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

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On 4/10/20 2:06 PM, Andrea Bolognani wrote:
On Thu, 2020-04-09 at 12:30 +0200, Shalini Chellathurai Saroja wrote:
The ZPCI address validation or autogeneration does not work as
expected in the following scenarios
1. uid = 0 and fid = 0
2. uid = 0 and fid > 0
3. uid = 0 and fid not specified
4. uid not specified and fid > 0
5. 2 ZPCI devices with uid > 0 and fid not specified.

This is because of the following reasons
1. If uid = 0 or fid = 0 the code assumes that user has not specified
    the corresponding address
2. If either uid or fid is provided, the code assumes that both uid
    and fid addresses are specified by the user.

I'd have to dig up the old threads, but based on what I remember the
behaviors you describe are entirely intentional.

For PCI addresses, setting all parts of the address to zero or not
setting it at all is equivalent, and we wanted to be consistent with
that behavior for ZPCI; additionally, zero is not a valid value for
uid so of course neither is the address uid=0 fid=0, which means that
we're not preventing the user from specifying a valid address by
conflating the all-zero address with the unspecified address.

For partially-specified addresses, the behavior is also the same as
PCI: any part you don't specify is considered to be zero, which
results in

   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
         fid=x -> uid=0 fid=x -> address is rejected as invalid
   uid=x       -> uid=x fid=0 -> address is accepted

So, just like for PCI addresses, you have basically two reasonable
options: either don't specify any zPCI address and leave allocation
entirely up to libvirt, or specify all of the addresses completely:
anything in between will likely not work as you'd expect or want.

Again, this is based purely on my recollection of design discussions
that happened one and a half years ago, so I might have gotten some
of the details wrong - in which case by all means call me out on
that O:-)

Hi Andrea,
sorry for the delayed answer. I (and some others as well) lost some emails on my IMAP account and I just found your answer today. I can remember that you had a discussion with the original author of the zpci code. There are a few issues with the currently implemented "rules" which partially are not even working as you outlined above in more complex scenarios.

First: Setting uid=0 or uid='0x0000'
The architecture allows to do that BUT if you do than you are NOT using the uid mode which results for the guest OS to use the legacy mode for assigning PCI addresses starting with 0 increasing by one following an unpredictable order by which the pci device are presented to guest OS. Since we never ever wanted to support the legacy mode in KVM guests we decided to never allow uid=0. Allowing the uid to be set to 0 is a contradiction. Actually the user can also set uid='0x0000' which I consider very specific and one would end up with something like uid='0x0001' and even more confusing is that suddenly setting uid='0x0000' on more than one PCI device is allowed.

Besides that the current zpci code still contains at least one flaw that is simply caused by the fact that there is no knowledge about which value was specified by the user. In Shalini's and your list it is case 5: This scenario runs into errors when another PCI device already has a fid set to 0 OR another PCI device exists specified with a uid > 0 and without a fid. The user gets the error message for something he did not specify:
 error: Failed to define domain from pci-addr-test.xml
 error: internal error: zPCI fid 0 is already reserved

Regarding setting fid=0 or fid='0x00000000'
Since it is a legal value for fid specifying it should not be considered as a wildcard or set equivalent to not specifying it at all. Doing so things like this happen and for the user it certainly seems like a bug:
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'
Another example:
Specify this in the domain:
  pcidev1: fid='0x00000000'
  pcidev2: fid='0x00000000'
Results in a defined domain:
  pcidev1: uid='0x0002' fid='0x00000001'
  pcidev2: uid='0x0001' fid='0x00000000'
 BUT
Specify this in the domain:
  pcidev1: uid='0x0002' fid='0x00000000'
  pcidev2: uid='0x0001' fid='0x00000000'
Results in error:
  error: Failed to define domain from pci-addr-test.xml
  error: internal error: zPCI fid 0 is already reserved
(Btw remove one of the fids results in the flaw described above.)


I think that Shalini's patch series improves the zpci address generation to better meet the users expected behavior. It also removes a correlation between uid and fid that does not really exist.

--
Mit freundlichen Grüßen/Kind regards
   Boris Fiuczynski

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