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Fam Zheng ab27c3b5e7 mirror: Workaround for unexpected iohandler events during completion
Commit 5a7e7a0ba moved mirror_exit to a BH handler but didn't add any
protection against new requests that could sneak in just before the
BH is dispatched. For example (assuming a code base at that commit):

        main_loop_wait # 1
          os_host_main_loop_wait
            g_main_context_dispatch
              aio_ctx_dispatch
                aio_dispatch
                  ...
                    mirror_run
                      bdrv_drain
    (a)               block_job_defer_to_main_loop
          qemu_iohandler_poll
            virtio_queue_host_notifier_read
              ...
                virtio_submit_multiwrite
    (b)           blk_aio_multiwrite

        main_loop_wait # 2
          <snip>
                aio_dispatch
                  aio_bh_poll
    (c)             mirror_exit

At (a) we know the BDS has no pending request. However, the same
main_loop_wait call is going to dispatch iohandlers (EventNotifier
events), which may lead to a new I/O from guest. So the invariant is
already broken at (c). Data loss.

Commit f3926945c8 made iohandler to use aio API.  The order of
virtio_queue_host_notifier_read and block_job_defer_to_main_loop within
a main_loop_wait becomes unpredictable, and even worse, if the host
notifier event arrives at the next main_loop_wait call, the
unpredictable order between mirror_exit and
virtio_queue_host_notifier_read is also a trouble. As shown below, this
commit made the bug easier to trigger:

    - Bug case 1:

        main_loop_wait # 1
          os_host_main_loop_wait
            g_main_context_dispatch
              aio_ctx_dispatch (qemu_aio_context)
                ...
                  mirror_run
                    bdrv_drain
    (a)             block_job_defer_to_main_loop
              aio_ctx_dispatch (iohandler_ctx)
                virtio_queue_host_notifier_read
                  ...
                    virtio_submit_multiwrite
    (b)               blk_aio_multiwrite

        main_loop_wait # 2
          ...
                aio_dispatch
                  aio_bh_poll
    (c)             mirror_exit

    - Bug case 2:

        main_loop_wait # 1
          os_host_main_loop_wait
            g_main_context_dispatch
              aio_ctx_dispatch (qemu_aio_context)
                ...
                  mirror_run
                    bdrv_drain
    (a)             block_job_defer_to_main_loop

        main_loop_wait # 2
          ...
            aio_ctx_dispatch (iohandler_ctx)
              virtio_queue_host_notifier_read
                ...
                  virtio_submit_multiwrite
    (b)             blk_aio_multiwrite
              aio_dispatch
                aio_bh_poll
    (c)           mirror_exit

In both cases, (b) breaks the invariant wanted by (a) and (c).

Until then, the request loss has been silent. Later, 3f09bfbc7b added
asserts at (c) to check the invariant (in
bdrv_replace_in_backing_chain), and Max reported an assertion failure
first visible there, by doing active committing while the guest is
running bonnie++.

2.5 added bdrv_drained_begin at (a) to protect the dataplane case from
similar problems, but we never realize the main loop bug until now.

As a bandage, this patch disables iohandler's external events
temporarily together with bs->ctx.

