Re: [Qemu-devel] [PATCH 4/6] i386: Infrastructure for versioned CPU models

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Jun 25, 2019 at 03:56:51PM +0100, Dr. David Alan Gilbert wrote:
> * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > On Tue, Jun 25, 2019 at 03:32:16PM +0100, Dr. David Alan Gilbert wrote:
> > > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > > On Tue, Jun 25, 2019 at 10:32:01AM +0100, Dr. David Alan Gilbert wrote:
> > > > > * Eduardo Habkost (ehabkost@redhat.com) wrote:
> > > > > > Base code for versioned CPU models.  This will register a "-4.1"
> > > > > > version of all existing CPU models, and make the unversioned CPU
> > > > > > models be an alias for the -4.1 versions on the pc-*-4.1 machine
> > > > > > types.
> > > > > > 
> > > > > > On older machine types, the unversioned CPU models will keep the
> > > > > > old behavior.  This way, management software can use old machine
> > > > > > types while resolving aliases if compatibility with older QEMU
> > > > > > versions is required.
> > > > > > 
> > > > > > Using "-machine none", the unversioned CPU models will be aliases
> > > > > > to the latest CPU model version.
> > > > > > 
> > > > > > Includes a test case to ensure that:
> > > > > > old machine types won't report any alias to versioned CPU models;
> > > > > > "pc-*-4.1" will return aliases to -4.1 CPU models;
> > > > > > and "-machine none" will report aliases to some versioned CPU model.
> > > > > > 
> > > > > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > > > 
> > > > > What happens when we add the next new CPU model?  So say in 4.2 we add
> > > > > a new CPU, does that default to being newcpu-4.2 ?
> > > > 
> > > > We can choose between providing old versions of the CPU model
> > > > retroactively ("NewModel-4.1" and "NewModel-4.2"), or providing
> > > > only "NewModel-4.2".
> > > > 
> > > > The question is: if we provide only "NewModel-4.2", what should
> > > > be the behavior of "-machine pc-i440fx-4.1 -cpu NewModel"?
> > > 
> > > Perhaps the existing CPUs and the first instance of a new CPU
> > > we should use something non-numeric, e.g. 'orig' rather than 4.1;
> > > we only go numeric when we cause a divergence.
> > 
> > What would be the advantage of a non-numeric version identifier?
> > I believe it would be more confusing to have (e.g.)
> > ["NewModel-orig", "NewModel-4.3"] in QEMU 4.3 instead of
> > ["NewModel-4.2", "NewModel-4.3"].
> 
> To my mind it answers your question:
> > > > The question is: if we provide only "NewModel-4.2", what should
> > > > be the behavior of "-machine pc-i440fx-4.1 -cpu NewModel"?
> 
> NewModel-orig doesn't look weird in pc-i440fx-4.1

It doesn't look weird in the surface, but it doesn't change the
fact that "NewModel-orig" is the version of NewModel that was
shipped in in QEMU 4.2.  Why would we want to hide that fact?

What's the advantage of:

  "-machine pc-i440fx-4.1 -cpu NewModel" will use "NewModel-orig",
  which is the version of NewModel added in QEMU 4.2.

in relation to:

  "-machine pc-i440fx-4.1 -cpu NewModel" will use "NewModel-4.2",
  which is the version of NewModel added in QEMU 4.2.


-- 
Eduardo