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There is a reliable way to make a CPU hotunplug fail in the pseries machine. Hotplug a CPU A, then offline all other CPUs inside the guest but A. When trying to hotunplug A the guest kernel will refuse to do it, because A is now the last online CPU of the guest. PAPR has no 'error callback' in this situation to report back to the platform, so the guest kernel will deny the unplug in silent and QEMU will never know what happened. The unplug pending state of A will remain until the guest is shutdown or rebooted. Previous attempts of fixing it (see [1] and [2]) were aimed at trying to mitigate the effects of the problem. In [1] we were trying to guess which guest CPUs were online to forbid hotunplug of the last online CPU in the QEMU layer, avoiding the scenario described above because QEMU is now failing in behalf of the guest. This is not robust because the last online CPU of the guest can change while we're in the middle of the unplug process, and our initial assumptions are now invalid. In [2] we were accepting that our unplug process is uncertain and the user should be allowed to spam the IRQ hotunplug queue of the guest in case the CPU hotunplug fails. This patch presents another alternative, using the timeout infrastructure introduced in the previous patch. CPU hotunplugs in the pSeries machine will now timeout after 15 seconds. This is a long time for a single CPU unplug to occur, regardless of guest load - although the user is *strongly* encouraged to *not* hotunplug devices from a guest under high load - and we can be sure that something went wrong if it takes longer than that for the guest to release the CPU (the same can't be said about memory hotunplug - more on that in the next patch). Timing out the unplug operation will reset the unplug state of the CPU and allow the user to try it again, regardless of the error situation that prevented the hotunplug to occur. Of all the not so pretty fixes/mitigations for CPU hotunplug errors in pSeries, timing out the operation is an admission that we have no control in the process, and must assume the worst case if the operation doesn't succeed in a sensible time frame. [1] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg03353.html [2] https://lists.gnu.org/archive/html/qemu-devel/2021-01/msg04400.html Reported-by: Xujun Ma <xuma@redhat.com> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1911414 Reviewed-by: David Gibson <david@gibson.dropbear.id.au> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com> Message-Id: <20210222194531.62717-5-danielhb413@gmail.com> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
=========== 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: .. 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 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: * `<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>`_