perf mmap: Preserve the consumed cursor in forward mode

perf_mmap__read_event() updates prev as it consumes forward-buffer
records. perf_mmap__read_done() then overwrites prev with a fresh
producer head, even when unread records remain.

perf_kvm__mmap_read_idx() stops after 25 records per mmap and calls
read_done(). If more records are pending, prev advances past them. The
next drain starts at that new position and skips the unread remainder.

An event arriving after the last empty read_event() and before
read_done() can also be skipped. read_done() does not itself update the
kernel's data_tail.

Only refresh prev for overwrite buffers, which need the current head as
the end of the next backward read. Leave the forward cursor at the
consumed position.

Fixes: 53172f9057 ("perf kvm: Switch to new perf_mmap__read_event() interface")
Signed-off-by: Paul Moses <p@1g4.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/mmap.c

@@ -200,7 +200,7 @@ void perf_mmap__read_done(struct perf_mmap *map)
 	/*
 	 * Check if event was unmapped due to a POLLHUP/POLLERR.
 	 */
-	if (!refcount_read(&map->refcnt))
+	if (!refcount_read(&map->refcnt) || !map->overwrite)
 		return;
 
 	map->prev = perf_mmap__read_head(map);