Launchpad Bug: 1570134

Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2016-04-22 16:44:09 +02:00
audio util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
backends hostmem-file: plug a small leak 2016-04-15 17:56:06 +02:00
block mirror: Workaround for unexpected iohandler events during completion 2016-04-22 16:44:09 +02:00
bsd-user bsd-user: Suppress gcc 4.x -Wpointer-sign (included in -Wall) warning 2016-04-05 17:49:41 +01:00
contrib * Log filtering from Alex and Peter 2016-03-24 21:42:40 +00:00
crypto crypto: do an explicit check for nettle pbkdf functions 2016-03-30 14:55:11 +01:00
default-configs hw/gpio: Add the emulation of gpio_key 2016-03-30 17:27:22 +01:00
disas * Log filtering from Alex and Peter 2016-03-24 21:42:40 +00:00
docs fw_cfg: Adopt /opt/RFQDN convention 2016-04-19 16:09:50 +02:00
dtc@65cc4d2748 dtc: Update dtc / libfdt submodule to version 1.4.0 2015-06-03 23:56:49 +02:00
fpu target-tricore: Add FPU infrastructure 2016-03-23 09:22:48 +01:00
fsdev module: Rename machine_init() to opts_init() 2016-03-16 15:54:23 -03:00
gdb-xml target-ppc: gdbstub: Add VSX support 2016-01-30 23:37:38 +11:00
hw virtio: Mark host notifiers as external 2016-04-22 16:43:58 +02:00
include event-notifier: Add "is_external" parameter 2016-04-22 16:43:56 +02:00
io include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
libdecnumber libdecnumber: Clean up includes 2016-02-16 14:29:27 +00:00
linux-headers update Linux headers to 4.6 2016-04-05 11:46:52 +02:00
linux-user linux-user: arm: Handle (ignore) EXCP_YIELD in ARM cpu_loop() 2016-04-04 17:33:51 +01:00
migration util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
nbd nbd: Don't mishandle unaligned client requests 2016-04-22 11:55:35 +01:00
net filter-buffer: fix segfault when starting qemu with status=off property 2016-04-06 09:52:07 +08:00
pc-bios Update OpenBIOS images 2016-04-18 09:38:55 +01:00
pixman@87eea99e44
po Update language files for QEMU 2.6.0 2016-04-19 18:41:25 +01:00
qapi block: add generic full disk encryption driver 2016-03-30 12:11:26 +02:00
qga qga: Workaround for console redirection from non-interactive qemu-ga service 2016-04-07 11:43:54 -05:00
qobject util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
qom util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
replay replay: introduce block devices record/replay 2016-03-30 12:15:57 +02:00
roms Update OpenBIOS images 2016-04-18 09:38:55 +01:00
scripts checkpatch: add target_ulong to typelist 2016-04-05 11:46:52 +02:00
slirp wxx: Fix broken TCP networking (regression) 2016-04-15 19:35:17 +02:00
stubs event-notifier: Add "is_external" parameter 2016-04-22 16:43:56 +02:00
target-alpha include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-arm target-arm: Make the 64-bit version of VTCR do the migration 2016-04-04 17:33:52 +01:00
target-cris include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-i386 event-notifier: Add "is_external" parameter 2016-04-22 16:43:56 +02:00
target-lm32 include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-m68k include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-microblaze include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-mips target-mips: add MAAR, MAARI register 2016-03-30 09:14:00 +01:00
target-moxie include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-openrisc include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-ppc ppc: Fix migration of the XER register 2016-04-18 15:14:38 +10:00
target-s390x util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
target-sh4 include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-sparc target-sparc: fix Trap Based Address Register behavior for sparc64 2016-04-15 09:30:40 +01:00
target-tilegx include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-tricore * Log filtering from Alex and Peter 2016-03-24 21:42:40 +00:00
target-unicore32 include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
target-xtensa include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
