Re: [RFC PATCH v4 2/4] vfio-ccw: Check workqueue before doing START

[Eric Farman <farman@linux.ibm.com>]

On Thu, 2021-04-15 at 18:19 +0200, Cornelia Huck wrote:
> On Thu, 15 Apr 2021 09:48:37 -0400
> Eric Farman <farman@linux.ibm.com> wrote:
> 
> > On Thu, 2021-04-15 at 12:51 +0200, Cornelia Huck wrote:
> > > I'm wondering what we should do for hsch. We probably want to
> > > return
> > > -EBUSY for a pending condition as well, if I read the PoP
> > > correctly...  
> > 
> > Ah, yes...  I agree that to maintain parity with ssch and pops, the
> > same cc1/-EBUSY would be applicable here. Will make that change in
> > next
> > version.
> 
> Yes, just to handle things in the same fashion consistently.
> 
> > > the only problem is that QEMU seems to match everything to 0; but
> > > that
> > > is arguably not the kernel's problem.
> > > 
> > > For clear, we obviously don't have busy conditions. Should we
> > > clean
> > > up
> > > any pending conditions?  
> > 
> > By doing anything other than issuing the csch to the subchannel?  I
> > don't think so, that should be more than enough to get the css and
> > vfio-ccw in sync with each other.
> 
> Hm, doesn't a successful csch clear any status pending? 

Yep.

> That would mean
> that invoking our csch backend implies that we won't deliver the
> status
> pending that is already pending via the workqueue, which therefore
> needs to be flushed out in some way? 

Ah, so I misunderstood the direction you were going... I'm not aware of
a way to "purge" items from a workqueue, as the flush_workqueue()
routine is documented as picking them off and running them.

Perhaps an atomic flag in (private? cp?) that causes
vfio_ccw_sch_io_todo() to just exit rather than doing all its stuff?

> I remember we did some special
> csch handling, but I don't immediately see where; might have been
> only
> in QEMU.
> 

Maybe.  I don't see anything jumping out at me though. :(