On Tue, 5 May 2020 20:15:44 -0400 Jared Rossi <[email protected]> wrote:
> 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 <[email protected]> > --- > Documentation/s390/vfio-ccw.rst | 6 ++++++ > drivers/s390/cio/vfio_ccw_cp.c | 19 ++++++++++++------- > 2 files changed, 18 insertions(+), 7 deletions(-) > (...) > @@ -625,23 +626,27 @@ static int ccwchain_fetch_one(struct ccwchain *chain, > * the target channel program from @orb->cmd.iova to the new ccwchain(s). > * > * Limitations: > - * 1. Supports only prefetch enabled mode. > - * 2. Supports idal(c64) ccw chaining. > - * 3. Supports 4k idaw. > + * 1. Supports idal(c64) ccw chaining. > + * 2. Supports 4k idaw. > * > * Returns: > * %0 on success and a negative error value on failure. > */ > int cp_init(struct channel_program *cp, struct device *mdev, union orb *orb) > { > + static DEFINE_RATELIMIT_STATE(ratelimit_state, 5 * HZ, 1); > int ret; > > /* > - * XXX: > - * Only support prefetch enable mode now. > + * We only support prefetching the channel program. We assume all > channel > + * programs executed by supported guests (i.e. Linux) likewise support > + * prefetching. Even if prefetching is not specified the channel program > + * is still executed using prefetch. Executing a channel program that > + * does not specify prefetching will typically not cause an error, but a > + * warning is issued to help identify the problem if something does > break. > */ > - if (!orb->cmd.pfch) > - return -EOPNOTSUPP; /* custom ratelimiting to avoid flood during boot */ (to avoid people stumbling over this) > + if (!orb->cmd.pfch && __ratelimit(&ratelimit_state)) > + dev_warn(mdev, "executing channel program with prefetch, but > prefetch isn't specified"); hmm... what about "prefetching channel program even though prefetch not specified in orb"? > > INIT_LIST_HEAD(&cp->ccwchain_list); > memcpy(&cp->orb, orb, sizeof(*orb)); (...) Apart from the bikeshedding, looks sane to me; but would like more opinions.
