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

From: Cornelia Huck <cohuck@redhat.com>
To: Eric Farman <farman@linux.ibm.com>
Cc: Halil Pasic <pasic@linux.ibm.com>,
	Matthew Rosato <mjrosato@linux.ibm.com>,
	Jared Rossi <jrossi@linux.ibm.com>,
	linux-s390@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [RFC PATCH v4 2/4] vfio-ccw: Check workqueue before doing START
Date: Fri, 16 Apr 2021 16:41:37 +0200	[thread overview]
Message-ID: <20210416164137.23f4631b.cohuck@redhat.com> (raw)
In-Reply-To: <577e873506ef60dd988653b8b28898e306e7493f.camel@linux.ibm.com>

On Thu, 15 Apr 2021 14:42:21 -0400
Eric Farman <farman@linux.ibm.com> wrote:

> 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?

Yes, maybe something like that.

Maybe we should do that on top once we have a good idea, if the current
series already fixes the problems that are actually happening now and
then.

> 
> > 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. :(

I might have misremembered; it only really applies to passthrough, as
emulated subchannels are handled synchronously anyway.