tcg tcg: check for CONFIG_DEBUG_TCG instead of NDEBUG 2016-04-21 15:43:20 +01:00
tests iotests: Test case for drive-mirror with unaligned image size 2016-04-20 16:52:55 +02:00
trace trace: do not always call exit() in trace_enable_events 2016-03-31 09:48:59 +01:00
ui input-linux: refine mouse detection 2016-04-13 15:52:28 +02:00
util event-notifier: Add "is_external" parameter 2016-04-22 16:43:56 +02:00
.dir-locals.el Add .dir-locals.el file to configure emacs coding style 2015-10-08 19:46:01 +03:00
.exrc
.gitignore maint: Ignore ivshmem binaries 2015-11-06 15:42:38 +03:00
.gitmodules
.mailmap
.travis.yml .travis.yml: make -j3 2016-04-05 10:08:15 +01:00
accel.c all: Clean up includes 2016-02-04 17:41:30 +00:00
aio-posix.c aio-posix: Skip external nodes in aio_dispatch 2016-04-22 16:43:59 +02:00
aio-win32.c all: Clean up includes 2016-02-04 17:41:30 +00:00
arch_init.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
async.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
balloon.c all: Clean up includes 2016-02-04 17:41:30 +00:00
block.c block: Forbid I/O throttling on nodes with multiple parents for 2.6 2016-04-05 09:22:28 +02:00
blockdev-nbd.c nbd: enable use of TLS with nbd-server-start command 2016-02-16 17:17:49 +01:00
blockdev.c block: Forbid I/O throttling on nodes with multiple parents for 2.6 2016-04-05 09:22:28 +02:00
blockjob.c blockjob: Fix hang in block_job_finish_sync 2016-02-09 13:52:26 +00:00
bootdevice.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
bt-host.c all: Clean up includes 2016-02-04 17:41:30 +00:00
bt-vhci.c all: Clean up includes 2016-02-04 17:41:30 +00:00
Changelog
CODING_STYLE CODING_STYLE: update mixed declaration rules 2015-09-09 15:34:54 +02:00
configure block/gluster: prevent data loss after i/o error 2016-04-19 12:24:59 -04:00
COPYING
COPYING.LIB
cpu-exec-common.c exec: Clean up includes 2016-01-29 15:07:22 +00:00
cpu-exec.c qemu-log: dfilter-ise exec, out_asm, op and opt_op 2016-03-22 22:20:18 +01:00
cpus.c cpus: don't use atomic_read for vm_clock_warp_start 2016-04-05 11:46:52 +02:00
cputlb.c cputlb: modernise the debug support 2016-03-22 22:20:18 +01:00
device_tree.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
device-hotplug.c blockdev: Split monitor reference from BB creation 2016-03-17 15:47:56 +01:00
disas.c all: Clean up includes 2016-02-04 17:41:30 +00:00
dma-helpers.c all: Clean up includes 2016-02-04 17:41:30 +00:00
dump.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
exec.c memory: fix segv on qemu_ram_free(block=0x0) 2016-04-05 11:46:52 +02:00
gdbstub.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
HACKING HACKING: Add a section on error handling and reporting 2016-02-09 13:19:49 +01:00
hmp-commands-info.hx Dump: add hmp command "info dump" 2016-02-22 18:40:28 +01:00
hmp-commands.hx hmp: 'drive_add -n' for creating a node without BB 2016-03-14 16:46:43 +01:00
hmp.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
hmp.h Dump: add hmp command "info dump" 2016-02-22 18:40:28 +01:00
iohandler.c iohandler: Introduce iohandler_get_aio_context 2016-04-22 16:43:42 +02:00
ioport.c all: Clean up includes 2016-02-04 17:41:30 +00:00
iothread.c all: Clean up includes 2016-02-04 17:41:30 +00:00
kvm-all.c kvm: add kvm_device_supported() helper function 2016-03-30 17:27:24 +01:00
kvm-stub.c all: Clean up includes 2016-02-04 17:41:30 +00:00
LICENSE
main-loop.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
MAINTAINERS MAINTAINERS: Avoid using K: for NUMA section 2016-04-21 11:40:57 +01:00
Makefile osdep: add wrappers for socket functions 2016-03-10 17:19:07 +00:00
Makefile.objs crypto: add cryptographic random byte source 2016-03-17 09:49:01 +00:00
Makefile.target io: add abstract QIOChannel classes 2015-12-18 12:18:05 +00:00
memory_mapping.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
memory.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
module-common.c all: Clean up includes 2016-02-04 17:41:30 +00:00
