Re: [PATCH libvirt v1 0/6] Fix ZPCI address auto-generation on s390
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
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.
--
Andrea Bolognani / Red Hat / Virtualization