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authorJared Rossi <jrossi@linux.ibm.com>2020-05-07 00:24:40 +0300
committerCornelia Huck <cohuck@redhat.com>2020-06-02 14:14:08 +0300
commit725b94d74128cd208bfdd446ad6b5f4b38cf5395 (patch)
tree394d06fb3b404e9dc4e293fedaa4c18bc186049a /Documentation/s390
parente1750a3d9abbea2ece29cac8dc5a6f5bc19c1492 (diff)
downloadlinux-725b94d74128cd208bfdd446ad6b5f4b38cf5395.tar.xz
vfio-ccw: Enable transparent CCW IPL from DASD
Remove the explicit prefetch check when using vfio-ccw devices. This check does not trigger in practice as all Linux channel programs are intended to use prefetch. It is expected that all ORBs issued by Linux will request prefetch. Although non-prefetching ORBs are not rejected, they will prefetch nonetheless. A warning is issued up to once per 5 seconds when a forced prefetch occurs. A non-prefetch ORB does not necessarily result in an error, however frequent encounters with non-prefetch ORBs indicate that channel programs are being executed in a way that is inconsistent with what the guest is requesting. While there is currently no known case of an error caused by forced prefetch, it is possible in theory that forced prefetch could result in an error if applied to a channel program that is dependent on non-prefetch. Signed-off-by: Jared Rossi <jrossi@linux.ibm.com> Reviewed-by: Eric Farman <farman@linux.ibm.com> Message-Id: <20200506212440.31323-2-jrossi@linux.ibm.com> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Diffstat (limited to 'Documentation/s390')
-rw-r--r--Documentation/s390/vfio-ccw.rst6
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/s390/vfio-ccw.rst b/Documentation/s390/vfio-ccw.rst
index fca9c4f5bd9c..23e7d136f8b4 100644
--- a/Documentation/s390/vfio-ccw.rst
+++ b/Documentation/s390/vfio-ccw.rst
@@ -335,6 +335,12 @@ device.
The current code allows the guest to start channel programs via
START SUBCHANNEL, and to issue HALT SUBCHANNEL and CLEAR SUBCHANNEL.
+Currently all channel programs are prefetched, regardless of the
+p-bit setting in the ORB. As a result, self modifying channel
+programs are not supported. For this reason, IPL has to be handled as
+a special case by a userspace/guest program; this has been implemented
+in QEMU's s390-ccw bios as of QEMU 4.1.
+
vfio-ccw supports classic (command mode) channel I/O only. Transport
mode (HPF) is not supported.