Improve performance of large restore commands

I noticed a large (100MB) restore took hours to complete. The problem
is memory_xfer_partial repeatedly mallocs and memcpys the entire
100MB buffer for breakpoint shadow handling only to find a small
portion of it is actually written.

The testcase that originally took hours now takes 50 seconds.

gdb/
2013-07-29  Anton Blanchard  <anton@samba.org>

	* target.c (memory_xfer_partial): Cap write to 4KB.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2013-11-04  Anton Blanchard  <anton@samba.org>
+
+	* target.c (memory_xfer_partial): Cap write to 4KB.
+
 2013-11-01  Tiago Stürmer Daitx  <tdaitx@linux.vnet.ibm.com>
 
 	* breakpoint.c (create_longjmp_master_breakpoint): Allow libc

gdb/target.c

@@ -1678,6 +1678,13 @@ memory_xfer_partial (struct target_ops *ops, enum target_object object,
       void *buf;
       struct cleanup *old_chain;
 
+      /* A large write request is likely to be partially satisfied
+	 by memory_xfer_partial_1.  We will continually malloc
+	 and free a copy of the entire write request for breakpoint
+	 shadow handling even though we only end up writing a small
+	 subset of it.  Cap writes to 4KB to mitigate this.  */
+      len = min (4096, len);
+
       buf = xmalloc (len);
       old_chain = make_cleanup (xfree, buf);
       memcpy (buf, writebuf, len);