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

Eric Farman
2019-02-04T21:29:40Z

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_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
> index a10cec0e86eb..0b3b9de45c60 100644
> --- a/drivers/s390/cio/vfio_ccw_drv.c
> +++ b/drivers/s390/cio/vfio_ccw_drv.c
> @@ -72,20 +72,24 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
> {
> struct vfio_ccw_private *private;
> struct irb *irb;
> + bool is_final;
>
> private = container_of(work, struct vfio_ccw_private, io_work);
> irb = &private->irb;
>
> + is_final = !(scsw_actl(&irb->scsw) &
> + (SCSW_ACTL_DEVACT | SCSW_ACTL_SCHACT));
> if (scsw_is_solicited(&irb->scsw)) {
> cp_update_scsw(&private->cp, &irb->scsw);
> - cp_free(&private->cp);
> + if (is_final)
> + cp_free(&private->cp);
> }
> memcpy(private->io_region->irb_area, irb, sizeof(*irb));
>
> if (private->io_trigger)
> eventfd_signal(private->io_trigger, 1);
>
> - if (private->mdev)
> + if (private->mdev && is_final)
> private->state = VFIO_CCW_STATE_IDLE;
> }
>
> 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

Besides that, I think this looks good to me.

- Eric

> trace_vfio_ccw_io_fctl(scsw->cmd.fctl, get_schid(private),
> io_region->ret_code, errstr);
> }
> @@ -221,7 +225,12 @@ fsm_func_t *vfio_ccw_jumptable[NR_VFIO_CCW_STATES][NR_VFIO_CCW_EVENTS] = {
> [VFIO_CCW_EVENT_IO_REQ] = fsm_io_request,
> [VFIO_CCW_EVENT_INTERRUPT] = fsm_irq,
> },
> - [VFIO_CCW_STATE_BUSY] = {
> + [VFIO_CCW_STATE_CP_PROCESSING] = {
> + [VFIO_CCW_EVENT_NOT_OPER] = fsm_notoper,
> + [VFIO_CCW_EVENT_IO_REQ] = fsm_io_retry,
> + [VFIO_CCW_EVENT_INTERRUPT] = fsm_irq,
> + },
> + [VFIO_CCW_STATE_CP_PENDING] = {
> [VFIO_CCW_EVENT_NOT_OPER] = fsm_notoper,
> [VFIO_CCW_EVENT_IO_REQ] = fsm_io_busy,
> [VFIO_CCW_EVENT_INTERRUPT] = fsm_irq,
> diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
> index f673e106c041..3fdcc6dfe0bf 100644
> --- a/drivers/s390/cio/vfio_ccw_ops.c
> +++ b/drivers/s390/cio/vfio_ccw_ops.c
> @@ -193,8 +193,6 @@ static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
> return -EINVAL;
>
> private = dev_get_drvdata(mdev_parent_dev(mdev));
> - if (private->state != VFIO_CCW_STATE_IDLE)
> - return -EACCES;
>
> region = private->io_region;
> if (copy_from_user((void *)region + *ppos, buf, count))
> diff --git a/drivers/s390/cio/vfio_ccw_private.h b/drivers/s390/cio/vfio_ccw_private.h
> index 08e9a7dc9176..50c52efb4fcb 100644
> --- a/drivers/s390/cio/vfio_ccw_private.h
> +++ b/drivers/s390/cio/vfio_ccw_private.h
> @@ -63,7 +63,8 @@ enum vfio_ccw_state {
> VFIO_CCW_STATE_NOT_OPER,
> VFIO_CCW_STATE_STANDBY,
> VFIO_CCW_STATE_IDLE,
> - VFIO_CCW_STATE_BUSY,
> + VFIO_CCW_STATE_CP_PROCESSING,
> + VFIO_CCW_STATE_CP_PENDING,
> /* last element! */
> NR_VFIO_CCW_STATES
> };
>