On Tue, 5 May 2020 20:15:44 -0400
Jared Rossi <jrossi@xxxxxxxxxxxxx> 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 <jrossi@xxxxxxxxxxxxx>
> ---
> 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.
