[xen] Wait for and clear XenStore event before receiving data

Older, out-of-tree Xen kernel modules (such as those provided with SuSE Linux Enterprise Server 11) do not clear the leftover "event pending" bit when opening an event channel. Consequently, no event is ever delivered to indicate that there is information in the XenStore ring buffer, and the system hangs shortly after loading the xen-platform-pci kernel module. Work around this problem by always waiting for the XenStore event channel to be signalled, and clearing the event before processing the received data. Signed-off-by: Michael Brown <mcb30@ipxe.org>
ipxe · Jun 30, 2015 · 211529a · 211529a
1 parent a0f60d2
commit 211529a
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diff --git a/src/arch/x86/include/bits/xen.h b/src/arch/x86/include/bits/xen.h
@@ -161,4 +161,23 @@ xen_hypercall_5 ( struct xen_hypervisor *xen, unsigned int hypercall,
 	return retval;
 }
 
+/**
+ * Test and clear pending event
+ *
+ * @v xen		Xen hypervisor
+ * @v port		Event channel port
+ * @ret pending		Event was pending
+ */
+static inline __attribute__ (( always_inline )) uint8_t
+xenevent_pending ( struct xen_hypervisor *xen, evtchn_port_t port ) {
+	uint8_t pending;
+
+	__asm__ __volatile__ ( "lock btr %2, %0\n\t"
+			       "setc %1\n\t"
+			       : "+m" ( xen->shared->evtchn_pending ),
+				 "=a"  ( pending )
+			       : "Ir" ( port ) );
+	return pending;
+}
+
 #endif /* _BITS_XEN_H */
diff --git a/src/interface/xen/xenstore.c b/src/interface/xen/xenstore.c
@@ -242,6 +242,10 @@ static int xenstore_response ( struct xen_hypervisor *xen, uint32_t req_id,
 	char *string;
 	int rc;
 
+	/* Wait for response to become available */
+	while ( ! xenevent_pending ( xen, xen->store.port ) )
+		cpu_nap();
+
 	/* Receive message header */
 	xenstore_recv ( xen, &msg, sizeof ( msg ) );
 	*len = msg.len;