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

On 5/14/20 10:37 AM, Daniel P. Berrangé wrote: > 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. Maybe I am repeating myself but I find e.g the below example confusing if I take into consideration that uid=0 is NOT a valid value and fid is a valid value. Please note that the valid fid is dislocated from its original device! Specify this in the domain: pcidev1: uid='0x0000' fid='0x00000000' pcidev2: uid='0x0000' Results in a defined domain: pcidev1: uid='0x0002' fid='0x00000001' pcidev2: uid='0x0001' fid='0x00000000' If the user would be tying to fix the dislocating fid... one would very likely try this: Specify this in the domain: pcidev1: uid='0x0000' fid='0x00000000' pcidev2: uid='0x0000' fid='0x00000001' Result: error: Failed to define domain from mini-pcis.xml error: XML error: Invalid PCI address uid='0x0000', must be > 0x0000 and <= 0xffff Btw setting uid=0 is defined by architecture for a mode that we do not support in qemu AND setting fid=0 is an architectural valid assignment which in the example above is not granted to the device it was defined for.