Re: [Qemu-devel] [PATCH 2/9] s390x: fix invalid use of cc 1 for SSCH

[Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com>]

* Cornelia Huck <cohuck@redhat.com> [2017-09-06 13:25:38 +0200]:

> On Wed, 6 Sep 2017 16:27:20 +0800
> Dong Jia Shi <bjsdjshi@linux.vnet.ibm.com> wrote:
> 
> > * Halil Pasic <pasic@linux.vnet.ibm.com> [2017-09-05 19:20:43 +0200]:
> > 
> > > 
> > > 
> > > On 09/05/2017 05:46 PM, Cornelia Huck wrote:  
> > > > On Tue, 5 Sep 2017 17:24:19 +0200
> > > > Halil Pasic <pasic@linux.vnet.ibm.com> wrote:
> > > >   
> > > >> My problem with a program check (indicated by SCSW word 2 bit 10) is
> > > >> that, in my reading of the architecture, the semantic behind it is: The
> > > >> channel subsystem (not the cu or device) has detected, that the 
> > > >> the channel program (previously submitted as an ORB) is erroneous. Which
> > > >> programs are erroneous is specified by the architecture. What we have
> > > >> here does not qualify.
> > > >>
> > > >> My idea was to rather blame the virtual hardware (device) and put no blame
> > > >> on the program nor he channel subsystem. This could be done using device
> > > >> status (unit check with command reject, maybe unit exception) or interface
> > > >> check. My train of thought was, the problem is not consistent across a
> > > >> device type, so it has to be device specific.  
> > > > 
> > > > Unit exception might be a better way to express what is happening here.
> > > > At least, it moves us away from cc 1 and not towards cc 3 :)
> > > >   
> > > 
> > > I will do a follow up patch pursuing device exception.
> > >   
> > > >>
> > > >> Of course blaming the device could mislead the person encountering the
> > > >> problem, and make him believe it's an non-virtual hardware problem.
> > > >>
> > > >> About the misleading, I think the best we can do is log out a message
> > > >> indicating what really happened.  
> > > > 
> > > > Just document it in the code? If it doesn't happen with Linux as a
> > > > guest, it is highly unlikely to be seen in the wild.
> > > >   
> > > 
> > > 
> > > Well we have two problems here:
> > > 1) Unit exception can be already defined by the device type for the
> > > command (reference: http://publibfp.dhe.ibm.com/cgi-bin/bookmgr/BOOKS/dz9ar110/2.6.10?DT=19920904110920).
> > > I think this one is what you mean. And I agree that's best handled
> > > with comment in code.  
> > Using unit check, with bit 3 byte 0 of the sense data set to 1, to
> > indicate an 'Equipment check', sounds a bit more proper than unit
> > exception.
> 
> I don't agree: Equipment check sounds a lot more dire (and seems to
> imply a malfunction). I like unit exception better.
Got the point. Fair enough!

-- 
Dong Jia Shi