From: Sebastian Ott <sebott@linux.vnet.ibm.com>
Date: Thu, 12 Oct 2017 11:57:26 +0000 (+0200)
Subject: s390/debug: only write data once
X-Git-Tag: v4.15-rc1~166^2~34
X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=0dcd91a9e6cc0a401416bbdd34b0a255cde0aee1;p=linux.git

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 df916738fbd8..2df679d75454 100644
--- 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;