[Qemu-devel] [RFC PATCH 0/3] vfio: ccw: basic channel path event handling

[Dong Jia Shi]

Hi Folks,

Background
==========

Some days ago, we had a discussion on the topic of channel path virtualization.
Ref:
Subject: [PATCH 0/3] Channel Path realted CRW generation
Message-Id: <address@hidden>
URL: https://lists.nongnu.org/archive/html/qemu-devel/2017-07/msg08414.html

Indeed that thread is not short and discussed many aspects in a
non-concentrated manner. The parts those are most valuable to me are:
1. a re-modelling for channel path is surely the best offer, but is not
   possible to have in the near future.
2. to enhance the path related functionalities, using PNO and PNOM might
   be something we can do for now. This may be something that I could improve
   without model related arguments.

So here I have this series targeting to add basic channel path event handling
for vfio-ccw -- no touch of the channel path modelling in both the kernel and
the QEMU side, but find a way to sync path status change to guest lazily using
SCSW_FLAGS_MASK_PNO and pmcw->pnom.  In short, I want to enhance path related
stuff (to be more specific: sync up path status to the guest) on a best effort
basis, which means in a way that won't get us invloed to do channel path
re-modelling.

What benifit can we get from this? The administrator of a virtual machine can
get uptodate (in some extent) status of the current using channel paths, so
he/she can monitor paths status and get path problem noticed timely (see the
example below).

I think we can start a new round discussion based on this series. So reviewers
can give their comments based on some code, and then we can decide if this is
we want or not.

As flagged with RFC, the intention of this series is to show what I have for
now, and what could the code look like in general. Thus I can get some early
feedbacks. I would expect to see opinions on:
- is the target (mentioned above) of this series welcomed or not.
- is the approach of this series good or bad.
So I can either move on with this (or with other suggested approach) or leave
it alone.

Basic Introduction of The Patches
=================================

This is the kernel counterpart, which mainly does:
1. add a schib vfio region for userland to _store_ subchannel information.
2. add a channel path vfio irq to notify userland with chp status change event.
3. add .chp_event handler for vfio-ccw driver, so the driver handles chp event,
   and signals userland about the event.

With the above work, userland can be signaled with chp related event, and then
it can read out uptodate SCHIB from the new region, and sync up path related
information to the corresponding virtual subchannel. So a guest can sense the
path update in some extent.

For the QEMU counterpart, please ref:
[RFC PATCH 0/5] vfio/ccw: basic channel path event handling

The QEMU counterpart mainly does:
1. add handling of the schib region, so that it can read out the newest schib
   information.
2. add handling of the chp irq, so that it can get notification of channel path
   status change.
3. once there is a chp status event, synchronize related information from the
   newest schib information to guest lazily.

What are still missing, thus need to be offered in the next version are:
- I/O termination and FSM state handling if currently we have I/O on the status
  switched path.
- Vary on/off event is not sensible to a guest.

Example
=======

With both the kernel and Qemu parts applied, we can notice some new behaviors
of a channel path when we have a guest with a passed through vfio-ccw device
using it. The guest can reflect the chp status change of the host side lazily,
and synchronize the updated information.

For example:
0. Prepare a vfio subchannel on the host:
address@hidden ~]# lscss --vfio 013f
MDEV                                  Subchan.  PIM PAM POM  CHPIDs
------------------------------------------------------------------------------
6dfd3ec5-e8b3-4e18-a6fe-57bc9eceb920  0.0.013f  f0  f0  ff   42434445 00000000

1. Pass-through subchannel 0.0.013f to a guest:
-device vfio-ccw,sysfsdev="$MDEV_FILE_PATH",devno=0.0.3f3f

2. Start the guest and check the device and path information:
address@hidden ~]# lscss 0002
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.3f3f 0.0.0002  3390/0c 3990/e9      f0  f0  ff   42434445 00000000 
address@hidden ~]# lschp 
CHPID  Vary  Cfg.  Type  Cmg  Shared  PCHID
============================================
0.00   1     -     32    -    -       -    
0.42   1     3     1b    -    -       -    
0.43   1     3     1b    -    -       -    
0.44   1     3     1b    -    -       -    
0.45   1     3     1b    -    -       -

3. On the host, configure off one path.
address@hidden ~]# chchp -c 0 42

4. On the guest, check the status:
address@hidden ~]# lscss 0002
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.3f3f 0.0.0002  3390/0c 3990/e9      f0  f0  ff   42434445 00000000 
#Notice: No change!

address@hidden ~]# chccwdev -e 3f3f
Setting device 0.0.3f3f online
dasd-eckd 0.0.3f3f: A channel path to the device has become operational
dasd-eckd 0.0.3f3f: New DASD 3390/0C (CU 3990/01) with 30051 cylinders, 15 
heads, 224 sectors
dasd-eckd 0.0.3f3f: DASD with 4 KB/block, 21636720 KB total size, 48 KB/track, 
compatible disk layout
 dasda:VOL1/  0X3F3F: dasda1
Done

address@hidden ~]# lscss 0002
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.3f3f 0.0.0002  3390/0c 3990/e9      f0  70  ff   42434445 00000000 
#Notice: PAM value of path 0.42 changed.

5. On the host, configure on one path.
address@hidden ~]# chchp -c 1 42

6. On the guest, check the status again:
address@hidden ~]# lscss 0002
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.3f3f 0.0.0002  3390/0c 3990/e9      f0  70  ff   42434445 00000000 
#Notice: No change!

address@hidden ~]# chccwdev -d 3f3f
Setting device 0.0.3f3f offline
Done

address@hidden ~]# lscss 0002
Device   Subchan.  DevType CU Type Use  PIM PAM POM  CHPIDs
----------------------------------------------------------------------
0.0.3f3f 0.0.0002  3390/0c 3990/e9      f0  f0  ff   42434445 00000000 
#Notice: PAM changed again.

Dong Jia Shi (3):
  vfio: ccw: introduce schib region
  vfio: ccw: introduce channel path irq
  vfio: ccw: handle chp event

 drivers/s390/cio/vfio_ccw_drv.c     |  51 +++++++++++++++++
 drivers/s390/cio/vfio_ccw_fsm.c     |  43 ++++++++++++++
 drivers/s390/cio/vfio_ccw_ops.c     | 108 ++++++++++++++++++++++++++----------
 drivers/s390/cio/vfio_ccw_private.h |   8 +++
 include/uapi/linux/vfio.h           |   2 +
 include/uapi/linux/vfio_ccw.h       |   6 ++
 6 files changed, 190 insertions(+), 28 deletions(-)

-- 
2.13.5