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The LoPAR spec provides no way for the guest kernel to report failure of hotplug/hotunplug events. This wouldn't be bad if those operations were granted to always succeed, but that's far for the reality. What ends up happening is that, in the case of a failed hotunplug, regardless of whether it was a QEMU error or a guest misbehavior, the pSeries machine is retaining the unplug state of the device in the running guest. This state is cleanup in machine reset, where it is assumed that this state represents a device that is pending unplug, and the device is hotunpluged from the board. Until the reset occurs, any hotunplug operation of the same device is forbid because there is a pending unplug state. This behavior has at least one undesirable side effect. A long standing pending unplug state is, more often than not, the result of a hotunplug error. The user had to dealt with it, since retrying to unplug the device is noy allowed, and then in the machine reset we're removing the device from the guest. This means that we're failing the user twice - failed to hotunplug when asked, then hotunplugged without notice. Solutions to this problem range between trying to predict when the hotunplug will fail and forbid the operation from the QEMU layer, from opening up the IRQ queue to allow for multiple hotunplug attempts, from telling the users to 'reboot the machine if something goes wrong'. The first solution is flawed because we can't fully predict guest behavior from QEMU, the second solution is a trial and error remediation that counts on a hope that the unplug will eventually succeed, and the third is ... well. This patch introduces a crude, but effective solution to hotunplug errors in the pSeries machine. For each unplug done, we'll timeout after some time. If a certain amount of time passes, we'll cleanup the hotunplug state from the machine. During the timeout period, any unplug operations in the same device will still be blocked. After that, we'll assume that the guest failed the operation, and allow the user to try again. If the timeout is too short we'll prevent legitimate hotunplug situations to occur, so we'll need to overestimate the regular time an unplug operation takes to succeed to account that. The true solution for the hotunplug errors in the pSeries machines is a PAPR change to allow for the guest to warn the platform about it. For now, the work done in this timeout design can be used for the new PAPR 'abort hcall' in the future, given that for both cases we'll need code to cleanup the existing unplug states of the DRCs. At this moment we're adding the basic wiring of the timer into the DRC. Next patch will use the timer to timeout failed CPU hotunplugs. Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com> Message-Id: <20210222194531.62717-4-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>`_