[Qemu-devel] [v2 Patch 0/9]block: Image file reopen and dynamic host pagecache change

[Supriya Kannery]

 For changing host pagecache setting of a running VM, it is
important to have a safe way of reopening its image file.

V1 introduced:
 * a generic way to reopen image files safely. 
        In this approach, before reopening an image, for each
    block driver, its state will be stashed. Incase preparation,
    (bdrv_reopen_prepare) for reopening returns success, the stashed 
    state will be cleared (bdrv_reopen_commit) and reopened state will 
    be used further. Incase preparation of reopening returns failure, 
    the state of the driver will be rolled back (bdrv_reopen_abort) 
    to the stashed state. This approach is implemented for raw-posix, 
    raw-win32, vmdk, qcow, qcow2 and qed block drivers.
  
 * qmp and hmp command 'block_set_hostcache' using which host 
   pagecache setting for a block device can be changed 
   when the VM is running.

 * BDRVReopenState, a generic structure which can be 
   extended by each of the block drivers to reopen 
   respective image files.

V2:
 * Changed ordering of patches such that code changes related to 
   generic framework for safely reopening images gets applied first.

 * For block drivers not having bdrv_reopen_xx functions 
   implemented, return "feature not supported" error.

Testing:
=======
[Thanks! to Yoganananth Subramanian for helping out with testing]

Steps:
1) boot up guest image of different formats qed, raw, qcow2, vmdk
2) run iozone in these guests
   command: iozone -a
3) view cache setting of image file through qemu monitor
   command: info block
            "hostcache =" 0/1 should be displayed
4) Toggle hostcache value using block_set_hostcache
   command: block_set_hostcache virtio0 on
5) Disable and enable hostcache at randon intervals while iozone is
   running inside guest.
   command: block_set_hostcache virtio0 on/off
6) Info block should reflect toggled hostcache value
   and iozone should complete without any issue

Results:
  Verified above steps for raw-posix, qcow2, qed and vmdk images.
  raw-posix, qed and vmdk images (split files) worked fine. With
  qcow2 image getting an error of double free after iozone running
  for a while.

To Do:
======
* Debug the issue with qcow2 image and resolve asap.
* Enhance code around dup3 in raw-posix to fall back to
  dup2/dup when dup3 is not supported by OS.
* Do some more extensive testing, especially with qcow2 and
  qed drivers.

 New block command added:
"block_set_hostcache"
    -- Sets hostcache parameter for block device  while guest is running.

Usage:
 block_set_hostcache  <device> <option>
   <device> = block device
   <option>  = on/off

 qemu/block.c           |   79 ++++++++++++++++++++++
 qemu/block.h           |    5 +
 qemu/block/qcow.c      |  108 ++++++++++++++++++++++++++++++
 qemu/block/qcow2.c     |  175 +++++++++++++++++++++++++++++++++++++++++++++++++
 qemu/block/qed.c       |  103 ++++++++++++++++++++++++++++
 qemu/block/raw-posix.c |  121 +++++++++++++++++++++++++++++++++
 qemu/block/raw-win32.c |   96 ++++++++++++++++++++++++++
 qemu/block/raw.c       |   20 +++++
 qemu/block/vmdk.c      |  103 ++++++++++++++++++++++++++++
 qemu/block_int.h       |   12 +++
 qemu/blockdev.c        |   19 +++++
 qemu/hmp-commands.hx   |   15 ++++
 qemu/hmp.c             |   11 ++
 qemu/hmp.h             |    1
 qemu/qapi-schema.json  |   21 ++++-
 qemu/qemu-common.h     |    1
 qemu/qmp-commands.hx   |   24 ++++++
 17 files changed, 912 insertions(+), 2 deletions(-)