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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [Qemu-devel] [PATCH] RFC: chardev: mark the calls that allow an implicit mux monitor
Hi
On Fri, Aug 24, 2018 at 9:37 AM Markus Armbruster <armbru@xxxxxxxxxx> wrote:
>
> Marc-André Lureau <marcandre.lureau@xxxxxxxxxx> 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 <marcandre.lureau@xxxxxxxxxx>
> > ---
> > 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
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