qemu-trace-stap: changing SYSTEMTAP_TAPSET considered harmful.

Setting SYSTEMTAP_TAPSET to some value other than
/usr/share/systemtap/tapsets results in systemtap not finding the
standard tapset library any more, which in turn breaks tracing because
pid() and other standard systemtap functions are not available any more.

So using SYSTEMTAP_TAPSET to point systemtap to the qemu probes will
only work for the prefix=/usr installs because both qemu and system
tapsets in the same directory then.  All other prefixes are broken.

Fix that by using the "-I $tapsetdir" command line switch instead.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210601132414.432430-2-kraxel@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/qemu-trace-stap

@@ -55,11 +55,6 @@ def tapset_dir(binary):
     return os.path.realpath(tapset)
 
 
-def tapset_env(tapset_dir):
-    tenv = copy.copy(os.environ)
-    tenv["SYSTEMTAP_TAPSET"] = tapset_dir
-    return tenv
-
 def cmd_run(args):
     prefix = probe_prefix(args.binary)
     tapsets = tapset_dir(args.binary)
@@ -81,11 +76,11 @@ def cmd_run(args):
 
     # We request an 8MB buffer, since the stap default 1MB buffer
     # can be easily overflowed by frequently firing QEMU traces
-    stapargs = ["stap", "-s", "8"]
+    stapargs = ["stap", "-s", "8", "-I", tapsets ]
     if args.pid is not None:
         stapargs.extend(["-x", args.pid])
     stapargs.extend(["-e", script])
-    subprocess.call(stapargs, env=tapset_env(tapsets))
+    subprocess.call(stapargs)
 
 
 def cmd_list(args):
@@ -101,10 +96,9 @@ def cmd_list(args):
 
         if verbose:
             print("Listing probes with name '%s'" % script)
-        proc = subprocess.Popen(["stap", "-l", script],
+        proc = subprocess.Popen(["stap", "-I", tapsets, "-l", script],
                                 stdout=subprocess.PIPE,
-                                universal_newlines=True,
-                                env=tapset_env(tapsets))
+                                universal_newlines=True)
         out, err = proc.communicate()
         if proc.returncode != 0:
             print("No probes found, are the tapsets installed in %s" % tapset_dir(args.binary))