[Minios-devel] [UNIKRAFT PATCH] plat/xen: Do not discard characters on console

[Simon Kuenzer <simon.kuenzer@xxxxxxxxx>]

Retry sending characters to console ring instead of discarding
them whenever the ring is full (busy wait).

Signed-off-by: Simon Kuenzer <simon.kuenzer@xxxxxxxxx>
---
 plat/xen/console.c | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/plat/xen/console.c b/plat/xen/console.c
index f2f0e78..2a776ee 100644
--- a/plat/xen/console.c
+++ b/plat/xen/console.c
@@ -132,6 +132,21 @@ static int emergency_output(const char *str, unsigned int 
len)
 }
 #endif
 
+/*
+ * hvconsole_output can operate in two modes: buffered and intialized.
+ * The buffered mode is automatically activated after
+ * _libxenplat_prepare_console() was called and we know where the console ring
+ * is. The output string is put to the console ring until the ring is full. Any
+ * further characters are discarded. Since the event channel is not initialized
+ * yet, the backend is not notified. This mode is introduced to support early
+ * printing, even before events are not intialized.
+ * _libxenplat_init_console() finalizes the initialization and enables
+ * the event channel. From now on, the console backend is notified whenever
+ * we put characters on the console ring. Whenever this ring is full and there
+ * are still characters that should be printed, we are entering a busy loop and
+ * wait for the backend to make us space again. Of course this is slow: do not
+ * print so much! ;-)
+ */
 static int hvconsole_output(const char *str, unsigned int len)
 {
        unsigned int sent = 0;
@@ -140,6 +155,7 @@ static int hvconsole_output(const char *str, unsigned int 
len)
        if (unlikely(!console_ring))
                return sent;
 
+retry:
        cons = console_ring->out_cons;
        prod = console_ring->out_prod;
 
@@ -148,12 +164,13 @@ static int hvconsole_output(const char *str, unsigned int 
len)
 
        while ((sent < len) && ((prod - cons) < sizeof(console_ring->out))) {
                if (str[sent] == '\n') {
-                       /* Convert: '\n' -> '\r''\n' */
+                       /* prepend '\r' for converting '\n' to '\r''\n' */
+                       if ((prod + 1 - cons) >= sizeof(console_ring->out))
+                               break; /* not enough space for '\r' and '\n'! */
+
                        console_ring->out[MASK_XENCONS_IDX(prod++,
                                                           console_ring->out)] =
                                '\r';
-                       if ((prod - cons) >= sizeof(console_ring->out))
-                               break; /* no more space left */
                }
 
                console_ring->out[MASK_XENCONS_IDX(prod++, console_ring->out)] =
@@ -163,8 +180,18 @@ static int hvconsole_output(const char *str, unsigned int 
len)
        wmb(); /* ensure characters are written before increasing out_prod */
        console_ring->out_prod = prod;
 
-       if (likely(console_ready && console_evtchn))
+       /* Is the console fully initialized?
+        * Are we able to notify the backend?
+        */
+       if (likely(console_ready && console_evtchn)) {
                notify_remote_via_evtchn(console_evtchn);
+
+               /* There are still bytes left to send out? If yes, do not
+                * discard them, retry sending (enters busy waiting)
+                */
+               if (sent < len)
+                       goto retry;
+       }
        return sent;
 }
 
-- 
2.7.4


_______________________________________________
Minios-devel mailing list
Minios-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/minios-devel