Re: [Qemu-devel] [PATCH 3/4] s390x/css: remove dubious error handling branch

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 11 Sep 2017 15:08:52 +0200
Halil Pasic <pasic@linux.vnet.ibm.com> wrote:

> On 09/11/2017 11:48 AM, Cornelia Huck wrote:
> > On Fri,  8 Sep 2017 17:24:45 +0200
> > Halil Pasic <pasic@linux.vnet.ibm.com> wrote:

> >> The case in question actually never happens. Let us get rid of the dead
> >> code.  
> > 
> > I had tried to be complete in my initial implementation. With the
> > current implementation, it can never happen, yes. We can easily
> > resurrect it from git history should we need it in the future.
> >   
> 
> I've tried to understand the branch but I failed. Weather the subchannel
> is busy or not should be already decided, and if the device is busy
> that is probably supposed to be indicated via device status (word 2 bit 3).
> 
> So I wrote dubious because I could not figure it out. But unused
> is certainly nicer.
> 
> I think shall the need emerge the preferred way of handling would
> be to use the custom handling (-EIO) branch.

What this is trying to do is deferred cc 1 handling. On a real system,
a ssch may return with cc 0 to indicate that the channel program has
been accepted; but when the channel subsystem actually tries to execute
it, a status is already pending. Basically, it's a race condition:
Therefore the 'deferred' cc 1.

Maybe I should have called this 'deferred subchannel busy'... but
anyway, this cannot happen in our current implementation.