Re: [PATCH] xen/evtchn: Introduce new IOCTL to bind static evtchn

[Stefano Stabellini]

On Fri, 28 Apr 2023, Rahul Singh wrote:
> Xen 4.17 supports the creation of static evtchns. To allow user space
> application to bind static evtchns introduce new ioctl
> "IOCTL_EVTCHN_BIND_STATIC". Existing IOCTL doing more than binding
> that’s why we need to introduce the new IOCTL to only bind the static
> event channels.
> 
> Also, static evtchns to be available for use during the lifetime of the
> guest. When the application exits, __unbind_from_irq() end up
> being called from release() fop because of that static evtchns are
> getting closed. To avoid closing the static event channel, add the new
> bool variable "is_static" in "struct irq_info" to mark the event channel
> static when creating the event channel to avoid closing the static
> evtchn.
> 
> Signed-off-by: Rahul Singh <rahul.singh@xxxxxxx>

I think the patch is OK but evtchn_bind_to_user on the error path calls
EVTCHNOP_close. Could that be a problem for static evtchns? I wonder if
we need to skip that EVTCHNOP_close call too.


err:
	/* bind failed, should close the port now */
	close.port = port;
	if (HYPERVISOR_event_channel_op(EVTCHNOP_close, &close) != 0)
		BUG();
	del_evtchn(u, evtchn);


