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Re: [qemu-s390x] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling

From: Cornelia Huck
Subject: Re: [qemu-s390x] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling
Date: Mon, 28 Jan 2019 18:24:24 +0100

On Fri, 25 Jan 2019 10:57:38 -0500
Eric Farman <address@hidden> wrote:

> On 01/25/2019 07:58 AM, Cornelia Huck wrote:
> > On Fri, 25 Jan 2019 11:24:37 +0100
> > Cornelia Huck <address@hidden> wrote:
> >   
> >> On Thu, 24 Jan 2019 21:37:44 -0500
> >> Eric Farman <address@hidden> 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.  
> That's good!  Maybe it helps us with the circles we're on :)


> > 
> > 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.   
> These two seem to contradict one another.  I think you're saying is that 
> we don't _want_ userspace to issue another channel program, even though 
> its _allowed_ to as far as vfio-ccw is concerned.

What I'm trying to say is that we want to distinguish two things:
- The code is currently doing translation etc. We probably want to keep
  that atomic, in order not to make things too complicated.
- We have sent the ssch() to the hardware, but have not yet received
  the final interrupt for that request (that's what I meant with "in
  flight"). It's easier for the first shot to disallow a second ssch()
  as that would need handling of more than one cp request, but we may
  want to allow it in the future.
  A hsch()/csch() (which does not generate a new cp) should be fine.

(see also my reply to Halil's mail)

> 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  
> I liked CP_PENDING, since it corresponds to the subchannel being marked 
> "start pending" as described in POPS, but this statement suggests that 
> the BUSY/PENDING state to be swapped, such that state=PENDING returns 
> -EAGAIN and state=BUSY returns -EBUSY.  Not super-concerned with the 
> terminology though.

What about s/BUSY/CP_PROCESSING/ ?

> , 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?
> >   
> I think so, and I think I like it.  So you want to distinguish between 
> (I have swapped BUSY/PENDING in this example per my above comment):
> A) SSCH issued by userspace (IDLE->PENDING)
> B) SSCH issued (successfully) by kernel (PENDING->BUSY)
> B') SSCH issued (unsuccessfully) by kernel (PENDING->IDLE?)

I think so.

> C) Interrupt received by kernel (no change?)
> D) Interrupt given to userspace (BUSY->IDLE)

Only if that is the final interrupt for that cp.

> If we receive A and A, the second A gets EAGAIN
> If we do A+B and A, the second A gets EBUSY (unless async, which is 
> processed)


> Does the boundary of "in flight" in the interrupt side (C and D) need to 
> be defined, such that we go BUSY->PENDING->IDLE instead of BUSY->IDLE ?

I don't think we can go BUSY->PENDING (in your terminology), at that
would imply a retry of the ssch()?

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