xentrace: Allow xentrace to handle >4G of trace data.

It was previously assert'ing when it hit 4G.

Also, because the trace buffer is not a power of 2 in size,
using modulo arithmetic to address the buffer does not work
when the index wraps around 2^32.

This patch fixes both issues, and as a side effect, removes all
integer division from the hypervisor side of the trace mechanism.

Signed-off-by: Michael A Fetterman <Michael.Fetterman@cl.cam.ac.uk>

1 files changed, 11 insertions, 2 deletions

diff --git a/tools/xentrace/xentrace.c b/tools/xentrace/xentrace.c
index 26415bdb7d..bd276cc3ed 100644
--- a/tools/xentrace/xentrace.c
+++ b/tools/xentrace/xentrace.c
@@ -362,9 +362,18 @@ int monitor_tbufs(int outfd)
             if ( cons == prod )
                 continue;
            
-            assert(prod > cons);
+            assert(cons < 2*data_size);
+            assert(prod < 2*data_size);
+
+            // NB: if (prod<cons), then (prod-cons)%data_size will not yield
+            // the correct answer because data_size is not a power of 2.
+            if ( prod < cons )
+                window_size = (prod + 2*data_size) - cons;
+            else
+                window_size = prod - cons;
+            assert(window_size > 0);
+            assert(window_size <= data_size);
 
-            window_size = prod - cons;
             start_offset = cons % data_size;
             end_offset = prod % data_size;