Re: [Qemu-devel] [PATCH] RFC: chardev: mark the calls that allow an implicit mux monitor

[Marc-André Lureau]

Hi
On Fri, Aug 24, 2018 at 9:37 AM Markus Armbruster <address@hidden> wrote:
>
> Marc-André Lureau <address@hidden> writes:
>
> > This is mostly for readability of the code. Let's make it clear which
> > callers can create an implicit monitor when the chardev is muxed.
> >
> > This will also enforce a safer behaviour, as we don't really support
> > creating monitor anywhere/anytime at the moment.
> >
> > There are documented cases, such as: -serial/-parallel/-virtioconsole
> > and to less extent -debugcon.
> >
> > Less obvious and questionnable ones are -gdb and Xen console. Add a
> > FIXME note for those, but keep the support for now.
> >
> > Other qemu_chr_new() callers either have a fixed parameter/filename
> > string, or do preliminary checks on the string (such as slirp), or do
> > not need it, such as -qtest.
> >
> > On a related note, the list of monitor creation places:
> >
> > - the chardev creators listed above: all from command line (except
> >   perhaps Xen console?)
> >
> > - -gdb & hmp gdbserver will create a "GDB monitor command" chardev
> >   that is wired to an HMP monitor.
> >
> > - -mon command line option
>
> Aside: the question "what does it mean to connect a monitor to a chardev
> that has an implicit monitor" crosses my mind.  Next thought is "the
> day's too short to go there".
>
> > From this short study, I would like to think that a monitor may only
> > be created in the main thread today, though I remain skeptical :)
>
> Hah!
>
> > Signed-off-by: Marc-André Lureau <address@hidden>
> > ---
> >  include/chardev/char.h | 18 +++++++++++++++++-
> >  chardev/char.c         | 21 +++++++++++++++++----
> >  gdbstub.c              |  6 +++++-
> >  hw/char/xen_console.c  |  5 ++++-
> >  vl.c                   |  8 ++++----
> >  5 files changed, 47 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/chardev/char.h b/include/chardev/char.h
> > index 6f0576e214..be2b9b817e 100644
> > --- a/
> > +++ b/include/chardev/char.h
> > @@ -105,6 +105,7 @@ ChardevBackend *qemu_chr_parse_opts(QemuOpts *opts,
> >   * @qemu_chr_new:
> >   *
> >   * Create a new character backend from a URI.
> > + * Do not implicitely initialize a monitor if the chardev is muxed.
> >   *
> >   * @label the name of the backend
> >   * @filename the URI
> > @@ -113,6 +114,19 @@ ChardevBackend *qemu_chr_parse_opts(QemuOpts *opts,
> >   */
> >  Chardev *qemu_chr_new(const char *label, const char *filename);
> >
> > +/**
> > + * @qemu_chr_new_mux_mon:
> > + *
> > + * Create a new character backend from a URI.
> > + * Implicitely initialize a monitor if the chardev is muxed.
> > + *
> > + * @label the name of the backend
> > + * @filename the URI
>
> I'm no friend of GTK-Doc style, but where we use it, we should use it
> correctly: put a colon after @param.

I copied existing comment... Should I fixed all others in the headers?

>
> > + *
> > + * Returns: a new character backend
> > + */
> > +Chardev *qemu_chr_new_mux_mon(const char *label, const char *filename);
> > +
> >  /**
> >   * @qemu_chr_change:
> >   *
> > @@ -138,10 +152,12 @@ void qemu_chr_cleanup(void);
> >   *
> >   * @label the name of the backend
> >   * @filename the URI
> > + * @with_mux_mon if chardev is muxed, initialize a monitor
> >   *
> >   * Returns: a new character backend
> >   */
> > -Chardev *qemu_chr_new_noreplay(const char *label, const char *filename);
> > +Chardev *qemu_chr_new_noreplay(const char *label, const char *filename,
> > +                               bool with_mux_mon);
> >
> >  /**
> >   * @qemu_chr_be_can_write:
> > diff --git a/chardev/char.c b/chardev/char.c
> > index 76d866e6fe..2c295ad676 100644
> > --- a/chardev/char.c
> > +++ b/chardev/char.c
> > @@ -683,7 +683,8 @@ out:
> >      return chr;
> >  }
> >
> > -Chardev *qemu_chr_new_noreplay(const char *label, const char *filename)
> > +Chardev *qemu_chr_new_noreplay(const char *label, const char *filename,
> > +                               bool with_mux_mon)
> >  {
> >      const char *p;
> >      Chardev *chr;
> > @@ -702,17 +703,19 @@ Chardev *qemu_chr_new_noreplay(const char *label, 
> > const char *filename)
> >      if (err) {
> >          error_report_err(err);
> >      }
> > -    if (chr && qemu_opt_get_bool(opts, "mux", 0)) {
> > +    if (with_mux_mon && chr && qemu_opt_get_bool(opts, "mux", 0)) {
> >          monitor_init(chr, MONITOR_USE_READLINE);
> >      }
>
> When !chr, the function already failed, so that part of the condition is
> uninteresting here[*].

