Re: [PATCH v3 6/6] vfio: ccw: serialize the write system calls

From: Pierre Morel
Date: Fri Dec 14 2018 - 09:08:19 EST

On 13/12/2018 16:39, Cornelia Huck wrote:
On Wed, 28 Nov 2018 13:41:07 +0100
Pierre Morel <pmorel@xxxxxxxxxxxxx> wrote:

When the user program is QEMU we rely on the QEMU lock to serialize
the calls to the driver.

In the general case we need to make sure that two data transfer are
not started at the same time.
It would in the current implementation resul in a overwriting of the
IO region.

We also need to make sure a clear or a halt started after a data
transfer do not win the race agains the data transfer.
Which would result in the data transfer being started after the

Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
drivers/s390/cio/vfio_ccw_ops.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/s390/cio/vfio_ccw_ops.c b/drivers/s390/cio/vfio_ccw_ops.c
index eb5b49d..b316966 100644
--- a/drivers/s390/cio/vfio_ccw_ops.c
+++ b/drivers/s390/cio/vfio_ccw_ops.c
@@ -267,22 +267,31 @@ static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
unsigned int index = VFIO_CCW_OFFSET_TO_INDEX(*ppos);
struct vfio_ccw_private *private;
+ static atomic_t serialize = ATOMIC_INIT(0);
+ int ret = -EINVAL;
+ if (!atomic_add_unless(&serialize, 1, 1))
+ return -EBUSY;

I think that hammer is far too big: This serializes _all_ write
operations across _all_ devices.

Right, much too much.
This should go inside the device.
(Don't know what I was thinking of).

There are various cases of simultaneous writes that may happen
(assuming any userspace; QEMU locking will prevent some of them from

- One thread does a write for one mdev, another thread does a write for
another mdev. For example, if two vcpus issue an I/O instruction on
two different devices. This should be fine.
- One thread does a write for one mdev, another thread does a write for
the same mdev. Maybe a guest has confused/no locking and is trying to
do ssch on the same device from different vcpus. There, we want to
exclude simultaneous writes; the desired outcome may be that one ssch
gets forwarded to the hardware, and the second one either gets
forwarded after processing for the first one has finished, or
userspace gets an error immediately (hopefully resulting in a
appropriate condition code for that second ssch in any case). Both
handing the second ssch to the hardware or signaling device busy
immediately are probably sane in that case.

should be.

- If those writes for the same device involve hsch/csch, things get
more complicated. First, we have two different regions, and allowing
simultaneous writes to the I/O region and to the async region should
not really be a problem, so I don't think fencing should be done in
the generic write handler. Second, the semantics for device busy are
different: a hsch immediately after a ssch should not give device
busy, and csch cannot return device busy at all.

The lock should be done in the device and only for SSCH.
We did not have CSCH or HSCH at the moment I sent the patch.

It is clear that CSCH or HSCH should not be blocked.

I don't think we'll be able to get around some kind of "let's retry
sending this" logic for hsch/csch; maybe we should already do that for
ssch. (Like the -EINVAL logic I described in the other thread.)

As I wrote in the cover letter this (stupid) patch is decoupled from the series.
I made the mistake to attach it here but I hope you will consider the rest of the serie which I think is much more important.

The handling of a lock / busy waiting here should wait for your serie on HSCH/CSCH anyway.


Pierre Morel
Linux/KVM/QEMU in BÃblingen - Germany