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Hanna Reitz 0bc329fbb0 block: block-status cache for data regions
As we have attempted before
(https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg06451.html,
"file-posix: Cache lseek result for data regions";
https://lists.nongnu.org/archive/html/qemu-block/2021-02/msg00934.html,
"file-posix: Cache next hole"), this patch seeks to reduce the number of
SEEK_DATA/HOLE operations the file-posix driver has to perform.  The
main difference is that this time it is implemented as part of the
general block layer code.

The problem we face is that on some filesystems or in some
circumstances, SEEK_DATA/HOLE is unreasonably slow.  Given the
implementation is outside of qemu, there is little we can do about its
performance.

We have already introduced the want_zero parameter to
bdrv_co_block_status() to reduce the number of SEEK_DATA/HOLE calls
unless we really want zero information; but sometimes we do want that
information, because for files that consist largely of zero areas,
special-casing those areas can give large performance boosts.  So the
real problem is with files that consist largely of data, so that
inquiring the block status does not gain us much performance, but where
such an inquiry itself takes a lot of time.

To address this, we want to cache data regions.  Most of the time, when
bad performance is reported, it is in places where the image is iterated
over from start to end (qemu-img convert or the mirror job), so a simple
yet effective solution is to cache only the current data region.

(Note that only caching data regions but not zero regions means that
returning false information from the cache is not catastrophic: Treating
zeroes as data is fine.  While we try to invalidate the cache on zero
writes and discards, such incongruences may still occur when there are
other processes writing to the image.)

We only use the cache for nodes without children (i.e. protocol nodes),
because that is where the problem is: Drivers that rely on block-status
implementations outside of qemu (e.g. SEEK_DATA/HOLE).

