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 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. ToDo: * Currently driver state is stashed by field-by-field copy. Optimise this by stashing only the required fields. * Find out other drivers needing bdrv_reopen implementation. * Do some more extensive testing, especially with drivers like floppy, cdrom etc.. Earlier discussions on RFC for dynamic change of host pagecache can be found at: http://lists.gnu.org/archive/html/qemu-devel/2012-01/msg04139.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 | 127 ++++++++++++++++++++++++++++++++-- qemu/block.h | 5 + qemu/block/qcow.c | 108 +++++++++++++++++++++++++++++ qemu/block/qcow2.c | 177 +++++++++++++++++++++++++++++++++++++++++++++++++ qemu/block/qed.c | 103 ++++++++++++++++++++++++++++ qemu/block/raw-posix.c | 123 ++++++++++++++++++++++++++++++++++ qemu/block/raw-win32.c | 96 ++++++++++++++++++++++++++ qemu/block/raw.c | 20 +++++ qemu/block/vmdk.c | 103 ++++++++++++++++++++++++++++ qemu/block_int.h | 11 +++ qemu/blockdev.c | 24 ++++++ qemu/hmp-commands.hx | 16 ++++ qemu/hmp.c | 11 ++ qemu/hmp.h | 1 qemu/qapi-schema.json | 20 ++++- qemu/qemu-common.h | 1 qemu/qerror.c | 8 ++ qemu/qerror.h | 6 + qemu/qmp-commands.hx | 24 ++++++ 19 files changed, 971 insertions(+), 13 deletions(-)