[PATCH v2 2/5] vfio-ccw: concurrent I/O handling

[Cornelia Huck <cohuck@redhat.com>]

Rework handling of multiple I/O requests to return -EAGAIN if
we are already processing an I/O request. Introduce a mutex
to disallow concurrent writes to the I/O region.

The expectation is that userspace simply retries the operation
if it gets -EAGAIN.

We currently don't allow multiple ssch requests at the same
time, as we don't have support for keeping channel programs
around for more than one request.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
---
 drivers/s390/cio/vfio_ccw_drv.c     |  1 +
 drivers/s390/cio/vfio_ccw_fsm.c     |  8 +++-----
 drivers/s390/cio/vfio_ccw_ops.c     | 31 +++++++++++++++++++----------
 drivers/s390/cio/vfio_ccw_private.h |  2 ++
 4 files changed, 26 insertions(+), 16 deletions(-)

diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
index a10cec0e86eb..2ef189fe45ed 100644
--- a/drivers/s390/cio/vfio_ccw_drv.c
+++ b/drivers/s390/cio/vfio_ccw_drv.c
@@ -125,6 +125,7 @@ static int vfio_ccw_sch_probe(struct subchannel *sch)
 
 	private->sch = sch;
 	dev_set_drvdata(&sch->dev, private);
+	mutex_init(&private->io_mutex);
 
 	spin_lock_irq(sch->lock);
 	private->state = VFIO_CCW_STATE_NOT_OPER;
diff --git a/drivers/s390/cio/vfio_ccw_fsm.c b/drivers/s390/cio/vfio_ccw_fsm.c
index cab17865aafe..f6ed934cc565 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);
 
@@ -42,6 +41,8 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
 		 */
 		sch->schib.scsw.cmd.actl |= SCSW_ACTL_START_PEND;
 		ret = 0;
+		/* Don't allow another ssch for now */
+		private->state = VFIO_CCW_STATE_BUSY;
 		break;
 	case 1:		/* Status pending */
 	case 2:		/* Busy */
@@ -99,7 +100,7 @@ static void fsm_io_error(struct vfio_ccw_private *private,
 static void fsm_io_busy(struct vfio_ccw_private *private,
 			enum vfio_ccw_event event)
 {
-	private->io_region->ret_code = -EBUSY;
+	private->io_region->ret_code = -EAGAIN;
 }
 
 static void fsm_disabled_irq(struct vfio_ccw_private *private,
@@ -130,8 +131,6 @@ 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;
-
 	memcpy(scsw, io_region->scsw_area, sizeof(*scsw));
 
 	if (scsw->cmd.fctl & SCSW_FCTL_START_FUNC) {
@@ -176,7 +175,6 @@ static void fsm_io_request(struct vfio_ccw_private *private,
 	}
 
 err_out:
-	private->state = VFIO_CCW_STATE_IDLE;
 	trace_vfio_ccw_io_fctl(scsw->cmd.fctl, get_schid(private),
 			       io_region->ret_code, errstr);
 }
diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
index f673e106c041..3fa9fc570400 100644
--- a/drivers/s390/cio/vfio_ccw_ops.c
+++ b/drivers/s390/cio/vfio_ccw_ops.c
@@ -169,16 +169,20 @@ static ssize_t vfio_ccw_mdev_read(struct mdev_device *mdev,
 {
 	struct vfio_ccw_private *private;
 	struct ccw_io_region *region;
+	int ret;
 
 	if (*ppos + count > sizeof(*region))
 		return -EINVAL;
 
 	private = dev_get_drvdata(mdev_parent_dev(mdev));
+	mutex_lock(&private->io_mutex);
 	region = private->io_region;
 	if (copy_to_user(buf, (void *)region + *ppos, count))
-		return -EFAULT;
-
-	return count;
+		ret = -EFAULT;
+	else
+		ret = count;
+	mutex_unlock(&private->io_mutex);
+	return ret;
 }
 
 static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
@@ -188,25 +192,30 @@ static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
 {
 	struct vfio_ccw_private *private;
 	struct ccw_io_region *region;
+	int ret;
 
 	if (*ppos + count > sizeof(*region))
 		return -EINVAL;
 
 	private = dev_get_drvdata(mdev_parent_dev(mdev));
-	if (private->state != VFIO_CCW_STATE_IDLE)
+	if (private->state == VFIO_CCW_STATE_NOT_OPER ||
+	    private->state == VFIO_CCW_STATE_STANDBY)
 		return -EACCES;
+	if (!mutex_trylock(&private->io_mutex))
+		return -EAGAIN;
 
 	region = private->io_region;
-	if (copy_from_user((void *)region + *ppos, buf, count))
-		return -EFAULT;
+	if (copy_from_user((void *)region + *ppos, buf, count)) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
 
 	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_IO_REQ);
-	if (region->ret_code != 0) {
-		private->state = VFIO_CCW_STATE_IDLE;
-		return region->ret_code;
-	}
+	ret = (region->ret_code != 0) ? region->ret_code : count;
 
-	return count;
+out_unlock:
+	mutex_unlock(&private->io_mutex);
+	return ret;
 }
 
 static int vfio_ccw_mdev_get_device_info(struct vfio_device_info *info)
diff --git a/drivers/s390/cio/vfio_ccw_private.h b/drivers/s390/cio/vfio_ccw_private.h
index 08e9a7dc9176..e88237697f83 100644
--- a/drivers/s390/cio/vfio_ccw_private.h
+++ b/drivers/s390/cio/vfio_ccw_private.h
@@ -28,6 +28,7 @@
  * @mdev: pointer to the mediated device
  * @nb: notifier for vfio events
  * @io_region: MMIO region to input/output I/O arguments/results
+ * @io_mutex: protect against concurrent update of I/O structures
  * @cp: channel program for the current I/O operation
  * @irb: irb info received from interrupt
  * @scsw: scsw info
@@ -42,6 +43,7 @@ struct vfio_ccw_private {
 	struct mdev_device	*mdev;
 	struct notifier_block	nb;
 	struct ccw_io_region	*io_region;
+	struct mutex		io_mutex;
 
 	struct channel_program	cp;
 	struct irb		irb;
-- 
2.17.2