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/307
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20210812084148.14458-3-hreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
[hreitz: Added `local_file == bs` assertion, as suggested by Vladimir]
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
2021-09-15 15:54:06 +02:00
.github Update documentation to refer to new location for issues 2021-06-21 05:43:11 +02:00
.gitlab/issue_templates GitLab: Add "Feature Request" issue template. 2021-06-25 10:08:37 +01:00
.gitlab-ci.d gitlab-ci: Make more custom runner jobs manual, and don't allow failure 2021-09-14 17:03:03 +01:00
accel accel/tcg/user-exec: Fix read-modify-write of code on s390 hosts 2021-09-14 12:00:21 -07:00
audio audio: Never send migration section 2021-08-10 10:55:57 +02:00
authz configure, meson: convert pam detection to meson 2021-06-25 10:54:10 +02:00
backends migration: Unify failure check for migrate_add_blocker() 2021-08-26 17:15:28 +02:00
block block: block-status cache for data regions 2021-09-15 15:54:06 +02:00
bsd-user user: Remove cpu_get_pic_interrupt() stubs 2021-09-14 12:00:21 -07:00
capstone@f8b1b83301 capstone: Update to upstream "next" branch 2020-10-03 04:23:14 -05:00
chardev chardev: remove needless class method 2021-09-14 16:53:55 +04:00
configs hw/acpi: refactor acpi hp modules so that targets can just use what they need 2021-09-04 09:07:46 -04:00
contrib configure / meson: Move the GBM handling to meson.build 2021-09-06 10:00:14 +02:00
crypto crypto: add gnutls pbkdf provider 2021-07-14 14:15:52 +01:00
disas Hexagon (disas/hexagon.c) fix memory leak for early exit cases 2021-08-12 09:06:05 -05:00
docs docs: add supported host CPU architectures section 2021-09-14 16:51:58 +04:00
dtc@85e5d83984 Makefile: dtc: update, build the libfdt target 2020-06-16 14:49:05 +01:00
dump Do not include cpu.h if it's not really necessary 2021-05-02 17:24:51 +02:00
ebpf ebpf: Added eBPF RSS loader. 2021-06-04 15:25:46 +08:00
fpu softfloat: Remove assertion preventing silencing of NaN in default-NaN mode 2021-09-01 11:08:17 +01:00
fsdev meson: Declare have_virtfs_proxy_helper in main meson.build 2021-01-23 15:55:04 -05:00
gdb-xml target/riscv: Remove built-in GDB XML files for CSRs 2021-01-16 10:57:21 -08:00
hw target-arm queue: 2021-09-13 21:06:15 +01:00
include block: block-status cache for data regions 2021-09-15 15:54:06 +02:00
io io: use GDateTime for formatting timestamp for websock headers 2021-07-14 14:15:52 +01:00
libdecnumber qemu/: fix some comment spelling errors 2020-09-17 20:35:43 +02:00
linux-headers linux-headers: Update 2021-07-09 11:01:06 +10:00
linux-user user: Remove cpu_get_pic_interrupt() stubs 2021-09-14 12:00:21 -07:00
meson@776acd2a80 submodules: bump meson to 0.55.3 2020-10-17 10:45:42 -04:00
migration migration: Handle migration_incoming_setup() errors consistently 2021-08-26 17:15:28 +02:00
monitor arch_init.h: Don't include arch_init.h unnecessarily 2021-08-26 17:02:00 +01:00
nbd nbd/server: Mark variable unused in nbd_negotiate_meta_queries 2021-07-26 07:06:25 -10:00
net vhost-vdpa: remove the unncessary queue_index assignment 2021-09-04 17:34:05 -04:00
pc-bios meson: look up cp and dtrace with find_program() 2021-09-13 13:56:26 +02:00
plugins plugins/api: added a boolean parsing plugin api 2021-09-02 11:29:34 +01:00
po configure: move gettext detection to meson.build 2021-01-02 21:03:09 +01:00
python python:QEMUMachine: template typing for self returning methods 2021-09-01 14:03:47 +02:00
qapi qapi: Set boolean value correctly in examples 2021-09-03 17:03:05 +02:00
qga Remove superfluous ERRP_GUARD() 2021-08-26 17:15:28 +02:00
qobject qobject: braces {} are necessary for all arms of this statement 2021-02-04 13:20:29 +01:00
qom qom: use correct field name when getting/setting alias properties 2021-07-23 18:17:17 +02:00
replay replay: notify CPU on event 2021-04-01 10:37:20 +02:00
roms Update OpenBIOS images to d657b653 built from submodule. 2021-09-08 10:30:10 +01:00
scripts qapi: Fix bogus error for 'if': { 'not': '' } 2021-09-08 15:30:30 +02:00
