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

[Cornelia Huck]

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 <address@hidden>
---
 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