On Thu, 24 Jan 2019 14:16:21 -0500 Eric Farman <far...@linux.ibm.com> wrote:
> On 01/23/2019 08:34 AM, Cornelia Huck wrote: > > On Wed, 23 Jan 2019 14:06:01 +0100 > > Halil Pasic <pa...@linux.ibm.com> wrote: > > > >> On Wed, 23 Jan 2019 11:34:47 +0100 > >> Cornelia Huck <coh...@redhat.com> wrote: > > > >> Yes, one can usually think of interfaces as contracts: both sides need > >> to keep their end for things to work as intended. Unfortunately the > >> vfio-ccw iterface is not a very well specified one, and that makes > >> reasoning about right order so much harder. > > > > That's probably where our disconnect comes from. > > > >> > >> I was under the impression that the right ordering is dictated by the > >> SCSW in userspace. E.g. if there is an FC bit set there userspace is not > >> ought to issue a SSCH request (write to the io_region). The kernel part > >> however may say 'userspace read the actual SCSW' by signaling > >> the io_trigger eventfd. Userspace is supposed to read the IRB from the > >> region and update it's SCSW. > >> > >> Now if userspace reads a broken SCSW from the IRB, because of a race > >> (due to poorly written kernel part -- userspace not at fault), it is > >> going to make wrong assumptions about currently legal and illegal > >> operations (ordering). > > > > My understanding of the interface was that writing to the I/O region > > triggers a ssch (unless rejected with error) and that reading it just > > gets whatever the kernel wrote there the last time it updated its > > internal structures. The eventfd simply triggers to say "the region has > > been updated with an IRB", not to say "userspace, read this". > > > >> > >> Previously I described a scenario where IRB can break without userspace > >> being at fault (race between unsolicited interrupt -- can happen at any > >> time -- and a legit io request). I was under the impression we agreed on > >> this. > > > > There is a bug in there (clearing the cp for non-final interrupts), and > > it needs to be fixed. I'm not so sure if the unsolicited interrupt > > thing is a bug (beyond that the internal state machine is confused). > > > >> > >> This in turn could lead to userspace violating the contract, as perceived > >> by the kernel side. > > > > Which contract? ;) > > > > Also, I'm not sure if we'd rather get a deferred cc 1? > > As I'm encountering dcc=1 quite regularly lately, it's a nice error. > But we don't have a good way of recovering from it, and so my test tends > to go down in a heap quite quickly. This patch set will probably help; > I should really get it applied and try it out. The deferred cc 1 is probably more likely simply due to the overhead we get from intercepting the I/O calls. > > > > >> > >>> At this point, I'm mostly confused... I'd prefer to simply fix things > >>> as they come up so that we can finally move forward with the halt/clear > >>> handling (and probably rework the state machine on top of that.) > > +1 for fixing things as we go. I hear the complaints about this code > (and probably say them too), but remain convinced that a large rewrite > is unnecessary. Lots of opportunities for improvement, with lots of > willing and motivated participants, means it can only get better! Yeah, the code would probably look a bit different if I started writing it from scratch now, but I don't think the basic design is unfixably broken. > > >>> > >> > >> I understand. I guess you will want to send a new version because of the > >> stuff that got lost in the rebase, or? > > > > Yes, I'll send a new version; but I'll wait for more feedback for a bit. > > > > I'll try to provide some now. Still digging through the emails marked > "todo" :) Ok, I'll wait for a bit more :)
