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Re: [PULL 10/11] trace: document how to specify multiple --trace pattern
From: |
Markus Armbruster |
Subject: |
Re: [PULL 10/11] trace: document how to specify multiple --trace patterns |
Date: |
Tue, 02 Feb 2021 13:41:28 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) |
BALATON Zoltan <balaton@eik.bme.hu> writes:
> On Mon, 1 Feb 2021, Paolo Bonzini wrote:
>> On 01/02/21 17:54, Kevin Wolf wrote:
>>>> How does this option parsing work? Would then multiple patterns
>>>> separated by
>>>> comma as in -trace pattern1,pattern2 also work?
>>> This would be interpreted as an implied "enable" option with a value of
>>> "pattern1,pattern2". I don't think anything splits that string at the
>>> comma, so it would look for a trace event matching that string.
>>
>> Even worse, it would be interpreted as "-trace
>> enable=pattern1,pattern2=on" (and raise a warning since recently).
>
> Not very intuitive... What would -trace
> enable=pattern1,enable=pattern2 do then?
Welcome to the QemuOpts swamp. Bring your own mosquito net.
The argument of -trace is parsed with QemuOpts.
The option argument is specified in trace/control.c:
QemuOptsList qemu_trace_opts = {
.name = "trace",
.implied_opt_name = "enable",
.head = QTAILQ_HEAD_INITIALIZER(qemu_trace_opts.head),
.desc = {
{
.name = "enable",
.type = QEMU_OPT_STRING,
},
{
.name = "events",
.type = QEMU_OPT_STRING,
},{
.name = "file",
.type = QEMU_OPT_STRING,
},
{ /* end of list */ }
},
};
We generally refer to QemuOptsList by name. This one's name is "trace".
The non-empty .desc[] enumerates the recognized parameters.
Additionally, special parameter "id" is recognized.
.implied_opt_name enables "omitted first key defaults to implied key"
sugar. This is what makes "-trace PATTERN" shorthand for -trace
enable=PATTERN", where PATTERN contains neither '=' nor unescaped ','.
The QemuOpts parser parses an option argument string into a QemuOpts,
stores it for later use, and also returns it for immediate use.
Code can do whatever it wants with the stored parameters. This is a
wellspring of inconsistency and confusion.
Let's look at the code for -trace. In qemu_init(), we have:
case QEMU_OPTION_trace:
trace_opt_parse(optarg);
break;
This calls trace_opt_parse() for every -trace, in order. @optarg is the
argument string.
void trace_opt_parse(const char *optarg)
{
QemuOpts *opts = qemu_opts_parse_noisily(qemu_find_opts("trace"),
optarg, true);
qemu_opts_parse_noisily() parses @optarg into a QemuOpts, stores it for
later use, and also returns it for immediate use.
if (!opts) {
exit(1);
}
if (qemu_opt_get(opts, "enable")) {
trace_enable_events(qemu_opt_get(opts, "enable"));
}
Pass the last enable=PATTERN in @optarg to trace_enable_events().
trace_init_events(qemu_opt_get(opts, "events"));
Pass the the last events=FILENAME to trace_init_events(), which parses
patterns from file FILENAME and passes them to trace_enable_events().
Non-last enable=... ane events=... are silently ignored.
init_trace_on_startup = true;
Set a flag for trace_init_file().
qemu_opts_del(opts);
Delete the stored QemuOpts. We'll get back to this in jiffie.
}
Later in qemu_init(), we call trace_init_file(). Here it is:
void trace_init_file(void)
{
QemuOpts *opts = qemu_find_opts_singleton("trace");
This gets the first QemuOpts stored in the QemuOptsList named "trace"
without "id". If there is none, it creates an empty one for us.
Since trace_opt_parse() deletes, this always creates an empty one.
const char *file = qemu_opt_get(opts, "file");
This is always null.
#ifdef CONFIG_TRACE_SIMPLE
st_set_trace_file(file);
if (init_trace_on_startup) {
st_set_trace_file_enabled(true);
}
#elif defined CONFIG_TRACE_LOG
/*
* If both the simple and the log backends are enabled, "--trace file"
* only applies to the simple backend; use "-D" for the log
* backend. However we should only override -D if we actually have
* something to override it with.
*/
if (file) {
qemu_set_log_filename(file, &error_fatal);
}
#else
if (file) {
fprintf(stderr, "error: --trace file=...: "
"option not supported by the selected tracing backends\n");
exit(1);
}
#endif
}
Bug: option parameter "file" has no effect. I suspect this was broken
in commit 92eecfff32 "trace: remove argument from trace_init_file",
2020-11-11.
And now I'm ready to answer your question:
-trace enable=pattern1,enable=pattern2
is a confusing way to say
-trace enable=pattern2
To specify both patterns, use
-trace enable=pattern1 -trace enable=pattern2
Lovely, isn't it?
- [PULL 02/11] tracing: convert documentation to rST, (continued)
- [PULL 02/11] tracing: convert documentation to rST, Stefan Hajnoczi, 2021/02/01
- [PULL 11/11] trace: update docs with meson build information, Stefan Hajnoczi, 2021/02/01
- [PULL 10/11] trace: document how to specify multiple --trace patterns, Stefan Hajnoczi, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, BALATON Zoltan, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Kevin Wolf, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Philippe Mathieu-Daudé, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, BALATON Zoltan, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Kevin Wolf, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Paolo Bonzini, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, BALATON Zoltan, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns,
Markus Armbruster <=
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, BALATON Zoltan, 2021/02/02
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Paolo Bonzini, 2021/02/02
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Daniel P . Berrangé, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, BALATON Zoltan, 2021/02/01
- Re: [PULL 10/11] trace: document how to specify multiple --trace patterns, Stefan Hajnoczi, 2021/02/02
[PULL 09/11] simpletrace: build() missing 2 required positional arguments, Stefan Hajnoczi, 2021/02/01
[PULL 05/11] tracetool: also strip %l and %ll from systemtap format strings, Stefan Hajnoczi, 2021/02/01
Re: [PULL 00/11] Tracing patches, Peter Maydell, 2021/02/02