Re: [PATCH v3] machine: add missing doc for memory-backend option

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

On Wed, Jan 27, 2021 at 07:30:24PM +0100, Michal Privoznik wrote:
> On 1/27/21 6:56 PM, Daniel P. Berrangé wrote:
> > On Wed, Jan 27, 2021 at 04:35:22PM +0100, Igor Mammedov wrote:
> > > On Wed, 27 Jan 2021 15:24:26 +0100
> > > Michal Privoznik <mprivozn@redhat.com> wrote:
> > > 
> > > > On 1/27/21 11:54 AM, Daniel P. Berrangé wrote:
> > > > > On Wed, Jan 27, 2021 at 10:45:11AM +0000, Daniel P. Berrangé wrote:
> > > > > > On Thu, Jan 21, 2021 at 11:15:04AM -0500, Igor Mammedov wrote:
> > > > 
> > > > 
> > > > > > 
> > > > > > How does a mgmt app know which machine types need to use this
> > > > > > option ? The machine type names are opaque strings, and apps
> > > > > > must not attempt to parse or interpret the version number
> > > > > > inside the machine type name, as they can be changed by
> > > > > > distros.  IOW, saying to use it for machine types 4.0 and
> > > > > > older isn't a valid usage strategy IMHO.
> > > it's possible (but no necessary) to use knob with new machine types
> > > (defaults for these match suggested property value).
> > 
> > IIUC, this means that setting the property has no impact on
> > migration ABI for new machine types > 4.0....
> > 
> > > Limiting knob usage to 4.0 and older would allow us to drop
> > > without extra efforts once 4.0 is deprecated/removed.
> > 
> > ...so, even if we set the property unconditionally for *all*
> > machine types, then we can still remove it in future, becuase
> > its removal won't affect ABI of the 5.x, 6.x machine types.
> 
> Alright, so after all you agree with proposed patch? I'm a bit confused.

+        b) for machine types 4.0 and older, user shall
+        use ``x-use-canonical-path-for-ramblock-id=off`` backend option
+        if migration to/from old QEMU (<5.0) is expected.

instead of refering to machine type versions, I think it
could say something more like


      b) if the ``x-use-canonical-path-for-ramblock-id`` property
         exists, it should be set to ``off``, 
         if migration to/from old QEMU (<5.0) is expected.

Assuming it genuinely does not have any ABI impact on new machine
types.

Regards,
Daniel
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