scsi error: Use error_fatal to simplify obvious fatal errors (again) 2021-08-26 17:15:28 +02:00
semihosting Do not include cpu.h if it's not really necessary 2021-05-02 17:24:51 +02:00
slirp@a88d9ace23 Update libslirp to v4.6.1 2021-08-03 16:07:22 +04:00
softmmu qdev: Support marking individual buses as 'full' 2021-09-13 21:01:08 +01:00
storage-daemon storage-daemon: Add missing build dependency to the vhost-user-blk-test 2021-08-11 13:39:50 +02:00
stubs hw/pci: remove all references to find_i440fx function 2021-09-04 17:34:05 -04:00
subprojects/libvhost-user libvhost-user: fix -Werror=format= warnings with __u64 fields 2021-07-29 10:15:52 +02:00
target user: Remove cpu_get_pic_interrupt() stubs 2021-09-14 12:00:21 -07:00
tcg tcg/arm: More use of the TCGReg enum 2021-09-14 12:00:21 -07:00
tests tests/data/acpi/virt: Update IORT files for ITS 2021-09-13 21:01:08 +01:00
tools virtiofsd: Add missing newline in error message 2021-07-09 18:42:46 +02:00
trace meson: look up cp and dtrace with find_program() 2021-09-13 13:56:26 +02:00
ui meson: remove dead variable 2021-09-13 13:56:26 +02:00
util util: Suppress -Wstringop-overflow in qemu_thread_start 2021-09-13 13:56:26 +02:00
.cirrus.yml cirrus: delete FreeBSD and macOS jobs 2021-07-14 14:33:53 +01:00
.dir-locals.el Add .dir-locals.el file to configure emacs coding style 2015-10-08 19:46:01 +03:00
.editorconfig .editorconfig: update the automatic mode setting for Emacs 2021-03-10 15:34:11 +00:00
.exrc qemu: add .exrc 2012-09-07 09:02:44 +03:00
.gdbinit .gdbinit: load QEMU sub-commands when gdb starts 2017-06-07 14:38:45 +01:00
.gitattributes maint: Tell git that *.py files should use python diff hunks 2021-02-15 22:13:34 -05:00
.gitignore gitignore: Update with some filetypes 2021-07-23 17:22:15 +01:00
.gitlab-ci.yml docs: Document GitLab custom CI/CD variables 2021-07-29 07:56:01 +02:00
.gitmodules gitmodules: use GitLab repos instead of qemu.org 2021-02-09 20:53:56 +00:00
.gitpublish Add a git-publish configuration file 2018-03-05 09:03:17 +00:00
.mailmap MAINTAINERS: Name and email address change 2021-08-10 16:42:16 +01:00
.patchew.yml scripts/checkpatch: roll diff tweaking into checkpatch itself 2021-06-25 10:08:33 +01:00
.readthedocs.yml readthedocs: build with Python 3.6 2020-10-05 16:30:45 +01:00
.travis.yml hw/usb/ccid: remove references to NSS 2021-07-14 14:33:53 +01:00
block.c block: block-status cache for data regions 2021-09-15 15:54:06 +02:00
blockdev-nbd.c block/nbd: Use qcrypto_tls_creds_check_endpoint() 2021-06-29 18:29:47 +01:00
blockdev.c arch_init.h: Don't include arch_init.h unnecessarily 2021-08-26 17:02:00 +01:00
blockjob.c progressmeter: protect with a mutex 2021-06-25 14:24:24 +03:00
configure bsd-user: start to move target CPU functions to target_arch* 2021-09-10 14:13:06 -06:00
COPYING
COPYING.LIB COPYING.LIB: Synchronize the LGPL 2.1 with the version from gnu.org 2019-01-30 11:01:22 +01:00
cpu.c accel/tcg: Record singlestep_enabled in tb->cflags 2021-07-21 07:47:05 -10:00
cpus-common.c overall/alpha tcg cpus|hppa: Fix Lesser GPL version number 2020-11-15 16:43:54 +01:00
disas.c Do not include cpu.h if it's not really necessary 2021-05-02 17:24:51 +02:00
gdbstub.c gdbstub: Zero-initialize sockaddr structs 2021-08-26 17:02:00 +01:00
gitdm.config contrib/gitdm: add a new interns group-map for GSoC/Outreachy work 2021-07-23 17:22:16 +01:00
hmp-commands-info.hx monitor/tcg: move tcg hmp commands to accel/tcg, register them dynamically 2021-07-09 18:21:33 +02:00
hmp-commands.hx netdev: add more commands to preconfig mode 2021-06-11 10:30:13 +08:00
iothread.c iothread: add aio-max-batch parameter 2021-07-21 13:47:50 +01:00
job-qmp.c progressmeter: protect with a mutex 2021-06-25 14:24:24 +03:00
job.c progressmeter: protect with a mutex 2021-06-25 14:24:24 +03:00
Kconfig meson: Introduce target-specific Kconfig 2021-07-09 18:21:34 +02:00
Kconfig.host multi-process: Add config option for multi-process QEMU 2021-02-09 20:53:56 +00:00
LICENSE tcg/LICENSE: Remove out of date claim about TCG subdirectory licensing 2019-11-11 15:11:21 +01:00
