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

[Jeff Cody <jcody@redhat.com>]

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 <programmingkidx@gmail.com> 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  |

> 
> I definitely want to hear Andreas's and Paolo's opinion (also cc'ed), if
> they have one.