> ---
>  drivers/xen/events/events_base.c |  7 +++++--
>  drivers/xen/evtchn.c             | 22 +++++++++++++++++-----
>  include/uapi/xen/evtchn.h        |  9 +++++++++
>  include/xen/events.h             |  2 +-
>  4 files changed, 32 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/xen/events/events_base.c b/drivers/xen/events/events_base.c
> index c7715f8bd452..31f2d3634ad5 100644
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -112,6 +112,7 @@ struct irq_info {
>  	unsigned int irq_epoch; /* If eoi_cpu valid: irq_epoch of event */
>  	u64 eoi_time;           /* Time in jiffies when to EOI. */
>  	raw_spinlock_t lock;
> +	u8 is_static;           /* Is event channel static */
>  
>  	union {
>  		unsigned short virq;
> @@ -982,7 +983,8 @@ static void __unbind_from_irq(unsigned int irq)
>  		unsigned int cpu = cpu_from_irq(irq);
>  		struct xenbus_device *dev;
>  
> -		xen_evtchn_close(evtchn);
> +		if (!info->is_static)
> +			xen_evtchn_close(evtchn);
>  
>  		switch (type_from_irq(irq)) {
>  		case IRQT_VIRQ:
> @@ -1574,7 +1576,7 @@ int xen_set_irq_priority(unsigned irq, unsigned priority)
>  }
>  EXPORT_SYMBOL_GPL(xen_set_irq_priority);
>  
> -int evtchn_make_refcounted(evtchn_port_t evtchn)
> +int evtchn_make_refcounted(evtchn_port_t evtchn, bool is_static)
>  {
>  	int irq = get_evtchn_to_irq(evtchn);
>  	struct irq_info *info;
> @@ -1590,6 +1592,7 @@ int evtchn_make_refcounted(evtchn_port_t evtchn)
>  	WARN_ON(info->refcnt != -1);
>  
>  	info->refcnt = 1;
> +	info->is_static = is_static;
>  
>  	return 0;
>  }
> diff --git a/drivers/xen/evtchn.c b/drivers/xen/evtchn.c
> index c99415a70051..47681d4c696b 100644
> --- a/drivers/xen/evtchn.c
> +++ b/drivers/xen/evtchn.c
> @@ -366,7 +366,8 @@ static int evtchn_resize_ring(struct per_user_data *u)
>  	return 0;
>  }
>  
> -static int evtchn_bind_to_user(struct per_user_data *u, evtchn_port_t port)
> +static int evtchn_bind_to_user(struct per_user_data *u, evtchn_port_t port,
> +			bool is_static)
>  {
>  	struct user_evtchn *evtchn;
>  	struct evtchn_close close;
> @@ -402,7 +403,7 @@ static int evtchn_bind_to_user(struct per_user_data *u, evtchn_port_t port)
>  	if (rc < 0)
>  		goto err;
>  
> -	rc = evtchn_make_refcounted(port);
> +	rc = evtchn_make_refcounted(port, is_static);
>  	return rc;
>  
>  err:
> @@ -456,7 +457,7 @@ static long evtchn_ioctl(struct file *file,
>  		if (rc != 0)
>  			break;
>  
> -		rc = evtchn_bind_to_user(u, bind_virq.port);
> +		rc = evtchn_bind_to_user(u, bind_virq.port, false);
>  		if (rc == 0)
>  			rc = bind_virq.port;
>  		break;
> @@ -482,7 +483,7 @@ static long evtchn_ioctl(struct file *file,
>  		if (rc != 0)
>  			break;
>  
> -		rc = evtchn_bind_to_user(u, bind_interdomain.local_port);
> +		rc = evtchn_bind_to_user(u, bind_interdomain.local_port, false);
>  		if (rc == 0)
>  			rc = bind_interdomain.local_port;
>  		break;
> @@ -507,7 +508,7 @@ static long evtchn_ioctl(struct file *file,
>  		if (rc != 0)
>  			break;
>  
> -		rc = evtchn_bind_to_user(u, alloc_unbound.port);
> +		rc = evtchn_bind_to_user(u, alloc_unbound.port, false);
>  		if (rc == 0)
>  			rc = alloc_unbound.port;
>  		break;
> @@ -536,6 +537,17 @@ static long evtchn_ioctl(struct file *file,
>  		break;
>  	}
>  
> +	case IOCTL_EVTCHN_BIND_STATIC: {
> +		struct ioctl_evtchn_bind bind;
> +
> +		rc = -EFAULT;
> +		if (copy_from_user(&bind, uarg, sizeof(bind)))
> +			break;
> +
> +		rc = evtchn_bind_to_user(u, bind.port, true);
> +		break;
> +	}
> +
>  	case IOCTL_EVTCHN_NOTIFY: {
>  		struct ioctl_evtchn_notify notify;
>  		struct user_evtchn *evtchn;
> diff --git a/include/uapi/xen/evtchn.h b/include/uapi/xen/evtchn.h
> index 7fbf732f168f..aef2b75f3413 100644
> --- a/include/uapi/xen/evtchn.h
> +++ b/include/uapi/xen/evtchn.h
> @@ -101,4 +101,13 @@ struct ioctl_evtchn_restrict_domid {
>  	domid_t domid;
>  };
>  
> +/*
> + * Bind statically allocated @port.
> + */
> +#define IOCTL_EVTCHN_BIND_STATIC			\
> +	_IOC(_IOC_NONE, 'E', 7, sizeof(struct ioctl_evtchn_bind))
> +struct ioctl_evtchn_bind {
> +	unsigned int port;
> +};
> +
>  #endif /* __LINUX_PUBLIC_EVTCHN_H__ */
> diff --git a/include/xen/events.h b/include/xen/events.h
> index 44c2855c76d1..962f0bbc7ce1 100644
> --- a/include/xen/events.h
> +++ b/include/xen/events.h
> @@ -69,7 +69,7 @@ int xen_set_irq_priority(unsigned irq, unsigned priority);
>  /*
>   * Allow extra references to event channels exposed to userspace by evtchn
>   */
> -int evtchn_make_refcounted(evtchn_port_t evtchn);
> +int evtchn_make_refcounted(evtchn_port_t evtchn, bool is_static);
>  int evtchn_get(evtchn_port_t evtchn);
>  void evtchn_put(evtchn_port_t evtchn);
>  
> -- 
> 2.25.1
>