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

[Cornelia Huck <cohuck@redhat.com>]

On Tue, 29 Jan 2019 20:39:33 +0100
Halil Pasic <pasic@linux.ibm.com> wrote:

> On Tue, 29 Jan 2019 10:58:40 +0100
> Cornelia Huck <cohuck@redhat.com> wrote:
> 
> > > > > The problem I see with the let the hardware sort it out is that, for
> > > > > that to work, we need to juggle multiple translations simultaneously
> > > > > (or am I wrong?). Doing that does not appear particularly simple to
> > > > > me.      
> > > > 
> > > > None in the first stage, at most two in the second stage, I guess.
> > > >       
> > > 
> > > Expected benefit of the second stage over the first stage? (I see none.)    
> > 
> > Making something possible that is allowed by the architecture. Not
> > really important, though.
> >   
> 
> I had a chat with Farhan, and he suggested that by 'allowed by
> architecture' you mean " You can submit a new request if the subchannel
> is pending with primary, but not with secondary state." (from Message-ID:
> <20190125152154.05120461.cohuck@redhat.com>).

Yes. I might have mixed things up, though.

> 
> So I re-read the PoP.
> 
> From the description of the start subchannel instruction:
> """
> Special Conditions
> 
> Condition code 1 is set, and no other action is
> taken, when the subchannel is status pending when
> START SUBCHANNEL is executed. On some mod-
> els, condition code 1 is not set when the subchannel
> is status pending with only secondary status; instead,
> the status-pending condition is discarded.
> 
> Condition code 2 is set, and no other action is
> taken, when a start, halt, or clear function is currently
> in progress at the subchannel (see “Function Control
> (FC)” on page 13).
> 
> """
> 
> So I guess you mixed primary and secondary up and wanted to say:
> "You can submit a new request if the subchannel
> is pending with _secondary_, but not with _primary_ _status_."
> 
> But does that really mean architecture allows the subchannel
> to accept multiple ssch() instructions so that it ends up processing
> two or more channel programs in parallel.

That's not what I meant. The vfio-ccw driver still holds on to one cp,
while a second one could be submitted.

But let's just end discussing this here, and continue with discussing
the reworked state machine, ok? It's not really relevant for going
forward with halt/clear.