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Handling errors in memory hotunplug in the pSeries machine is more complex than any other device type, because there are all the complications that other devices has, and more. For instance, determining a timeout for a DIMM hotunplug must consider if it's a Hash-MMU or a Radix-MMU guest, because Hash guests takes longer to hotunplug DIMMs. The size of the DIMM is also a factor, given that longer DIMMs naturally takes longer to be hotunplugged from the kernel. And there's also the guest memory usage to be considered: if there's a process that is consuming memory that would be lost by the DIMM unplug, the kernel will postpone the unplug process until the process finishes, and then initiate the regular hotunplug process. The first two considerations are manageable, but the last one is a deal breaker. There is no sane way for the pSeries machine to determine the memory load in the guest when attempting a DIMM hotunplug - and even if there was a way, the guest can start using all the RAM in the middle of the unplug process and invalidate our previous assumptions - and in result we can't even begin to calculate a timeout for the operation. This means that we can't implement a viable timeout mechanism for memory unplug in pSeries. Going back to why we would consider an unplug timeout, the reason is that we can't know if the kernel is giving up the unplug. Turns out that, sometimes, we can. Consider a failed memory hotunplug attempt where the kernel will error out with the following message: 'pseries-hotplug-mem: Memory indexed-count-remove failed, adding any removed LMBs' This happens when there is a LMB that the kernel gave up in removing, and the LMBs previously marked for removal are now being added back. This happens in the pseries kernel in [1], dlpar_memory_remove_by_ic() into dlpar_add_lmb(), and after that update_lmb_associativity_index(). In this function, the kernel is configuring the LMB DRC connector again. Note that this is a valid usage in LOPAR, as stated in section "ibm,configure-connector RTAS Call": 'A subsequent sequence of calls to ibm,configure-connector with the same entry from the “ibm,drc-indexes” or “ibm,drc-info” property will restart the configuration of devices which were not completely configured.' We can use this kernel behavior in our favor. If a DRC connector reconfiguration for a LMB that we marked as unplug pending happens, this indicates that the kernel changed its mind about the unplug and is reasserting that it will keep using all the LMBs of the DIMM. In this case, it's safe to assume that the whole DIMM device unplug was cancelled. This patch hops into rtas_ibm_configure_connector() and, in the scenario described above, clear the unplug state for the DIMM device. This will not solve all the problems we still have with memory unplug, but it will cover this case where the kernel reconfigures LMBs after a failed unplug. We are a bit more resilient, without using an unreliable timeout, and we didn't make the remaining error cases any worse. [1] arch/powerpc/platforms/pseries/hotplug-memory.c Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com> Message-Id: <20210222194531.62717-6-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>`_