[RFC PATCH v4 0/4] vfio-ccw: Fix interrupt handling for HALT/CLEAR

[Eric Farman <farman@linux.ibm.com>]

Hi Conny, Halil,

Let's restart our discussion about the collision between interrupts for
START SUBCHANNEL and HALT/CLEAR SUBCHANNEL. It's been a quarter million
minutes (give or take), so here is the problematic scenario again:

	CPU 1			CPU 2
 1	CLEAR SUBCHANNEL
 2	fsm_irq()
 3				START SUBCHANNEL
 4	vfio_ccw_sch_io_todo()
 5				fsm_irq()
 6				vfio_ccw_sch_io_todo()

From the channel subsystem's point of view the CLEAR SUBCHANNEL (step 1)
is complete once step 2 is called, as the Interrupt Response Block (IRB)
has been presented and the TEST SUBCHANNEL was driven by the cio layer.
Thus, the START SUBCHANNEL (step 3) is submitted [1] and gets a cc=0 to
indicate the I/O was accepted. However, step 2 stacks the bulk of the
actual work onto a workqueue for when the subchannel lock is NOT held,
and is unqueued at step 4. That code misidentifies the data in the IRB
as being associated with the newly active I/O, and may release memory
that is actively in use by the channel subsystem and/or device. Eww.

In this version...

Patch 1 and 2 are defensive checks. Patch 2 was part of v3 [2], but I
would love a better option here to guard between steps 2 and 4.

Patch 3 is a subset of the removal of the CP_PENDING FSM state in v3.
I've obviously gone away from this idea, but I thought this piece is
still valuable.

Patch 4 collapses the code on the interrupt path so that changes to
the FSM state and the channel_program struct are handled at the same
point, rather than separated by a mutex boundary. Because of the
possibility of a START and HALT/CLEAR running concurrently, it does
not make sense to split them here.

With the above patches, maybe it then makes sense to hold the io_mutex
across the entirety of vfio_ccw_sch_io_todo(). But I'm not completely
sure that would be acceptable.

So... Thoughts?

Thanks,
Eric

Previous versions:
v3: https://lore.kernel.org/kvm/20200616195053.99253-1-farman@linux.ibm.com/
v2: https://lore.kernel.org/kvm/20200513142934.28788-1-farman@linux.ibm.com/
v1: https://lore.kernel.org/kvm/20200124145455.51181-1-farman@linux.ibm.com/

Footnotes:
[1] Halil correctly asserts that today's QEMU should prohibit this, but I
    still have not looked into why. The above is the sequence that is
    occurring in the kernel, and we shouldn't rely on a well-behaved
    userspace to enforce things for us. It is still on my list for further
    investigation, but it's lower in priority.
[2] https://lore.kernel.org/kvm/20200619134005.512fc54f.cohuck@redhat.com/

Eric Farman (4):
  vfio-ccw: Check initialized flag in cp_init()
  vfio-ccw: Check workqueue before doing START
  vfio-ccw: Reset FSM state to IDLE inside FSM
  vfio-ccw: Reset FSM state to IDLE before io_mutex

 drivers/s390/cio/vfio_ccw_cp.c  | 4 ++++
 drivers/s390/cio/vfio_ccw_drv.c | 7 +++----
 drivers/s390/cio/vfio_ccw_fsm.c | 6 ++++++
 drivers/s390/cio/vfio_ccw_ops.c | 2 --
 4 files changed, 13 insertions(+), 6 deletions(-)

-- 
2.25.1