Re: [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 25 Jan 2019 11:24:37 +0100
Cornelia Huck <cohuck@redhat.com> wrote:

> On Thu, 24 Jan 2019 21:37:44 -0500
> Eric Farman <farman@linux.ibm.com> wrote:
> 
> > On 01/24/2019 09:25 PM, Eric Farman wrote:  
> > > 
> > > 
> > > On 01/21/2019 06:03 AM, Cornelia Huck wrote:    
> 
> > > [1] I think these changes are cool.  We end up going into (and staying 
> > > in) state=BUSY if we get cc=0 on the SSCH, rather than in/out as we 
> > > bumble along.
> > > 
> > > But why can't these be separated out from this patch?  It does change 
> > > the behavior of the state machine, and seem distinct from the addition 
> > > of the mutex you otherwise add here?  At the very least, this behavior 
> > > change should be documented in the commit since it's otherwise lost in 
> > > the mutex/EAGAIN stuff.  
> 
> That's a very good idea. I'll factor them out into a separate patch.

And now that I've factored it out, I noticed some more problems.

What we basically need is the following, I think:

- The code should not be interrupted while we process the channel
  program, do the ssch etc. We want the caller to try again later (i.e.
  return -EAGAIN)
- We currently do not want the user space to submit another channel
  program while the first one is still in flight. As submitting another
  one is a valid request, however, we should allow this in the future
  (once we have the code to handle that in place).
- With the async interface, we want user space to be able to submit a
  halt/clear while a start request is still in flight, but not while
  we're processing a start request with translation etc. We probably
  want to do -EAGAIN in that case.

My idea would be:

- The BUSY state denotes "I'm busy processing a request right now, try
  again". We hold it while processing the cp and doing the ssch and
  leave it afterwards (i.e., while the start request is processed by
  the hardware). I/O requests and async requests get -EAGAIN in that
  state.
- A new state (CP_PENDING?) is entered after ssch returned with cc 0
  (from the BUSY state). We stay in there as long as no final state for
  that request has been received and delivered. (This may be final
  interrupt for that request, a deferred cc, or successful halt/clear.)
  I/O requests get -EBUSY, async requests are processed. This state can
  be removed again once we are able to handle more than one outstanding
  cp.

Does that make sense?