Re: [Qemu-devel] [PATCH 4/9] s390x: refactor error handling for SSCH and RSCH

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

* Halil Pasic <pasic@linux.vnet.ibm.com> [2017-09-06 16:43:42 +0200]:

> 
> 
> On 09/06/2017 04:20 PM, Cornelia Huck wrote:
> > On Wed, 6 Sep 2017 14:25:13 +0200
> > Halil Pasic <pasic@linux.vnet.ibm.com> wrote:
> > 
> >> We have basically two possibilities/options which ask for different
> >> handling:
> >> 1) EFAULT is due to a bug in the vfio-ccw implementation
> >> (can be QEMU or kernel).
> >> 2) EFAULT is due to buggy channel program.
> >>
> >> Option 2) is basically to be handled with a channel-program check and
> >> setting primary secondary and alert status. For reference see PoP page
> >> 15-59 ("Designation of Storage Area").  An exception may be an invalid
> >> channel program address in the ORB. There the channel-program check ain't
> >> explicitly stated (although) I would expect one. It may be implied by the
> >> things on page 15-59 though.
> >>
> >> Option 1) is however very similar to other we have figured out that the
> >> implementation is broken situations and should be handled consequently.
> >> The current state of the discussion is with a unit exception.
> >>
> >> Does that make sense?
> > 
> > I think the situation is slightly different here, though. For the orb
> > flags, we reject something out of hand because we have not implemented
> > it, and for that, unit exception sounds like a good fit. Processing
> > errors, however, are more similar to errors in the hardware, and as
> > such can probably be reported via something like equipment check.
> > 
> 
> Noted. Let's see what Dong Jia has to say, before we continuing a
> discussion on something (option 1) what may be irrelevant anyway.
> 
> >>
> >> Now, Dong Jia, I need your help to figure out do we have option 1 or
> >> option 2 here? After quick look at the kernel code, it appears to me that
> >> I've seen both option 1 and option 2 (I'm afraid) -- but my assessment
> >> was really very superficial.
There are three cases (all in the kernel) that generate a -EFAULT ret
code:
a. vfio_ccw_mdev_write: copy_from_user() fails.
  This is option 1.

b. ccwchain_fetch_tic
  It's mostly likely that the vfio-ccw driver processed the ccw chains
  wrongly. (Actually I can not think of any other reason.)
  This is option 1.

c. ccwchain_fetch_idal
  When we find that an IDAW contents an invalid address
  This is option 2.

> >>
> >> I would expect option 2 to be handled differently (kernel provides the
> >> SCSW) though.
> >>
> >> Regards,
> >> Halil
> >>
> > 
> > 

-- 
Dong Jia Shi