[PATCH libvirt v1 0/6] Fix ZPCI address auto-generation on s390

[Daniel P. Berrangé]

On Wed, May 13, 2020 at 07:41:34PM +0200, Andrea Bolognani wrote:
> On Wed, 2020-05-13 at 17:32 +0100, Daniel P. Berrangé wrote:
> > On Tue, May 12, 2020 at 12:13:22PM +0200, Boris Fiuczynski wrote:
> > > The behavior change would be
> > > Current code:
> > >  uid=0 fid=0 -> uid=0 fid=0 -> address gets autogenerated
> > >  uid=0 fid=x -> uid=0 fid=x -> address is rejected as invalid
> > >  uid=0       -> uid=0 fid=0 -> address gets autogenerated
> > 
> > IIUC, in the two cases here where the address gets auto-generated,
> > the resulting guest VM successfully boots & runs....
> > 
> > > With the series applied
> > >  uid=0 fid=0 -> uid=0 fid=0 -> address is rejected as invalid
> > >  uid=0 fid=x -> uid=0 fid=x -> address is rejected as invalid
> > >  uid=0       -> uid=0 fid=0 -> address is rejected as invalid
> > 
> > ...so this proposed change is a functional regression for the
> > user.
> > 
> > > The documentation already specifies the uid value range correctly.
> > > The fix for users hitting the two scenarios (uid=0 fid=0) and (uid=0) is
> > > simple: Remove the zpci definition completely.
> > 
> > This would be taking a users' currently working VM, intentionally
> > breaking it, and then making the user pick up the pieces. This is
> > an example of a behaviour regression that libvirt promises to not
> > do to users.
> 
> The bit of nuance that might be missing here is that existing guests
> already have a full zPCI address stored in the domain XML, which
> means the wouldn't be affected in any way; additionally, the case
> where no zPCI address is provided when defining a new guest, which I
> assume is the most common one, will keep working.
> 
> The only scenarios that would no longer work are:
> 
>   * the user manually specifies uid=0 fid=0;
>   * the user manually specifies uid=0 and doesn't specify fid.
> 
> In both cases the user would have gone out of their way to specify
> a value for the uid attribute that is documented as being invalid:
> 
>   PCI addresses for S390 guests will have a zpci child element, with
>   two attributes: uid (a hex value between 0x0001 and 0xffff [...]
> 
>   https://libvirt.org/formatdomain.html#elementsAddress

The effect of specifying zero though is that we perform allocation
to assign a non-zero address, which is then valid. The same happens
with regular PCI devices if you give slot="0".

> As a result, they'd now get a pretty clear error message at define
> time instead of confusing behavior across the board. I'm not really
> sure anyone would complain about such a change.

I don't see this existing behaviour as confusing. It looks like mostly
being a docs ommission about auto-allocation taking place.

Regards,
Daniel
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