[PATCH 1 of 4] xentrace: reduce trace buffer size to something mfn_offset can reach

[Olaf Hering <olaf@aepfle.de>]

# HG changeset patch
# User Olaf Hering <olaf@aepfle.de>
# Date 1305037380 -7200
# Node ID 8ac937fa527b28243227193bf4749feb3a234c2c
# Parent  19452acd23045f40c4e18437f0a60f016757e5bd
xentrace: reduce trace buffer size to something mfn_offset can reach

The start of the array which holds the list of mfns for each cpus
tracebuffer is stored in an unsigned short. This limits the total amount
of pages for each cpu as the number of active cpus increases.

Update the math in calculate_tbuf_size() to apply also this rule to the
max number of trace pages. Without this change the index can overflow.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

diff -r 19452acd2304 -r 8ac937fa527b xen/common/trace.c
--- a/xen/common/trace.c	Fri May 06 11:15:35 2011 +0100
+++ b/xen/common/trace.c	Tue May 10 16:23:00 2011 +0200
@@ -112,11 +112,14 @@ static int calculate_tbuf_size(unsigned 
     typeof(dummy_size.prod) max_size;
     struct t_info dummy_pages;
     typeof(dummy_pages.tbuf_size) max_pages;
+    typeof(dummy_pages.mfn_offset[0]) max_mfn_offset;
+    unsigned int max_cpus = num_online_cpus();
     unsigned int t_info_words;
 
     /* force maximum value for an unsigned type */
     max_size = -1;
     max_pages = -1;
+    max_mfn_offset = -1;
 
     /* max size holds up to n pages */
     max_size /= PAGE_SIZE;
@@ -124,6 +127,18 @@ static int calculate_tbuf_size(unsigned 
     if ( max_size < max_pages )
         max_pages = max_size;
 
+    /*
+     * max mfn_offset holds up to n pages per cpu
+     * The array of mfns for the highest cpu can start at the maximum value
+     * mfn_offset can hold. So reduce the number of cpus and also the mfn_offset.
+     */
+    max_mfn_offset -= t_info_first_offset - 1;
+    max_cpus--;
+    if ( max_cpus )
+        max_mfn_offset /= max_cpus;
+    if ( max_mfn_offset < max_pages )
+        max_pages = max_mfn_offset;
+
     if ( pages > max_pages )
     {
         printk(XENLOG_INFO "xentrace: requested number of %u pages "