yeah, hopefully err is always set if the return value is NULL.

> That leaves qemu_opt_get_bool(opts, "mux", 0).  Behavior changes when
> it's true and with_mux_mon is false: we no longer create a monitor.
>
> Can this happen?
>
> If it can, when exactly, and how does it affect externally visible
> behavior?

we could add an assert() for with_mux_mon || !opt("mux").
>
> I guess we'll see below.
>
> >      qemu_opts_del(opts);
> >      return chr;
> >  }
> >
> > -Chardev *qemu_chr_new(const char *label, const char *filename)
> > +static Chardev *qemu_chr_new_with_mux_mon(const char *label,
> > +                                          const char *filename,
> > +                                          bool with_mux_mon)
> >  {
> >      Chardev *chr;
> > -    chr = qemu_chr_new_noreplay(label, filename);
> > +    chr = qemu_chr_new_noreplay(label, filename, with_mux_mon);
> >      if (chr) {
> >          if (replay_mode != REPLAY_MODE_NONE) {
> >              qemu_chr_set_feature(chr, QEMU_CHAR_FEATURE_REPLAY);
> > @@ -726,6 +729,16 @@ Chardev *qemu_chr_new(const char *label, const char 
> > *filename)
> >      return chr;
> >  }
> >
> > +Chardev *qemu_chr_new(const char *label, const char *filename)
> > +{
> > +    return qemu_chr_new_with_mux_mon(label, filename, false);
> > +}
> > +
> > +Chardev *qemu_chr_new_mux_mon(const char *label, const char *filename)
> > +{
> > +    return qemu_chr_new_with_mux_mon(label, filename, true);
> > +}
> > +
> >  static int qmp_query_chardev_foreach(Object *obj, void *data)
> >  {
> >      Chardev *chr = CHARDEV(obj);
>
> Note for later: qemu_chr_new() changes behavior.  To avoid that, call
> qemu_chr_new_mux_mon() instead.
>
> > diff --git a/gdbstub.c b/gdbstub.c
> > index d6ab95006c..963531ea90 100644
> > --- a/gdbstub.c
> > +++ b/gdbstub.c
> > @@ -2038,7 +2038,11 @@ int gdbserver_start(const char *device)
> >              sigaction(SIGINT, &act, NULL);
> >          }
> >  #endif
> > -        chr = qemu_chr_new_noreplay("gdb", device);
> > +        /*
> > +         * FIXME: it's a bit weird to allow using a mux chardev here
> > +         * and setup implicitely a monitor. We may want to break this.
> > +         */
> > +        chr = qemu_chr_new_noreplay("gdb", device, true);
> >          if (!chr)
> >              return -1;
> >      }
>
> No behavioral change.  The patch does exactly what the commit message
> claims, namely mark potential implicit monitor creation in the source
> code.
>
> > diff --git a/hw/char/xen_console.c b/hw/char/xen_console.c
> > index 8b4b4bf523..6a231d487b 100644
> > --- a/hw/char/xen_console.c
> > +++ b/hw/char/xen_console.c
> > @@ -207,7 +207,10 @@ static int con_init(struct XenDevice *xendev)
> >      } else {
> >          snprintf(label, sizeof(label), "xencons%d", con->xendev.dev);
> >          qemu_chr_fe_init(&con->chr,
> > -                         qemu_chr_new(label, output), &error_abort);
> > +                         /*
> > +                          * FIXME: should it support implicit muxed 
> > monitors?
> > +                          */
> > +                         qemu_chr_new_mux_mon(label, output), 
> > &error_abort);
> >      }
>
> Likewise, with a terser comment.
>
> >
> >      xenstore_store_pv_console_info(con->xendev.dev,
> > diff --git a/vl.c b/vl.c
> > index 16b913f9d5..20b5f321ec 100644
> > --- a/vl.c
> > +++ b/vl.c
> > @@ -2425,7 +2425,7 @@ static int serial_parse(const char *devname)
> >      snprintf(label, sizeof(label), "serial%d", index);
> >      serial_hds = g_renew(Chardev *, serial_hds, index + 1);
> >
> > -    serial_hds[index] = qemu_chr_new(label, devname);
> > +    serial_hds[index] = qemu_chr_new_mux_mon(label, devname);
> >      if (!serial_hds[index]) {
> >          error_report("could not connect serial device"
> >                       " to character backend '%s'", devname);
>
> Likewise, without a comment, because implicit monitor is a feature
> here.  Correct?

