[PATCH RFC] evtchn: introduce EVTCHNOP_fifo_destroy hypercall

[Vitaly Kuznetsov <vkuznets@redhat.com>]

New hypercall is required to allow guest to perform kexec when FIFO-based
event channel ABI is being used. This hypercall simply exposes evtchn_fifo_destroy()
function which cleans up all control blocks for all vcpus and frees event array
for the domain.

With this hypercall guest can do something like:
       struct evtchn_destroy_fifo destroy;

       destroy.dom = DOMID_SELF;
       HYPERVISOR_event_channel_op(EVTCHNOP_destroy_fifo, &destroy);

before doing
       HYPERVISOR_event_channel_op(EVTCHNOP_init_control, &init_control);
for the first vcpu on init.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 xen/common/event_channel.c         | 30 ++++++++++++++++++++++++++++++
 xen/common/event_fifo.c            |  1 +
 xen/include/public/event_channel.h |  7 +++++++
 3 files changed, 38 insertions(+)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index db952af..9bc18b2 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -939,6 +939,26 @@ int evtchn_unmask(unsigned int port)
     return 0;
 }
 
+static long evtchn_fifo_destroy_all(domid_t dom)
+{
+	struct domain *d;
+	int rc;
+
+	d = rcu_lock_domain_by_any_id(dom);
+	if ( d == NULL )
+		return -ESRCH;
+
+	rc = xsm_evtchn_reset(XSM_TARGET, current->domain, d);
+	if ( rc )
+		goto out;
+
+	evtchn_fifo_destroy(d);
+
+out:
+	rcu_unlock_domain(d);
+
+	return rc;
+}
 
 static long evtchn_reset(evtchn_reset_t *r)
 {
@@ -1121,6 +1141,16 @@ long do_event_channel_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
         break;
     }
 
+	case EVTCHNOP_fifo_destroy: {
+		struct evtchn_fifo_destroy destroy;
+		if ( copy_from_guest(&destroy, arg, 1) != 0 )
+			return -EFAULT;
+		if ( destroy.dom != DOMID_SELF )
+			return -EPERM;
+		rc = evtchn_fifo_destroy_all(destroy.dom);
+		break;
+	}
+
     default:
         rc = -ENOSYS;
         break;
diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 1fce3f1..51b4ff6 100644
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -451,6 +451,7 @@ static void cleanup_event_array(struct domain *d)
     for ( i = 0; i < EVTCHN_FIFO_MAX_EVENT_ARRAY_PAGES; i++ )
         unmap_guest_page(d->evtchn_fifo->event_array[i]);
     xfree(d->evtchn_fifo);
+    d->evtchn_fifo = NULL;
 }
 
 static void setup_ports(struct domain *d)
diff --git a/xen/include/public/event_channel.h b/xen/include/public/event_channel.h
index 49ac8cc..79b6e86 100644
--- a/xen/include/public/event_channel.h
+++ b/xen/include/public/event_channel.h
@@ -74,6 +74,7 @@
 #define EVTCHNOP_init_control    11
 #define EVTCHNOP_expand_array    12
 #define EVTCHNOP_set_priority    13
+#define EVTCHNOP_fifo_destroy    14
 /* ` } */
 
 typedef uint32_t evtchn_port_t;
@@ -308,6 +309,12 @@ struct evtchn_set_priority {
 };
 typedef struct evtchn_set_priority evtchn_set_priority_t;
 
+struct evtchn_fifo_destroy {
+	/* IN parameters. */
+	domid_t dom;
+};
+typedef struct evtchn_fifo_destroy evtchn_fifo_destroy_t;
+
 /*
  * ` enum neg_errnoval
  * ` HYPERVISOR_event_channel_op_compat(struct evtchn_op *op)
-- 
1.9.3