Re: [Qemu-devel] Should we auto-generate IDs?

[Programmingkid]

On Aug 27, 2015, at 8:32 AM, Jeff Cody wrote:

> (Added Eric back in to the CC list.  Looks like he got dropped
> somewhere along the way)
> 
> On Wed, Aug 26, 2015 at 11:22:08PM -0400, Programmingkid wrote:
>> 
>> On Aug 26, 2015, at 6:01 PM, Jeff Cody wrote:
>> 
>>> On Wed, Aug 26, 2015 at 02:17:17PM -0400, Programmingkid wrote:
>>>> 
>>>> On Aug 26, 2015, at 2:08 PM, Jeff Cody wrote:
>>>> 
>>>>> On Wed, Aug 26, 2015 at 01:29:04PM -0400, Programmingkid wrote:
>>>>>> 
>>>>>> On Aug 26, 2015, at 1:25 PM, Jeff Cody wrote:
>>>>>> 
>>>>>>> On Wed, Aug 26, 2015 at 06:31:57PM +0200, Markus Armbruster wrote:
>>>>>>>> Did you drop cc's intentionally?  I put them right back.
>>>>>>>> 
>>>>>>>> Programmingkid <address@hidden> writes:
>>>>>>>> 
>>>>>>>>> On Aug 25, 2015, at 8:38 AM, Markus Armbruster wrote:
>>>>>>>>> 
>>>>>>>>>> You're proposing to revise a qdev design decision, namely the 
>>>>>>>>>> purpose of
>>>>>>>>>> IDs.  This has been discussed before, and IDs remained unchanged.
>>>>>>>>>> Perhaps it's time to revisit this issue.  Cc'ing a few more people.
>>>>>>>>>> 
>>>>>>>>>> Relevant prior threads:
>>>>>>>>>> * [PATCH] qdev: Reject duplicate and anti-social device IDs
>>>>>>>>>> http://thread.gmane.org/gmane.comp.emulators.qemu/71230/focus=72272
>>>>>>>>>> * [PATCH 6/6] qdev: Generate IDs for anonymous devices
>>>>>>>>>> http://thread.gmane.org/gmane.comp.emulators.qemu/114853/focus=114858
>>>>>>>>>> * [PATCH] qdev: Assign a default device ID when none is provided.
>>>>>>>>>> http://thread.gmane.org/gmane.comp.emulators.qemu/249702
>>>>>>>>>> * IDs in QOM (was: [PATCH] util: Emancipate id_wellformed() from 
>>>>>>>>>> QemuOpt
>>>>>>>>>> http://thread.gmane.org/gmane.comp.emulators.qemu/299945/focus=300381
>>>>>>>>>> 
>>>>>>>>> 
>>>>>>>>> After reading all the threads, I realize why all the attempts to
>>>>>>>>> accept a device ID patch failed.
>>>>>>>>> It is because it was assumed everyone would agree on one patch to
>>>>>>>>> accept. This is
>>>>>>>>> very unlikely. It would take someone in a leadership position to
>>>>>>>>> decide which patch
>>>>>>>>> should be accepted. From one of the threads above, I saw Anthony
>>>>>>>>> Liguori participate.
>>>>>>>>> He was in the perfect position to make the choice. The person who is
>>>>>>>>> in his position now
>>>>>>>>> is Peter Maydell. Maybe we should just ask him to look at all the
>>>>>>>>> candidate patches and
>>>>>>>>> have him pick one to use. 
>>>>>>>> 
>>>>>>>> Yes, when no consensus emerges, problems tend to go unsolved.
>>>>>>>> 
>>>>>>>> Before we appeal to authority to break the deadlock, we should make
>>>>>>>> another attempt at finding consensus.
>>>>>>>> 
>>>>>>>> I know that we've entertained the idea of automatically generated IDs
>>>>>>>> for block layer objects (that's why I cc'ed some block guys).
>>>>>>> 
>>>>>>> Yeah, I was one of the ones that proposed some auto-generated IDs for
>>>>>>> the block layer, specifically for BlockDriverState, making use of the
>>>>>>> node-name field that Benoit introduced a while ago.  Here is my patch
>>>>>>> (not sure if this is the latest version, but it is sufficient for this
>>>>>>> discussion):
>>>>>>> 
>>>>>>> http://patchwork.ozlabs.org/patch/355990/
>>>>>>> 
>>>>>>> I'm not sure about the requirements needed by device ID names, and
>>>>>>> they may of course differ from what I was thinking for BDS entries.
>>>>>>> 
>>>>>>> Here is what I was after with my patch for node-name auto-generation:
>>>>>>> 
>>>>>>> * Identifiable as QEMU generated / reserved namespace
>>>>>>> 
>>>>>>> * Guaranteed uniqueness
>>>>>>> 
>>>>>>> * Non-predictable (don't want users trying to guess / assume
>>>>>>> generated node-names)
>>>>>>> 
>>>>>>> My approach was overkill in some ways (24 characters!).  But for
>>>>>>> better or worse, what I had was:
>>>>>>> 
>>>>>>>             __qemu##00000000IAIYNXXR
>>>>>>>             ^^^^^^^^
>>>>>>> QEMU namespace ----|    ^^^^^^^^
>>>>>>>                       |     ^^^^^^^^^
>>>>>>> Increment counter, unique |         |
