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Original Xbox Emulator for Windows, macOS, and Linux (Active Development)
abf754fe40
When we try to allocate new clusters we first look for available ones
starting from s->free_cluster_index and once we find them we increase
their reference counts. Before we get to call update_refcount() to do
this last step s->free_cluster_index is already pointing to the next
cluster after the ones we are trying to allocate.
During update_refcount() it may happen however that we also need to
allocate a new refcount block in order to store the refcounts of these
new clusters (and to complicate things further that may also require
us to grow the refcount table). After all this we don't know if the
clusters that we originally tried to allocate are still available, so
we return -EAGAIN to ask the caller to restart the search for free
clusters.
This is what can happen in a common scenario:
1) We want to allocate a new cluster and we see that cluster N is
free.
2) We try to increase N's refcount but all refcount blocks are full,
so we allocate a new one at N+1 (where s->free_cluster_index was
pointing at).
3) Once we're done we return -EAGAIN to look again for a free
cluster, but now s->free_cluster_index points at N+2, so that's
the one we allocate. Cluster N remains unallocated and we have a
hole in the qcow2 file.
This can be reproduced easily:
qemu-img create -f qcow2 -o cluster_size=512 hd.qcow2 1M
qemu-io -c 'write 0 124k' hd.qcow2
After this the image has 132608 bytes (256 clusters), and the refcount
block is full. If we write 512 more bytes it should allocate two new
clusters: the data cluster itself and a new refcount block.
qemu-io -c 'write 124k 512' hd.qcow2
However the image has now three new clusters (259 in total), and the
first one of them is empty (and unallocated):
dd if=hd.qcow2 bs=512c skip=256 count=1 | hexdump -C
If we write larger amounts of data in the last step instead of the 512
bytes used in this example we can create larger holes in the qcow2
file.
What this patch does is reset s->free_cluster_index to its previous
value when alloc_refcount_block() returns -EAGAIN. This way the caller
will try to allocate again the original clusters if they are still
free.
The output of iotest 026 also needs to be updated because now that
images have no holes some tests fail at a different point and the
number of leaked clusters is different.
Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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| accel | ||
| audio | ||
| backends | ||
| block | ||
| bsd-user | ||
| capstone@22ead3e0bf | ||
| chardev | ||
| contrib | ||
| crypto | ||
| default-configs | ||
| disas | ||
| docs | ||
| dtc@e54388015a | ||
| fpu | ||
| fsdev | ||
| gdb-xml | ||
| hw | ||
| include | ||
| io | ||
| libdecnumber | ||
| linux-headers | ||
| linux-user | ||
| migration | ||
| nbd | ||
| net | ||
| pc-bios | ||
| po | ||
| qapi | ||
| qga | ||
| qobject | ||
| qom | ||
| replay | ||
| roms | ||
| scripts | ||
| scsi | ||
| slirp | ||
| stubs | ||
| target | ||
| tcg | ||
| tests | ||
| trace | ||
| ui | ||
| util | ||
| .dir-locals.el | ||
| .editorconfig | ||
| .exrc | ||
| .gdbinit | ||
| .gitignore | ||
| .gitmodules | ||
| .gitpublish | ||
| .mailmap | ||
| .shippable.yml | ||
| .travis.yml | ||
| arch_init.c | ||
| balloon.c | ||
| block.c | ||
| blockdev-nbd.c | ||
| blockdev.c | ||
| blockjob.c | ||
| bootdevice.c | ||
| bt-host.c | ||
| bt-vhci.c | ||
| Changelog | ||
| CODING_STYLE | ||
| configure | ||
| COPYING | ||
| COPYING.LIB | ||
| COPYING.PYTHON | ||
| cpus-common.c | ||
| cpus.c | ||
| device_tree.c | ||
| device-hotplug.c | ||
| disas.c | ||
| dma-helpers.c | ||
| dump.c | ||
| exec.c | ||
| gdbstub.c | ||
| HACKING | ||
| hmp-commands-info.hx | ||
| hmp-commands.hx | ||
| hmp.c | ||
| hmp.h | ||
| ioport.c | ||
| iothread.c | ||
| LICENSE | ||
| MAINTAINERS | ||
| Makefile | ||
| Makefile.objs | ||
| Makefile.target | ||
| memory_ldst.inc.c | ||
| memory_mapping.c | ||
| memory.c | ||
| module-common.c | ||
| monitor.c | ||
| numa.c | ||
| os-posix.c | ||
| os-win32.c | ||
| qdev-monitor.c | ||
| qdict-test-data.txt | ||
| qemu-bridge-helper.c | ||
| qemu-doc.texi | ||
| qemu-ga.texi | ||
| qemu-img-cmds.hx | ||
| qemu-img.c | ||
| qemu-img.texi | ||
| qemu-io-cmds.c | ||
| qemu-io.c | ||
| qemu-keymap.c | ||
| qemu-nbd.c | ||
| qemu-nbd.texi | ||
| qemu-option-trace.texi | ||
| qemu-options-wrapper.h | ||
| qemu-options.h | ||
| qemu-options.hx | ||
| qemu-seccomp.c | ||
| qemu-tech.texi | ||
| qemu.nsi | ||
| qemu.sasl | ||
| qmp.c | ||
| qtest.c | ||
| README | ||
| replication.c | ||
| replication.h | ||
| rules.mak | ||
| thunk.c | ||
| tpm.c | ||
| trace-events | ||
| VERSION | ||
| version.rc | ||
| vl.c | ||
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
Additional information can also be found online via the QEMU website:
https://qemu.org/Hosts/Linux
https://qemu.org/Hosts/Mac
https://qemu.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu.org/qemu.git
When submitting patches, one common 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
https://qemu.org/Contribute/SubmitAPatch
https://qemu.org/Contribute/TrivialPatches
The QEMU website is also maintained under source control.
git clone git://git.qemu.org/qemu-web.git
https://www.qemu.org/2017/02/04/the-new-qemu-website-is-up/
A 'git-publish' utility was created to make above process less
cumbersome, and is highly recommended for making regular contributions,
or even just for sending consecutive patch series revisions. It also
requires a working 'git send-email' setup, and by default doesn't
automate everything, so you may want to go through the above steps
manually for once.
For installation instructions, please go to
https://github.com/stefanha/git-publish
The workflow with 'git-publish' is:
$ git checkout master -b my-feature
$ # work on new commits, add your 'Signed-off-by' lines to each
$ git publish
Your patch series will be sent and tagged as my-feature-v1 if you need to refer
back to it in the future.
Sending v2:
$ git checkout my-feature # same topic branch
$ # making changes to the commits (using 'git rebase', for example)
$ git publish
Your patch series will be sent with 'v2' tag in the subject and the git tip
will be tagged as my-feature-v2.
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:
https://qemu.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
https://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:
https://qemu.org/Contribute/StartHere
-- End