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Re: [qemu-s390x] [PATCH v3 2/6] vfio-ccw: rework ssch state handling

From: Eric Farman
Subject: Re: [qemu-s390x] [PATCH v3 2/6] vfio-ccw: rework ssch state handling
Date: Tue, 5 Feb 2019 09:31:55 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 02/05/2019 07:10 AM, Cornelia Huck wrote:
On Mon, 4 Feb 2019 16:29:40 -0500
Eric Farman <address@hidden> 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 <address@hidden>
   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;


        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;


        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,
-       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.

Besides that, I think this looks good to me.


You're welcome!  Here, have a thing to add to this patch:

Reviewed-by: Eric Farman <address@hidden>

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