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Re: [Qemu-devel] [PATCH V6 3/4] qmp, nmi: convert do_inject_nmi() to QOb

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH V6 3/4] qmp, nmi: convert do_inject_nmi() to QObject
Date: Thu, 24 Feb 2011 11:12:29 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.15) Gecko/20101027 Lightning/1.0b1 Thunderbird/3.0.10 
On 02/24/2011 10:20 AM, Markus Armbruster wrote:

Anthony Liguori<address@hidden>  writes:


On 02/24/2011 02:33 AM, Markus Armbruster wrote:

Anthony Liguori<address@hidden>   writes:



On 01/27/2011 02:20 AM, Lai Jiangshan wrote:


Make we can inject NMI via qemu-monitor-protocol.
We use "inject-nmi" for the qmp command name, the meaning is clearer.

Signed-off-by:  Lai Jiangshan<address@hidden>
---
diff --git a/hmp-commands.hx b/hmp-commands.hx
index ec1a4db..e763bf9 100644
--- a/hmp-commands.hx
+++ b/hmp-commands.hx
@@ -725,7 +725,8 @@ ETEXI
            .params     = "[cpu]",
            .help       = "Inject an NMI on all CPUs if no argument is given, "
                          "otherwise inject it on the specified CPU",
-        .mhandler.cmd = do_inject_nmi,
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_inject_nmi,
        },
    #endif
    STEXI
diff --git a/monitor.c b/monitor.c
index 387b020..1b1c0ba 100644
--- a/monitor.c
+++ b/monitor.c
@@ -2542,7 +2542,7 @@ static void do_wav_capture(Monitor *mon, const QDict 
*qdict)
    #endif

    #if defined(TARGET_I386)
-static void do_inject_nmi(Monitor *mon, const QDict *qdict)
+static int do_inject_nmi(Monitor *mon, const QDict *qdict, QObject **ret_data)
    {
        CPUState *env;
        int cpu_index;
@@ -2550,7 +2550,7 @@ static void do_inject_nmi(Monitor *mon, const QDict 
*qdict)
        if (!qdict_haskey(qdict, "cpu-index")) {
            for (env = first_cpu; env != NULL; env = env->next_cpu)
                cpu_interrupt(env, CPU_INTERRUPT_NMI);
-        return;
+        return 0;
        }

        cpu_index = qdict_get_int(qdict, "cpu-index");
@@ -2560,8 +2560,10 @@ static void do_inject_nmi(Monitor *mon, const QDict 
*qdict)
                    kvm_inject_interrupt(env, CPU_INTERRUPT_NMI);
                else
                    cpu_interrupt(env, CPU_INTERRUPT_NMI);
-            break;
+            return 0;
            }
+
+    return -1;
    }
    #endif

diff --git a/qmp-commands.hx b/qmp-commands.hx
index 56c4d8b..a887dd5 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -429,6 +429,34 @@ Example:

    EQMP

+#if defined(TARGET_I386)
+    {
+        .name       = "inject-nmi",
+        .args_type  = "cpu-index:i?",
+        .params     = "[cpu]",
+        .help       = "Inject an NMI on all CPUs if no argument is given, "
+                      "otherwise inject it on the specified CPU",
+        .user_print = monitor_user_noop,
+        .mhandler.cmd_new = do_inject_nmi,
+    },
+#endif
+SQMP
+inject-nmi
+----------
+
+Inject an NMI on all CPUs or the given CPU (x86 only).
+
+Arguments:
+
+- "cpu-index": the index of the CPU to be injected NMI (json-int, optional)
+
+Example:
+
+->    { "execute": "inject-nmi", "arguments": { "cpu-index": 0 } }
+<- { "return": {} }



Please describe all expected errors.


Quoting qmp-commands.hx:

      3. Errors, in special, are not documented. Applications should NOT check
         for specific errors classes or data (it's strongly recommended to only
         check for the "error" key)

Indeed, not a single error is documented there.  This is intentional.


Yeah, but we're not 0.14 anymore and for 0.15, we need to document
errors.  If you are suggesting I send a patch to remove that section,
I'm more than happy to.

Two separate issues here: 1. Are we ready to commit to the current
design of errors, and 2. Is it fair to reject Lai's patch now because he
doesn't document his errors.

I'm not commenting on 1. here.

Regarding 2.: rejecting a patch because it doesn't document an aspect
that current master intentionally leaves undocumented is not how you
treat contributors.  At least not if you want any other than certified
masochists who enjoy pain, and professionals who get adequately
compensated for it.

Lead by example, not by fiat.


http://repo.or.cz/w/qemu/aliguori.git/blob/refs/heads/glib:/qmp-schema.json
I am in the process of documenting the errors of every command. It's a royal pain but I'm going to document everything we have right now. It's actually the last bit of work I need to finish before sending QAPI out.
So for new commands being added, it would be hugely helpful for the authors to document the errors such that I don't have to reverse engineer all of the possible error conditions. 


Once we have an error design in place that has a reasonable hope to
stand the test of time, and have errors documented for at least some of
the commands here, we can start to require proper error documentation
for new commands.  But not now.


I'm quite happy with the error design we have today.  The only problem
is that we don't propagate errors in a sane way but I've got that all
but fixed in my qapi tree.

I don't think error propagation is the only problem we have with QError.

QError makes it way too hard to emit error messages fit for human
consumption.  The consequence is that we get errors unfit for humans.
Take a look at http://repo.or.cz/w/qemu/aliguori.git/blob/refs/heads/glib:/hmp.c#l64
Automatically generating an error string based on an error and no additional context is problematic. Given the context of the caller and the right fields in the error, very high quality error messages can be constructed. 

Regards,

Anthony Liguori




                                        Don't hide this command for
!defined(TARGET_I386), instead have it throw an error in the
implementation.


Works for me.



Don't have commands that multiple behavior based on the presence or
absence of arguments.  Make it take a list of cpus if you want the
ability to inject the NMI to more than one CPU.


Having optional arguments is fine.  It's good taste to give them
"default semantics", i.e. "no argument" is shorthand for one specific
argument value.

Luiz already pointed to the thread where we discussed this command
before.  Executive summary:

* Real hardware's NMI button injects all CPUs.  This is the primary use
    case.

* Lai said injecting a single CPU can be useful for debugging.  Was
    deemed acceptable as secondary use case.

    Lai also pointed out that the human monitor's nmi command injects a
    single CPU.  That was dismissed as irrelevant for QMP.

* No other use cases have been presented.

Therefore, the "list of CPUs" idea was shot down as overly general.


That's fine, then we should do two commands.  Think of it from the
perspective of the client.  This appears as:

in C:

qmp_inject_nmi(sess, false, 0,&err);

in Python:

sess.inject_nmi()

The first example doesn't tell you at all what's happening.  The
second API does look really nice until you see the following later:

sess.inject_nmi(0)

What's the difference between these two functions?  You might say this
is bad form and that an explicit named argument should be given but I
wouldn't count on it.

Having two commands, nmi_inject and nmi_inject_on_cpu, would result in
a much more readable API in both C and Python:

in C:

qmp_nmi_inject(sess,&err);
qmp_nmi_inject_on_cpu(sess, 3,&err);

in Python:

sess.nmi_inject()
sess.nmi_inject_on_cpu(3)

I'm fine with Lai's patch.  I'd be just as fine with Lai's patch less
the "inject on one CPU" feature.  And I'd be fine with your interface as
well.

Lai has been trying to get this simple feature in for almost three
months.  And we're still here, debating which way he should change his
arguments, and whether we should require him to document his errors
before anyone else.  For crying out loud, what's wrong with us?
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