlock_irq(&kvm->irqfds.lock);
> +	eventfd_ctx_put(eventfd);
> +
> +	/*
> +	 * Block until we know all outstanding shutdown jobs have completed
> +	 * so that we guarantee there will not be any more interrupts on this
> +	 * gsi once this deassign function returns.
> +	 */
> +	flush_workqueue(irqfd_cleanup_wq);
> +
> +	return 0;
> +}
> +
> +int
> +kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
> +{
> +	if (flags & KVM_IRQFD_FLAG_DEASSIGN)
> +		return kvm_irqfd_deassign(kvm, fd, gsi);
> +
> +	return kvm_irqfd_assign(kvm, fd, gsi);
> +}
> +
> +/*
>   * This function is called as the kvm VM fd is being released. Shutdown all
>   * irqfds that still remain open
>   */