Re: [RFC] Design changes for bidirectional interface compatibility.

[Paul Durrant <xadimgnik@xxxxxxxxx>]

On 26/08/2022 15:25, Paul Durrant wrote:
> On 24/08/2022 16:31, Martin Harvey wrote:
> > I have a request for comments on some design changes which would 
> > considerably ease our support load, and work towards solving VM 
> > upgrade issues with PV drivers installed. Since it was originally a 
> > wiki, I’ve had to convert it to PDF, which I hope is a format that 
> > most readers of this list find acceptable.
> > Design doc v1 in PDF format:
> >
> > https://citrix.sharefile.com/d-s722cae4e280f4148ae689f972a2cea10 
> > <https://citrix.sharefile.com/d-s722cae4e280f4148ae689f972a2cea10>
> > Paul, there is further documentation concerning details of 
> > shortcomings of the current system which I shall send to you separately.
> Can you send it to the list too? I don't see a problem statement in your 
> doc. What precisely is the problem with the current mechanism? The 
> currently implemented use of PDO names to match available interface 
> versions was very deliberately chosen to avoid the need for any child 
> driver to maintain compatibility with an older parent. As long as the 
> rule is followed (i.e a new interface combo in the parent requires that 
> the PDO rev is bumped) there really should not be any compatibility 
> problem to fix.
I guess I should elaborate by saying *if* a new child driver is 
installed in system that requires an interface that the parent can't 
provide (because it is older) then the PDO naming should mean that it 
does not load (yet), and the older version of the child driver currently 
running in the system continues to be used. Thus, another mistake that 
can lead to problems, is removal of 'old' drivers when new ones are 
installed. Just don't do that. Leave the currently installed set alone.
Paul