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Peter Xu
8597af7615
This patch proposes a flag to maintain disk activation status globally. It mostly rewrites disk activation mgmt for QEMU, including COLO and QMP command xen_save_devices_state. Backgrounds =========== We have two problems on disk activations, one resolved, one not. Problem 1: disk activation recover (for switchover interruptions) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When migration is either cancelled or failed during switchover, especially when after the disks are inactivated, QEMU needs to remember re-activate the disks again before vm starts. It used to be done separately in two paths: one in qmp_migrate_cancel(), the other one in the failure path of migration_completion(). It used to be fixed in different commits, all over the places in QEMU. So these are the relevant changes I saw, I'm not sure if it's complete list: - In 2016, commitfe904ea824("migration: regain control of images when migration fails to complete") - In 2017, commit1d2acc3162("migration: re-active images while migration been canceled after inactive them") - In 2023, commit6dab4c93ec("migration: Attempt disk reactivation in more failure scenarios") Now since we have a slightly better picture maybe we can unify the reactivation in a single path. One side benefit of doing so is, we can move the disk operation outside QMP command "migrate_cancel". It's possible that in the future we may want to make "migrate_cancel" be OOB-compatible, while that requires the command doesn't need BQL in the first place. This will already do that and make migrate_cancel command lightweight. Problem 2: disk invalidation on top of invalidated disks ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This is an unresolved bug for current QEMU. Link in "Resolves:" at the end. It turns out besides the src switchover phase (problem 1 above), QEMU also needs to remember block activation on destination. Consider two continuous migration in a row, where the VM was always paused. In that scenario, the disks are not activated even until migration completed in the 1st round. When the 2nd round starts, if QEMU doesn't know the status of the disks, it needs to try inactivate the disk again. Here the issue is the block layer API bdrv_inactivate_all() will crash a QEMU if invoked on already inactive disks for the 2nd migration. For detail, see the bug link at the end. Implementation ============== This patch proposes to maintain disk activation with a global flag, so we know: - If we used to inactivate disks for migration, but migration got cancelled, or failed, QEMU will know it should reactivate the disks. - On incoming side, if the disks are never activated but then another migration is triggered, QEMU should be able to tell that inactivate is not needed for the 2nd migration. We used to have disk_inactive, but it only solves the 1st issue, not the 2nd. Also, it's done in completely separate paths so it's extremely hard to follow either how the flag changes, or the duration that the flag is valid, and when we will reactivate the disks. Convert the existing disk_inactive flag into that global flag (also invert its naming), and maintain the disk activation status for the whole lifecycle of qemu. That includes the incoming QEMU. Put both of the error cases of source migration (failure, cancelled) together into migration_iteration_finish(), which will be invoked for either of the scenario. So from that part QEMU should behave the same as before. However with such global maintenance on disk activation status, we not only cleanup quite a few temporary paths that we try to maintain the disk activation status (e.g. in postcopy code), meanwhile it fixes the crash for problem 2 in one shot. For freshly started QEMU, the flag is initialized to TRUE showing that the QEMU owns the disks by default. For incoming migrated QEMU, the flag will be initialized to FALSE once and for all showing that the dest QEMU doesn't own the disks until switchover. That is guaranteed by the "once" variable. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2395 Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Fabiano Rosas <farosas@suse.de> Message-Id: <20241206230838.1111496-7-peterx@redhat.com> Signed-off-by: Fabiano Rosas <farosas@suse.de>
=========== 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://wiki.qemu.org/Hosts/Linux>`_ * `<https://wiki.qemu.org/Hosts/Mac>`_ * `<https://wiki.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://wiki.qemu.org/Contribute/SubmitAPatch>`_ * `<https://wiki.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://wiki.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://wiki.qemu.org/Contribute/StartHere>`_