[Qemu-devel] [RFC Patch 0/7]Qemu: 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.

Following patchset introduces:
 * 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 extended to raw-posix, raw-win32 and vmdk block 
    drivers in this patchset. Once this is reviewed and finalised, I will 
    extend the implementation to other drivers like qcow2, qed etc..
  
 * 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.

ToDo:
* memcpy is used to save driver state. Replace this with copying
  individual fields of driver state (?)
* Extend this implementation to other block drivers.
* Build and verify raw-win32 driver changes in windows
 
Earlier discussions related to dynamic change of host pagecache can be found 
at: http://lists.gnu.org/archive/html/qemu-devel/2011-11/msg01482.html

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           |  112 +++++++++++++++++++++++++++++++++++++++++++++----
 qemu/block.h           |    5 +
 qemu/block/raw-posix.c |   74 ++++++++++++++++++++++++++++++++
 qemu/block/raw-win32.c |   95 +++++++++++++++++++++++++++++++++++++++++
 qemu/block/raw.c       |   20 ++++++++
 qemu/block/vmdk.c      |   80 +++++++++++++++++++++++++++++++++--
 qemu/block_int.h       |   11 ++++
 qemu/blockdev.c        |   26 +++++++++++
 qemu/blockdev.h        |    2
 qemu/hmp-commands.hx   |   14 ++++++
 qemu/hmp.c             |    2
 qemu/qapi-schema.json  |    4 +
 qemu/qemu-common.h     |    1
 qemu/qerror.c          |    8 +++
 qemu/qerror.h          |    6 ++
 qemu/qmp-commands.hx   |   27 +++++++++++
 18 files changed, 474 insertions(+), 13 deletions(-)                           
                                                                                
                                                                                
                                                                                
                                           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
          
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
           
                                                                                
                                                                                
       
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
~                                                                               
                                                                                
            
-- INSERT --