[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>

diff --git a/src/arch/x86/include/bits/xen.h b/src/arch/x86/include/bits/xen.h
index 3433cea1f95813cfe96adf12f4244938f742b113..fc065ea388ae054dc13e09b79e608340145171ec 100644 (file)
--- 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
index 731af779c7145412caa48d4bfd16525c21907d78..23424a926e217e471d4a2164218cf51b4e79bf06 100644 (file)
--- 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;