This patch changes dirty bitmaps from following a BlockBackend in graph
changes to sticking with the node they were created at. For the full
discussion, read the following mailing list thread:
[Qemu-block] block: Dirty bitmaps and COR in bdrv_move_feature_fields()
https://lists.nongnu.org/archive/html/qemu-block/2016-02/msg00745.html
In summary, the justification for this change is:
* When moving the dirty bitmap to the top of the tree was introduced in
bdrv_append() in commit a9fc4408, it didn't actually have any effect
because there could never be a bitmap in use when bdrv_append() was
called (op blockers would prevent this). This is still true today for
all internal uses of dirty bitmaps.
* Support for user-defined dirty bitmaps was introduced in 2.4, but we
discouraged users from using it because we didn't consider it ready
yet.
Moreover, in 2.5, the bdrv_swap() removal introduced a bug that left
dangling pointers if a dirty bitmap was present (the anchors of the
dirty bitmap were swapped, but the back link in the first element
wasn't updated), so it didn't even work correctly.
* block-dirty-bitmap-add takes an arbitrary node name, even if no
BlockBackend is attached. This suggests that it is a node level
operation and not a BlockBackend one. Consequently, there is no reason
for dirty bitmaps to stay with a BlockBackend that was attached to the
node they were created for.
* It was suggested that block-dirty-bitmap-add could track the node if a
node name was specified, and track the BlockBackend if the device name
was specified. This would however be inconsistent with other QMP
commands. Commands that accept both device and node names currently
interpret the device name just as an alias for the current root node
of that BlockBackend.
* Dirty bitmaps have a name that is only unique amongst the bitmaps in a
specific node. Moving bitmaps could lead to name clashes. Automatic
renaming would involve too much magic.
* Persistent bitmaps are stored in a specific node. Moving them around
automatically might be at least surprising, but it would probably also
become a real problem because that would have to happen atomically
without the management tool knowing of the operation.
At the end of the day it seems to be very clear that it was a mistake to
include dirty bitmaps in bdrv_move_feature_fields(). The functionality
of moving bitmaps and/or attaching them to a BlockBackend instead will
probably be needed, but it should be done with a new explicit QMP
command or option.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Complete details of the process for building and configuring QEMU for
all supported host platforms can be found in the qemu-tech.html file.
Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu-project.org/qemu.git
When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
http://qemu-project.org/Contribute/SubmitAPatch
http://qemu-project.org/Contribute/TrivialPatches
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
http://qemu-project.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
http://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
http://qemu-project.org/Contribute/StartHere
-- End
7a827aaec8