>>>>>>>                                 |
>>>>>>> Random string, to spoil prediction  |
>>>>>> 
>>>>>> Yikes! 24 characters long. That is a bit much to type. Thank you very 
>>>>>> much
>>>>>> for your effort.
>>>>> 
>>>>> IMO, the number of characters to type is pretty low on the list of
>>>>> requirements, although it can still be addressed secondary to other
>>>>> concerns.
>>>>> 
>>>>> I should have made this in reply to Markus' other email, because the
>>>>> important part of this is try and address his point #2:
>>>>> 
>>>>> (from Markus' other email):
>>>>>> 2. The ID must be well-formed.
>>>>> 
>>>>> To have a well-formed ID, we need to have know requirements of the ID
>>>>> structure (i.e. the why and what of it all)
>>>>> 
>>>>> I don't know if the three requirements I had above apply to all areas
>>>>> in QEMU, but I expect they do, in varying degrees of importance.  The
>>>>> length itself can be tweaked.
>>>>> 
>>>>> Talking with John Snow over IRC (added to the CC), one thing he
>>>>> suggested was adding in sub-domain spaces; e.g.:
>>>>> 
>>>>> __qemu#bn#00000000#IAIYNXXR
>>>>> 
>>>>> Where the 'bn' in this case would be for Block Nodes, etc..
>>>>> 
>>>>> This may make the scheme extensible through QEMU, where auto-generated
>>>>> IDs are desired.
>>>>> 
>>>>> (sorry to say, this lengthens things, rather than shortening them!)
>>>>> 
>>>>> We can, of course, make the string shorter - if the random characters
>>>>> are just there for spoiling predictability, then 2-3 should be
>>>>> sufficient. We could then end up with something like this:
>>>>> 
>>>>> __qemu#bn#00000000#XR
>>>>> 
>>>>> The "__qemu" part of the namespace could be shortened as well, but it
>>>>> would be nice if it was easy recognizable as being from QEMU.
>>>> 
>>>> If this ID format was supported, I'm thinking being able to copy and paste 
>>>> from
>>>> the monitor is a necessary feature. 
>>>> 
>>>> Any way it could be shorted? I was hoping no more than three characters 
>>>> long. 
>>>> 
>>> 
>>> Likely could be shorter, but something in the realm of three
>>> characters doesn't seem very realistic.
>> 
>> Sure it is. Just set device id's like this: 0, 1, 2, 3, 4, 5, 6....
> 
> I'm not married to the ID generation scheme I proposed.  
> 
> What I am trying to do, however, is have a technical discussion on
> generating an ID in a well-formed manner.  And hopefully, in a way
> that is useful to all interested subsystems, if possible.
> 
> Do you disagree with the requirements I listed above?  If so, it would
> be useful to begin the discussion around that.  For ease of
> discussion, I'll list them again:
> 
> * Reserved namespaces
> * Uniqueness
> * Non-predictable (to avoid inadvertently creating a de facto ABI)

Uniqueness is a must. Reserve namespaces? Why do we need to do this?
What is wrong with having a predictable ID?

Maybe we need to discuss where this ID is going to be used. I know I 
need it for the device_del monitor command. Any other places you or
anyone else knows it is used?

>  . .
> 
> On the generation scheme proposed above:
> 
> I understand that something you desire is an ID that is easier to
> type.
> 
> If we wanted to make it shorter, perhaps we could have the number
> counter be variable length:
> 
>            qemu#ss#D#XY
>              |   | | |
> qemu reserved -   | | |
>                  | | |
> subsystem name ---| | |
>                    | |
>    counter --------| |
>                      |
>    2-digit random ---|
> 
> 
> The counter would just grow to however many digits are needed.  There
> is another benefit to growing that number as well - we can use
> whatever integer size we think is adequate in the code, without
> affecting the generation scheme.
> 
> -Jeff

This system does seem easy to type. Do we need the "qemu" part?
It seems unnecessary. Maybe we could do this:

<subsystem name><counter>

Examples:
For the third block device it would look like this: bl3
For the seventh USB device it would look like this: ub7

Each subsystem would receive a two character code.