MAINTAINERS MAINTAINERS: Added myself as a reviewer for acpi/smbios subsystem 2021-09-04 17:34:05 -04:00
Makefile Makefile: ignore long options 2021-07-29 10:15:51 +02:00
memory_ldst.c.inc exec/memory_ldst: Use correct type sizes 2021-05-26 08:35:51 -07:00
meson_options.txt configure, meson: convert libxml2 detection to meson 2021-07-06 08:33:51 +02:00
meson.build meson.build: fix comment typo 2021-09-14 16:52:02 +04:00
module-common.c all: Clean up includes 2016-02-04 17:41:30 +00:00
os-posix.c remove qemu-options* from root directory 2021-05-26 14:49:46 +02:00
os-win32.c remove qemu-options* from root directory 2021-05-26 14:49:46 +02:00
page-vary-common.c exec: Build page-vary-common.c with -fno-lto 2021-03-23 19:36:47 -06:00
page-vary.c exec: Build page-vary-common.c with -fno-lto 2021-03-23 19:36:47 -06:00
qemu-bridge-helper.c qemu-bridge-helper: relocate path to default ACL 2020-09-30 19:11:36 +02:00
qemu-edid.c qemu-edid: use qemu_edid_size() 2021-05-10 11:41:02 +02:00
qemu-img-cmds.hx qemu-img: add support for rate limit in qemu-img convert 2020-10-27 15:26:20 +01:00
qemu-img.c error: Use error_fatal to simplify obvious fatal errors (again) 2021-08-26 17:15:28 +02:00
qemu-io-cmds.c block: Acquire AioContexts during bdrv_reopen_multiple() 2021-07-09 13:19:11 +02:00
qemu-io.c error: Use error_fatal to simplify obvious fatal errors (again) 2021-08-26 17:15:28 +02:00
qemu-keymap.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qemu-nbd.c error: Use error_fatal to simplify obvious fatal errors (again) 2021-08-26 17:15:28 +02:00
qemu-options.hx softmmu/vl: Deprecate the old grab options 2021-09-06 10:00:14 +02:00
qemu.nsi nsis: adjust for new MinGW paths 2021-01-23 15:55:05 -05:00
qemu.sasl sasl: remove comment about obsolete kerberos versions 2021-06-14 13:28:50 +01:00
README.rst Update documentation to refer to new location for issues 2021-06-21 05:43:11 +02:00
replication.c replication: move include out of root directory 2021-05-26 14:49:46 +02:00
thunk.c overall usermode...: Fix Lesser GPL version number 2020-11-15 16:43:40 +01:00
trace-events cpu: Add breakpoint tracepoints 2021-07-09 21:31:11 -07:00
VERSION Open 6.2 development tree 2021-08-25 10:25:12 +01:00
version.rc configure: remove CONFIG_FILEVERSION and CONFIG_PRODUCTVERSION 2021-01-02 21:03:37 +01:00

===========
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.


Documentation
=============

Documentation can be found hosted online at
`<https://www.qemu.org/documentation/>`_. The documentation for the
current development version that is available at
`<https://www.qemu.org/docs/master/>`_ is generated from the ``docs/``
folder in the source tree, and is built by `Sphinx
<https://www.sphinx-doc.org/en/master/>_`.


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:


.. code-block:: shell

  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.

.. code-block:: shell

   git clone https://gitlab.com/qemu-project/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 `style section
<https://www.qemu.org/docs/master/devel/style.html>` of
the Developers Guide.

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.

.. code-block:: shell

  git clone https://gitlab.com/qemu-project/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:

.. code-block:: shell

  $ 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:

.. code-block:: shell

  $ 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 GitLab issues to track bugs. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:

* `<https://gitlab.com/qemu-project/qemu/-/issues>`_

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 GitLab.

For additional information on bug reporting consult:

* `<https://qemu.org/Contribute/ReportABug>`_


ChangeLog
=========

For version history and release notes, please visit
`<https://wiki.qemu.org/ChangeLog/>`_ or look at the git history for
more detailed information.


Contact
=======

The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC

* `<mailto: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>`_