Re: [Qemu-devel] [PATCH 2/3] QMP: Introduce Human Monitor passthrough command

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 10 Nov 2010 14:20:12 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Luiz Capitulino <lcapitulino@redhat.com> writes:
> 
> > This command allows QMP clients to execute HMP commands.
> >
> > Please, check the documentation added to the qmp-commands.hx file
> > for additional details about the interface and its limitations.
> >
> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > ---
> >  monitor.c       |   42 ++++++++++++++++++++++++++++++++++++++++++
> >  qmp-commands.hx |   45 +++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 87 insertions(+), 0 deletions(-)
> >
> > diff --git a/monitor.c b/monitor.c
> > index 61607c5..4fe7f5d 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -490,6 +490,48 @@ static int do_qmp_capabilities(Monitor *mon, const QDict *params,
> >      return 0;
> >  }
> >  
> > +static int mon_set_cpu(int cpu_index);
> > +static void handle_user_command(Monitor *mon, const char *cmdline);
> > +
> > +static int do_hmp_passthrough(Monitor *mon, const QDict *params,
> > +                              QObject **ret_data)
> > +{
> > +    int ret = 0;
> > +    QString *qs;
> > +    Monitor *old_mon, hmp;
> > +    CharDriverState bufchr;
> > +
> > +    memset(&hmp, 0, sizeof(hmp));
> > +    hmp.chr = &bufchr;
> > +    qemu_chr_init_buffered(hmp.chr);
> > +
> > +    old_mon = cur_mon;
> > +    cur_mon = &hmp;
> > +
> > +    if (qdict_haskey(params, "cpu-index")) {
> > +        ret = mon_set_cpu(qdict_get_int(params, "cpu-index"));
> > +        if (ret < 0) {
> > +            goto out;
> > +        }
> > +    }
> > +
> > +    handle_user_command(&hmp, qdict_get_str(params, "command-line"));
> > +
> > +    qs = qemu_chr_buffered_to_qs(hmp.chr);
> > +    if (qs) {
> > +        *ret_data = QOBJECT(qs);
> > +    }
> > +
> > +out:
> > +    cur_mon = old_mon;
> > +    qemu_chr_close_buffered(hmp.chr);
> > +
> > +    if (ret < 0) {
> > +        qerror_report(QERR_INVALID_PARAMETER_VALUE, "cpu-index","a CPU number");
> > +    }
> 
> Emit the error right where ret becomes negative.  Cleaner, and allows
> for other errors, should they become necessary.
> 
> > +    return ret;
> > +}
> > +
> >  static int compare_cmd(const char *name, const char *list)
> >  {
> >      const char *p, *pstart;
> > diff --git a/qmp-commands.hx b/qmp-commands.hx
> > index 793cf1c..b344096 100644
> > --- a/qmp-commands.hx
> > +++ b/qmp-commands.hx
> > @@ -761,6 +761,51 @@ Example:
> >  
> >  Note: This command must be issued before issuing any other command.
> >  
> > +EQMP
> > +
> > +    {
> > +        .name       = "hmp_passthrough",
> > +        .args_type  = "command-line:s,cpu-index:i?",
> > +        .params     = "",
> > +        .help       = "",
> > +        .user_print = monitor_user_noop,
> > +        .mhandler.cmd_new = do_hmp_passthrough,
> > +    },
> > +
> > +SQMP
> > +hmp_passthrough
> > +---------------
> > +
> > +Execute a Human Monitor command.
> > +
> > +Arguments: 
> > +
> > +- command-line: the command name and its arguments, just like the
> > +                Human Monitor's shell (json-string)
> > +- cpu-index: select the CPU number to be used by commands which access CPU
> > +             data, like 'info registers'. The Monitor selects CPU 0 if this
> > +             argument is not provided (json-int, optional)
> > +
> > +Example:
> > +
> > +-> { "execute": "hmp_passthrough", "arguments": { "command-line": "info kvm" } }
> > +<- { "return": "kvm support: enabled\r\n" }
> > +
> > +Notes:
> > +
> > +(1) The Human Monitor is NOT an stable interface, this means that command
> > +    names, arguments and responses can change or be removed at ANY time.
> > +    Applications that rely on long term stability guarantees should NOT
> > +    use this command
> > +
> > +(2) Limitations:
> > +
> > +    o This command is stateless, this means that commands that depend
> > +      on state information (such as getfd) might not work
> > +
> > +    o Commands that prompt the user for data (eg. 'cont' when the block
> > +      device is encrypted) don't currently work
> > +
> >  3. Query Commands
> >  =================
> 
> In the real human monitor, cpu-index is state (Monitor member mon_cpu).
> For pass through, you shift that state into the client (argument
> cpu-index).  Is there any other state that could need shifting?  You
> mention getfd.

Surprisingly or not, this is a very important question for QMP itself.

Anthony has said that we should make it stateless, and I do think this
is good because it seems to simplify things considerably.

However, I haven't thought about how to make things like getfd stateless.

> A possible alternative would be to keep the state around instead of
> creating a new one for each pass through command.  Dunno, haven't
> thought that through.  Maybe you did?

It's above ;) becoming stateless seems to be the way for QMP.

> Regardless, this is clearly better than nothing.  We can work on
> limitations later, should they turn out to be bothersome.

Agreed.