Re: [Qemu-devel] [libvirt] [qemu RFC v2] qapi: add "firmware.json"

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Apr 20, 2018 at 10:11:08AM +0200, Laszlo Ersek wrote:
> On 04/19/18 11:12, Daniel P. Berrangé wrote:
> > On Thu, Apr 19, 2018 at 10:39:32AM +0200, Laszlo Ersek wrote:
> >> On 04/19/18 09:56, Daniel P. Berrangé wrote:
> >>> On Thu, Apr 19, 2018 at 09:48:36AM +0200, Markus Armbruster wrote:
> >>>> Laszlo Ersek <lersek@redhat.com> writes:
> >>>>
> >>>>> On 04/18/18 10:47, Markus Armbruster wrote:
> >>>>>> Laszlo Ersek <lersek@redhat.com> writes:
> >>>> Replacing CpuInfoArch by such an enum will change the discriminator
> >>>> value from "other" to the real architecture, with the obvious
> >>>> compatibility concerns.  But we've accepted similar changes twice
> >>>> already: commit 9d0306dfdfb and commit 25fa194b7b1, both v2.12.0-rc0.
> >>>>
> >>>> "other" was a bad idea.  Hindsight 20/20.
> >>>>
> >>>> Getting rid of it in one go rather than piecemeal seems like the least
> >>>> bad way out.  Too late for 2.12, though.  Eric, what do you think?
> >>>
> >>> Given the context in which this "other" value is used, I think it is
> >>> reasonable to kill it and put a full arch list in there.
> >>>
> >>> No app is likely to be accessing the struct under "other" because it
> >>> is just an empty placeholder.
> >>
> >> Commit 9d0306dfdfb added "s390" and "CpuInfoS390", which I guess had the
> >> potential to confuse QMP clients that didn't expect "s390", but
> >> otherwise it didn't mess with preexistent enum values / structures.
> > 
> > NB, qemu-system-s390x would previously have returned "other" in
> > this field, and now it returns "s390".  So while it didn't
> > remove "other" from the list of things that could potentially
> > exist, it did change what the s390x binary will actually report.
> > 
> >> The same applies to commit 25fa194b7b1, just with "riscv" /
> >> "CpuInfoRISCV" substituted.
> >>
> >> Removing "other" might confuse QMP clients that expect it, except
> >> (according to Daniel) no such client exists, probably.
> > 
> > When I say removing "other", I imply that we add an explicit arch
> > for all those which we currently are missing. IOW, all qemu-system-XXX
> > binaries which currently report "other" would change to report their
> > respective "XXX" values.
> > 
> > So in this way, it is exactly the same as what we did when we
> > introduced "s390" as an option.
> > 
> > The only difference is that once we have every binary reporting the
> > correct arch, we can now also remove "other" from the schema itself
> > as it will then be unused.
> 
> Can we please translate this into more actionable items for me, because
> I'm getting confused :)
> 
> First, if I add "i386" and "x86_64" to the enum list, we'll have all
> three of "i386", "x86_64" and "x86". Is that useful? How will that work?

Hmm, yes, on closer look this is a big mess as it is. We've been using
generic terms for covering multiple architectures :-(  'x86' for both
i386 and x86_64,  'sparc' for sparc and sparc64, etc. If we try to fix
that we'll be entering a world of backcompat hurt :-(

Since your schema is likely to end up just being a file in docs/specs,
rather than directly part of our existnig qapi schema, I suggest we just
ignore whats there. Just define an arch enum in your spec which is right,
and let someone else worry about fixing the mess

> Second, assuming I add constants for the ~10 (?) softmmu arches, can I
> still use @CpuInfoOther as the type for the corresponding new members in
> @CpuInfo? What C code changes will be necessary?

Yes, we could still use the CpuInfoOther struct, since struct names are
invisible to consumers, but as above, lets ignore the mess

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|