monitor.c arm: qmp: add query-gic-capabilities interface 2016-03-30 17:27:24 +01:00
numa.c qapi: Don't special-case simple union wrappers 2016-03-18 10:29:26 +01:00
os-posix.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
os-win32.c all: Clean up includes 2016-02-04 17:41:30 +00:00
page_cache.c all: Clean up includes 2016-02-04 17:41:30 +00:00
qapi-schema.json slirp: Allow disabling IPv4 or IPv6 2016-04-01 17:51:55 +02:00
qdev-monitor.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
qdict-test-data.txt
qemu-bridge-helper.c all: Clean up includes 2016-02-04 17:41:30 +00:00
qemu-char.c char: ensure all clients are in non-blocking mode 2016-04-05 11:46:52 +02:00
qemu-doc.texi ivshmem: Require master to have ID zero 2016-03-21 21:29:03 +01:00
qemu-ga.texi docs: Style the command and its options in the synopsis 2016-01-26 15:58:11 +01:00
qemu-img-cmds.hx qemu-img: allow specifying image as a set of options args 2016-02-22 09:50:04 +01:00
qemu-img.c block: initialize qcrypto API at startup 2016-04-12 18:06:51 +02:00
qemu-img.texi qemu-img: allow specifying image as a set of options args 2016-02-22 09:50:04 +01:00
qemu-io-cmds.c qemu-io: Support 'aio_write -z' 2016-04-15 17:22:11 +02:00
qemu-io.c block: initialize qcrypto API at startup 2016-04-12 18:06:51 +02:00
qemu-nbd.c nbd: fix assert() on qemu-nbd stop 2016-04-15 17:56:56 +02:00
qemu-nbd.texi qemu-nbd: allow specifying image as a set of options args 2016-02-22 09:50:04 +01:00
qemu-options-wrapper.h
qemu-options.h
qemu-options.hx fw_cfg: Adopt /opt/RFQDN convention 2016-04-19 16:09:50 +02:00
qemu-seccomp.c seccomp: adding sysinfo system call to whitelist 2016-04-16 20:27:44 +02:00
qemu-tech.texi tcg: Rename tcg-target.c to tcg-target.inc.c 2016-02-23 08:30:38 -08:00
qemu-timer.c icount: decouple warp calls 2016-03-15 18:23:45 +01:00
qemu.nsi nsis: Add QEMU version information to Windows registry 2015-09-24 20:52:28 +02:00
qemu.sasl
qjson.c all: Clean up includes 2016-02-04 17:41:30 +00:00
qmp-commands.hx arm: qmp: add query-gic-capabilities interface 2016-03-30 17:27:24 +01:00
qmp.c util: move declarations out of qemu-common.h 2016-03-22 22:20:17 +01:00
qtest.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
README README: fill out some useful quickstart information 2015-10-13 18:48:46 +02:00
rules.mak rules: filter out irrelevant files 2016-02-17 16:59:36 +02:00
softmmu_template.h exec.c: Pass MemTxAttrs to iotlb_to_region so it uses the right AS 2016-01-21 14:15:05 +00:00
spice-qemu-char.c qapi: Don't special-case simple union wrappers 2016-03-18 10:29:26 +01:00
tcg-runtime.c all: Clean up includes 2016-02-04 17:41:30 +00:00
tci.c tci: Fix build regression 2016-04-07 19:01:21 +02:00
thread-pool.c all: Clean up includes 2016-02-04 17:41:30 +00:00
thunk.c all: Clean up includes 2016-02-04 17:41:30 +00:00
tpm.c qapi: Don't special-case simple union wrappers 2016-03-18 10:29:26 +01:00
trace-events ui/virtio-gpu: add and use qemu_create_displaysurface_pixman 2016-04-11 12:32:01 +02:00
translate-all.c translate-all: add missing fold of tb_ctx into tcg_ctx 2016-04-08 00:07:56 +02:00
translate-all.h translate-all: remove unnecessary argument to tb_invalidate_phys_range 2015-06-05 17:09:59 +02:00
translate-common.c exec: Clean up includes 2016-01-29 15:07:22 +00:00
user-exec.c all: Clean up includes 2016-02-04 17:41:30 +00:00
VERSION Update version for v2.6.0-rc3 release 2016-04-21 17:46:50 +01:00
version.rc
vl.c Sort the fw_cfg file list 2016-04-07 19:57:33 +03:00
xen-common-stub.c xen: Clean up includes 2016-01-29 15:07:23 +00:00
xen-common.c xen: drop XenXC and associated interface wrappers 2016-02-10 12:01:24 +00:00
xen-hvm-stub.c fix MSI injection on Xen 2016-02-06 20:44:10 +02:00
xen-hvm.c xen: Drop __XEN_LATEST_INTERFACE_VERSION__ checks from prior to Xen 4.2 2016-02-10 12:01:32 +00:00
xen-mapcache.c xen: Clean up includes 2016-01-29 15:07:23 +00:00

         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.


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