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 halt/clear. 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 happening): - 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.
Regards, Pierre -- Pierre Morel Linux/KVM/QEMU in Böblingen - Germany
