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Paolo Bonzini
c2b6428d38
While it is true that bdrv_set_aio_context only works on a single BlockDriverState subtree (see commit message for53ec73e, "block: Use bdrv_drain to replace uncessary bdrv_drain_all", 2015-07-07), it works at the AioContext level rather than the BlockDriverState level. Therefore, it is also necessary to trigger pending bottom halves too, even if no requests are pending. For NBD this ensures that the aio_co_schedule of a previous call to nbd_attach_aio_context is completed before detaching from the old AioContext; it fixes qemu-iotest 094. Another similar bug happens when the VM is stopped and the virtio-blk dataplane irqfd is torn down. In this case it's possible that guest I/O gets stuck if notify_guest_bh was scheduled but doesn't run. Calling aio_poll from another AioContext is safe if non-blocking; races such as the one mentioned in the commit message forc9d1a56("block: only call aio_poll on the current thread's AioContext", 2016-10-28) are a concern for blocking calls. I considered other options, including: - moving the bs->wakeup mechanism to AioContext, and letting the caller check. This might work for virtio which has a clear place to wakeup (notify_place_bh) and check the condition (virtio_blk_data_plane_stop). For aio_co_schedule I couldn't find a clear place to check the condition. - adding a dummy oneshot bottom half and waiting for it to trigger. This has the complication that bottom half list is LIFO for historical reasons. There were performance issues caused by bottom half ordering in the past, so I decided against it for 2.9. Fixes:9972354856Reported-by: Max Reitz <mreitz@redhat.com> Reported-by: Halil Pasic <pasic@linux.vnet.ibm.com> Tested-by: Halil Pasic <pasic@linux.vnet.ibm.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20170314111157.14464-2-pbonzini@redhat.com Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Max Reitz <mreitz@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
Additional information can also be found online via the QEMU website:
http://qemu-project.org/Hosts/Linux
http://qemu-project.org/Hosts/Mac
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