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[Qemu-devel] Re: [RFC 0/7] QError v1

From: Anthony Liguori
Subject: [Qemu-devel] Re: [RFC 0/7] QError v1
Date: Fri, 30 Oct 2009 08:09:02 -0500
User-agent: Thunderbird (X11/20090825)

Gerd Hoffmann wrote:
On 10/30/09 13:28, Luiz Capitulino wrote:
- qemu_error("Device \"%s\" not found. Try -device '?' for a list.\n",
-                   driver);
+ qemu_error_structed(QERR_DEV_NFOUND, "{ 'name': %s }", driver);

why not store the "{ 'name': %s }" in the qerror_table?  I guess you
plan to have different fields in some cases?

  The main reason is to have syntax checking, we can declare it in a
macro though, in case of generic errors which are going to be used in
other places.

I still feel the error reporting is too complex. IMHO there should be no need to edit two places for error reporting, which means I'd go the opposite direction: Zap qerror_table[], then have:

qemu_error_structed(QERR_DEV_NFOUND, "device %{name}s not found",
                    "{ 'name': %s }", driver);

Also I think the error codes should be more generic, so you don't need a new one for each and every error. Ideally we'll have a reasonable and stable set of error codes after the initial conversion, so you don't have to touch the management apps just to add new codes as qemu envolves. The error code must help the management app to decide how to deal with the error, but it shouldn't carry details not needed for that.

Okay, let's get more clever then and do:

#define QERR_DEV_NFOUND "{ 'code': 404, 'name': %s}"

So we can do:

qemu_error_structured(QERR_DEV_NFOUND, driver);

Such that we still get printf style parameter checking.

Picking the balloon errors (other patch in this thread): You have *two* error codes for ballooning not being available. I think a generic "service not available" error code would work for both (and for other error cases too) and would be good enougth. The management app will figure it can't balloon down the VM. It will not know the reason from the error code, but does it have to? I doubt it will react in a different way. And for manual trouble-shooting the text message which carries more information gets logged.

I think the trouble is Luiz is trying to preserve today's error messages. Honestly, if we need to break those, I don't mind so much because I really doubt anyone is depending on the exact text of the error messages.

I agree that a bit more generic error messages wouldn't be a bad thing.


Anthony Liguori

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