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

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

* 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.

> 2) The poor user/programmer is trying to figure out why things
> don't work (why are we getting the unit exception)? I think that's
> best remedied with producing something for the log (maybe a warning
> with warn_report which states that the implementation vfio-ccw requires
> the given flags).
Fine with me.

[...]

-- 
Dong Jia Shi