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btrace: read entire aux buffer 

The data_head of a perf event data buffer grows indefinitely.  Users are
expected to compute data_head % data_size to find the location inside the perf
event data buffer.

The aux_head of a perf event aux buffer wraps around and always stays within the
perf event aux buffer.

Well, at least that's the behaviour for BTS and PT - where BTS uses the data
buffer and PT the aux buffer.

GDB does not read beyond data_head or aux_head.  This is OK for BTS but wrong
for PT.  It causes only a portion of the trace to be considered by GDB.  In the
extreme case, the buffer may appear (almost) empty.

Thanks to Tim Wiederhake  <tim.wiederhake@intel.com> for reporting the anomaly.

Change it to read the entire aux buffer for PT.  The buffer is initially zero so
any extra zeroes we read before aux_head wraps around the first time will be
ignored when searching for the first PSB packet in order to synchronize onto the
trace stream.

gdb/
	* nat/linux-btrace.c (perf_event_read): Allow data_head < size.
	* nat/linux-btrace.c (perf_event_read_all): Do not adjust size.

Change-Id: If4f8049a2080a5f16f336309450b32a3eb1e3ec9
This commit is contained in:
Markus Metzger 2016-10-14 09:08:01 +02:00
parent 80bb3407f6
commit db58b3735f
2 changed files with 20 additions and 4 deletions
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5
gdb/ChangeLog
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@ -1,3 +1,8 @@
2016-11-14 Markus Metzger <markus.t.metzger@intel.com>
* nat/linux-btrace.c (perf_event_read): Allow data_head < size.
* nat/linux-btrace.c (perf_event_read_all): Do not adjust size.
2016-11-12 Tom Tromey <tom@tromey.com>
* rust-exp.y (super_name): Use std::vector.

19
gdb/nat/linux-btrace.c
View File

@ -119,10 +119,24 @@ perf_event_read (const struct perf_event_buffer *pev, __u64 data_head,
if (size == 0)
return NULL;
/* We should never ask for more data than the buffer can hold. */
buffer_size = pev->size;
gdb_assert (size <= buffer_size);
/* If we ask for more data than we seem to have, we wrap around and read
data from the end of the buffer. This is already handled by the %
BUFFER_SIZE operation, below. Here, we just need to make sure that we
don't underflow.
Note that this is perfectly OK for perf event buffers where data_head
doesn'grow indefinitely and instead wraps around to remain within the
buffer's boundaries. */
if (data_head < size)
data_head += buffer_size;
gdb_assert (size <= data_head);
data_tail = data_head - size;
buffer_size = pev->size;
begin = pev->mem;
start = begin + data_tail % buffer_size;
stop = begin + data_head % buffer_size;
@ -153,10 +167,7 @@ perf_event_read_all (struct perf_event_buffer *pev, gdb_byte **data,
__u64 data_head;
data_head = *pev->data_head;
size = pev->size;
if (data_head < size)
size = (size_t) data_head;
*data = perf_event_read (pev, data_head, size);
*psize = size;