On Thu, 24 May 2018 19:58:27 +0200 Halil Pasic <pa...@linux.ibm.com> wrote:
> There is at least one guest (OS) such that although it does not rely on > the guarantees provided by ORB 1 word 9 bit (aka unlimited prefetch, aka > P bit) not being set, it fails to tell this to the machine. > > Usually this ain't a big deal, as the original purpose of the P bit is to > allow for performance optimizations. vfio-ccw however can not provide the > guarantees required if the bit is not set. > > It is not possible to implement support for the P bit not set without > transitioning to lower level protocols for vfio-ccw. So let's give the > user the opportunity to force setting the P bit, if the user knows this > is safe. For self modifying channel programs forcing the P bit is not > safe. If the P bit is forced for a self modifying channel program things > are expected to break in strange ways. > > Let's also avoid warning multiple about P bit not set in the ORB in case > P bit is not told to be forced, and designate the affected vfio-ccw > device. > > Signed-off-by: Halil Pasic <pa...@linux.ibm.com> > Suggested-by: Dong Jia Shi <bjsdj...@linux.ibm.com> > Acked-by: Jason J. Herne <jjhe...@linux.ibm.com> > Tested-by: Jason J. Herne <jjhe...@linux.ibm.com> > --- > > @Jason: > I have kept the tags this time without consulting you because > all that changed is the logging. Scream if that's not OK with you. > > v2->v3: > * tweak commit message (Connie) > * designate subsystem (vfio-ccw) and devno in the log messages (Connie) > * turn WARN_ONCE macro into inline function > > --- > hw/s390x/css.c | 3 +-- > hw/vfio/ccw.c | 35 +++++++++++++++++++++++++++++++++++ > 2 files changed, 36 insertions(+), 2 deletions(-) How shall we proceed with this? I currently don't have much (understatement of the year...) queued, so we could wait until we hashed out where to go with _once logging (although the discussion seems to have died down a bit). If I merge a respin of David's tcg patches, however, I'd be inclined to merge this in the current form as well and transition to a common _once handling later.
