s390/debug: only write data once

debug_event_common memsets the active debug entry with zeros to
prevent stale data leakage. This is overwritten with the actual
debug data in the next step. Only write zeros to that part of the
debug entry that's not used by new debug data.

Micro benchmarks show a 2-10% reduction of cpu cycles with this
approach.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Acked-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

diff --git a/arch/s390/kernel/debug.c b/arch/s390/kernel/debug.c
index df916738fbd86bac3044f78743057924e6b67d28..2df679d754546328c4b430dbde377737f4c1d41e 100644 (file)
--- a/arch/s390/kernel/debug.c
+++ b/arch/s390/kernel/debug.c
@@ -902,8 +902,9 @@ debug_entry_t *debug_event_common(debug_info_t *id, int level, const void *buf,
        }
        do {
                active = get_active_entry(id);
-               memset(DEBUG_DATA(active), 0, id->buf_size);
                memcpy(DEBUG_DATA(active), buf, min(len, id->buf_size));
+               if (len < id->buf_size)
+                       memset((DEBUG_DATA(active)) + len, 0, id->buf_size - len);
                debug_finish_entry(id, active, level, 0);
                len -= id->buf_size;
                buf += id->buf_size;
@@ -934,8 +935,9 @@ debug_entry_t *debug_exception_common(debug_info_t *id, int level,
        }
        do {
                active = get_active_entry(id);
-               memset(DEBUG_DATA(active), 0, id->buf_size);
                memcpy(DEBUG_DATA(active), buf, min(len, id->buf_size));
+               if (len < id->buf_size)
+                       memset((DEBUG_DATA(active)) + len, 0, id->buf_size - len);
                debug_finish_entry(id, active, level, len <= id->buf_size);
                len -= id->buf_size;
                buf += id->buf_size;