Re: [PATCH] docs/designs: re-work the xenstore migration document...

[Jürgen Groß <jgross@xxxxxxxx>]

On 24.04.20 16:38, Julien Grall wrote:
> Hi,
>
> On 24/04/2020 15:26, Jürgen Groß wrote:
> > > +```
> > > +    0       1       2       3       4       5       6       7    octet
> > > ++-------+-------+-------+-------+-------+-------+-------+-------+
> > > +| type                          | len                           |
> > > ++-------------------------------+-------------------------------+
> > > +| body
> > > +...
> > > +|       | padding (0 to 7 octets)                               |
> > > ++-------+-------------------------------------------------------+
> > > +```
> > > +
> > > +NOTE: padding octets here and in all subsequent format 
> > > specifications must be
> > > +      zero, unless stated otherwise.
> > What about: "... are written as zero and should be ignored on read."
> I would rather not say "ignored" because it doesn't allow us to extend 
> the record if needed in a safe way. The read side should abort if it 
> sees an other value than 0 in the padding.
I'd rather ignore unknown fields. This allows to add optional data
without having to special case it. Writing zeroes is the important part
here, of course.

Ignoring those fields would e.g. enable a downgrade more easily, while
aborting could make that impossible.

And in case a version would write non-zero bytes (e.g. due to a bug)
this version could never be live-updated.

Juergen