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block: use transactions as a replacement of ->{can_}set_aio_context()
Emanuele Giuseppe Esposito authored 
Simplify the way the aiocontext can be changed in a BDS graph.
There are currently two problems in bdrv_try_set_aio_context:
- There is a confusion of AioContext locks taken and released, because
  we assume that old aiocontext is always taken and new one is
  taken inside.

- It doesn't look very safe to call bdrv_drained_begin while some
  nodes have already switched to the new aiocontext and others haven't.
  This could be especially dangerous because bdrv_drained_begin polls, so
  something else could be executed while graph is in an inconsistent
  state.

Additional minor nitpick: can_set and set_ callbacks both traverse the
graph, both using the ignored list of visited nodes in a different way.

Therefore, get rid of all of this and introduce a new callback,
change_aio_context, that uses transactions to efficiently, cleanly
and most importantly safely change the aiocontext of a graph.

This new callback is a "merge" of the two previous ones:
- Just like can_set_aio_context, recursively traverses the graph.
  Marks all nodes that are visited using a GList, and checks if
  they *could* change the aio_context.
- For each node that passes the above check, drain it and add a new transaction
  that implements a callback that effectively changes the aiocontext.
- Once done, the recursive function returns if *all* nodes can change
  the AioContext. If so, commit the above transactions.
  Regardless of the outcome, call transaction.clean() to undo all drains
  done in the recursion.
- The transaction list is scanned only after all nodes are being drained, so
  we are sure that they all are in the same context, and then
  we switch their AioContext, concluding the drain only after all nodes
  switched to the new AioContext. In this way we make sure that
  bdrv_drained_begin() is always called under the old AioContext, and
  bdrv_drained_end() under the new one.
- Because of the above, we don't need to release and re-acquire the
  old AioContext every time, as everything is done once (and not
  per-node drain and aiocontext change).

Note that the "change" API is not yet invoked anywhere.

Signed-off-by: default avatarEmanuele Giuseppe Esposito <eesposit@redhat.com>
Message-Id: <20221025084952.2139888-3-eesposit@redhat.com>
Reviewed-by: default avatarKevin Wolf <kwolf@redhat.com>
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
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.github/workflows
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accel
audio
authz
backends
block
bsd-user
chardev
common-user
configs
contrib
crypto
disas
docs
dump
ebpf
fpu
fsdev
gdb-xml
gdbstub
hw
include
io
libdecnumber
linux-headers
linux-user
migration
monitor
nbd
net
pc-bios
plugins
po
python
qapi
qga
qobject
qom
replay
roms
scripts
scsi
semihosting
softmmu
storage-daemon
stubs
subprojects
target
tcg
tests
tools
trace
ui
util
.cirrus.yml
.dir-locals.el
.editorconfig
.exrc
.gdbinit
.gitattributes
.gitignore
.gitlab-ci.yml
.gitmodules
.gitpublish
.mailmap
.patchew.yml
.readthedocs.yml
.travis.yml
COPYING
COPYING.LIB
Kconfig
Kconfig.host
LICENSE
MAINTAINERS
Makefile
README.rst
VERSION
block.c
blockdev-nbd.c
blockdev.c
blockjob.c
configure
cpu.c
cpus-common.c
disas.c
event-loop-base.c
gitdm.config
hmp-commands-info.hx
hmp-commands.hx
iothread.c
job-qmp.c
job.c
memory_ldst.c.inc
meson.build
meson_options.txt
module-common.c
os-posix.c
os-win32.c
README.rst

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).

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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.

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:

Submitting patches

The QEMU source code is maintained under the GIT version control system.

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 of the Developers Guide.

Additional information on submitting patches can be found online via the QEMU website

The QEMU website is also maintained under source control.

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

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

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.

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For additional information on bug reporting consult:

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