right

>
> > @@ -2461,7 +2461,7 @@ static int parallel_parse(const char *devname)
> >          exit(1);
> >      }
> >      snprintf(label, sizeof(label), "parallel%d", index);
> > -    parallel_hds[index] = qemu_chr_new(label, devname);
> > +    parallel_hds[index] = qemu_chr_new_mux_mon(label, devname);
> >      if (!parallel_hds[index]) {
> >          error_report("could not connect parallel device"
> >                       " to character backend '%s'", devname);
>
> Likewise.
>
> > @@ -2492,7 +2492,7 @@ static int virtcon_parse(const char *devname)
> >      qemu_opt_set(dev_opts, "driver", "virtconsole", &error_abort);
> >
> >      snprintf(label, sizeof(label), "virtcon%d", index);
> > -    virtcon_hds[index] = qemu_chr_new(label, devname);
> > +    virtcon_hds[index] = qemu_chr_new_mux_mon(label, devname);
> >      if (!virtcon_hds[index]) {
> >          error_report("could not connect virtio console"
> >                       " to character backend '%s'", devname);
>
> Likewise.
>
> > @@ -2508,7 +2508,7 @@ static int debugcon_parse(const char *devname)
> >  {
> >      QemuOpts *opts;
> >
> > -    if (!qemu_chr_new("debugcon", devname)) {
> > +    if (!qemu_chr_new_mux_mon("debugcon", devname)) {
> >          exit(1);
> >      }
> >      opts = qemu_opts_create(qemu_find_opts("device"), "debugcon", 1, NULL);
>
> Likewise.
>
> Not visible in the patch: calls of qemu_chr_new() not changed to
> qemu_chr_new_mux_mon().  These are:
>
> * qtest.c: qtest_init()
>
> * net/slirp.c: slirp_guestfwd()
>
> * hw/char/xen_console.c: con_init()
>
> * Several more in hw/, all with literal @filename argument that doesn't
>   enable mux.
>
> * tests/test-char.c
>
> * tests/vhost-user-test.c
>
> I figure these are meant to be covered by commit message paragraph
>
>     Other qemu_chr_new() callers either have a fixed parameter/filename
>     string, or do preliminary checks on the string (such as slirp), or do
>     not need it, such as -qtest.
>
> A non-RFC patch should add enough detail to let the reviewer understand
> each case without having to dig through the code himself.  Since I
> didn't do that, I can't give my R-by.  I can say I like the idea.

ok, I'll update the patch & commit message.

>
>
> [*] Aside: I prefer cleanly separated error paths, like this:
>
> Chardev *qemu_chr_new_noreplay(const char *label, const char *filename)
> {
>     const char *p;
>     Chardev *chr;
>     QemuOpts *opts;
>     Error *err = NULL;
>     bool mux;
>
>     if (strstart(filename, "chardev:", &p)) {
>         return qemu_chr_find(p);
>     }
>
>     opts = qemu_chr_parse_compat(label, filename);
>     if (!opts)
>         return NULL;
>
>     mux = qemu_opt_get_bool(opts, "mux", 0);
>     chr = qemu_chr_new_from_opts(opts, &err);
>     qemu_opts_del(opts);
>     if (!chr) {
>         error_report_err(err);
>         return NULL;
>     }
>     if (mux) {
>         monitor_init(chr, MONITOR_USE_READLINE);
>     }
>     return chr;
> }
>

thanks

-- 
Marc-André Lureau