Re: [PATCH v3 2/6] vfio-ccw: rework ssch state handling

Cornelia Huck
2019-02-05T16:32:23Z

On Tue, 5 Feb 2019 09:31:55 -0500
Eric Farman <farman@linux.ibm.com> wrote:

> On 02/05/2019 07:10 AM, Cornelia Huck wrote:
> > On Mon, 4 Feb 2019 16:29:40 -0500
> > Eric Farman <farman@linux.ibm.com> wrote:
> >
> >> On 01/30/2019 08:22 AM, Cornelia Huck wrote:
> >>> The flow for processing ssch requests can be improved by splitting
> >>> the BUSY state:
> >>>
> >>> - CP_PROCESSING: We reject any user space requests while we are in
> >>> the process of translating a channel program and submitting it to
> >>> the hardware. Use -EAGAIN to signal user space that it should
> >>> retry the request.
> >>> - CP_PENDING: We have successfully submitted a request with ssch and
> >>> are now expecting an interrupt. As we can't handle more than one
> >>> channel program being processed, reject any further requests with
> >>> -EBUSY. A final interrupt will move us out of this state; this also
> >>> fixes a latent bug where a non-final interrupt might have freed up
> >>> a channel program that still was in progress.
> >>> By making this a separate state, we make it possible to issue a
> >>> halt or a clear while we're still waiting for the final interrupt
> >>> for the ssch (in a follow-on patch).
> >>>
> >>> It also makes a lot of sense not to preemptively filter out writes to
> >>> the io_region if we're in an incorrect state: the state machine will
> >>> handle this correctly.
> >>>
> >>> Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> >>> ---
> >>> drivers/s390/cio/vfio_ccw_drv.c | 8 ++++++--
> >>> drivers/s390/cio/vfio_ccw_fsm.c | 19 ++++++++++++++-----
> >>> drivers/s390/cio/vfio_ccw_ops.c | 2 --
> >>> drivers/s390/cio/vfio_ccw_private.h | 3 ++-
> >>> 4 files changed, 22 insertions(+), 10 deletions(-)
> >
> >>> diff --git a/drivers/s390/cio/vfio_ccw_fsm.c b/drivers/s390/cio/vfio_ccw_fsm.c
> >>> index e7c9877c9f1e..b4a141fbd1a8 100644
> >>> --- a/drivers/s390/cio/vfio_ccw_fsm.c
> >>> +++ b/drivers/s390/cio/vfio_ccw_fsm.c
> >>> @@ -28,7 +28,6 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
> >>> sch = private->sch;
> >>>
> >>> spin_lock_irqsave(sch->lock, flags);
> >>> - private->state = VFIO_CCW_STATE_BUSY;
> >>>
> >>> orb = cp_get_orb(&private->cp, (u32)(addr_t)sch, sch->lpm);
> >>> if (!orb) {
> >>> @@ -46,6 +45,7 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
> >>> */
> >>> sch->schib.scsw.cmd.actl |= SCSW_ACTL_START_PEND;
> >>> ret = 0;
> >>> + private->state = VFIO_CCW_STATE_CP_PENDING;
> >>
> >> [1]
> >>
> >>> break;
> >>> case 1: /* Status pending */
> >>> case 2: /* Busy */
> >>> @@ -107,6 +107,12 @@ static void fsm_io_busy(struct vfio_ccw_private *private,
> >>> private->io_region->ret_code = -EBUSY;
> >>> }
> >>>
> >>> +static void fsm_io_retry(struct vfio_ccw_private *private,
> >>> + enum vfio_ccw_event event)
> >>> +{
> >>> + private->io_region->ret_code = -EAGAIN;
> >>> +}
> >>> +
> >>> static void fsm_disabled_irq(struct vfio_ccw_private *private,
> >>> enum vfio_ccw_event event)
> >>> {
> >>> @@ -135,8 +141,7 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> >>> struct mdev_device *mdev = private->mdev;
> >>> char *errstr = "request";
> >>>
> >>> - private->state = VFIO_CCW_STATE_BUSY;
> >>> -
> >>> + private->state = VFIO_CCW_STATE_CP_PROCESSING;
> >>
> >> [1]
> >>
> >>> memcpy(scsw, io_region->scsw_area, sizeof(*scsw));
> >>>
> >>> if (scsw->cmd.fctl & SCSW_FCTL_START_FUNC) {
> >>> @@ -181,7 +186,6 @@ static void fsm_io_request(struct vfio_ccw_private *private,
> >>> }
> >>>
> >>> err_out:
> >>> - private->state = VFIO_CCW_STATE_IDLE;
> >>
> >> [1] Revisiting these locations as from an earlier discussion [2]...
> >> These go IDLE->CP_PROCESSING->CP_PENDING if we get a cc=0 on the SSCH,
> >> but we stop in CP_PROCESSING if the SSCH gets a nonzero cc. Shouldn't
> >> we cleanup and go back to IDLE in this scenario, rather than forcing
> >> userspace to escalate to CSCH/HSCH after some number of retries (via FSM)?
> >>
> >> [2] https://patchwork.kernel.org/patch/10773611/#22447997
> >
> > It does do that (in vfio_ccw_mdev_write), it was not needed here. Or do
> > you think doing it here would be more obvious?
>
> Ah, my mistake, I missed that. (That function is renamed to
> vfio_ccw_mdev_write_io_region in patch 4.)
>
> I don't think keeping it here is necessary then. I got too focused
> looking at what you ripped out that I lost the things that stayed. Once
> this series gets in its entirety, and Pierre has a chance to rebase his
> FSM series on top of it all, this should be in great shape.

Yeah, it's probably easier to look at the end result.

>
> >
> >>
> >> Besides that, I think this looks good to me.
> >
> > Thanks!
> >
>
> You're welcome! Here, have a thing to add to this patch:
>
> Reviewed-by: Eric Farman <farman@linux.ibm.com>
>

Thanks a lot!