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d2a81ca8c6
The postcopy blocktime feature was tricky that it used quite some atomic operations over quite a few arrays and vars, without explaining how that would be thread safe. The thread safety here is about concurrency between the fault thread and the fault resolution threads, possible to access the same chunk of data. All these atomic ops can be expensive too before knowing clearly how it works. OTOH, postcopy has one page_request_mutex used to serialize the received bitmap updates. So far it's ok - we don't yet have a lot of threads contending the lock. It might change after multifd will be supported, but that's a separate story. What is important is, with that mutex, it's pretty lightweight to move all the blocktime maintenance into the mutex critical section. It's because the blocktime layer is lightweighted: almost "remember which vcpu faulted on which address", and "ok we get some fault resolved, calculate how long it takes". It's also an optional feature for now (but I have thought of changing that, maybe in the future). Let's push the blocktime layer into the mutex, so that it's always thread-safe even without any atomic ops. To achieve that, I'll need to add a tid parameter on fault path so that it'll start to pass the faulted thread ID into deeper the stack, but not too deep. When at it, add a comment for the shared fault handler (for example, vhost-user devices running with postcopy), to mention a TODO. One reason it might not be trivial is that vhost-user's userfaultfds should be opened by vhost-user process, so it's pretty hard to control making sure the TID feature will be around. It wasn't supported before, so keep it like that for now. Now we should be as ease when everything is protected by a mutex that we always take anyway. One side effect: we can finally remove one ramblock_recv_bitmap_test() in mark_postcopy_blocktime_begin(), which was pretty weird and which also includes a weird (but maybe necessary.. but maybe not?) operation to inject a blocktime entry then quickly erase it.. When we're with the mutex, and when we make sure it's invoked after checking the receive bitmap, it's not needed anymore. Instead, we assert. As another side effect, this paves way for removing all atomic ops in all the mem accesses in blocktime layer. Note that we need a stub for mark_postcopy_blocktime_begin() for Windows builds. Reviewed-by: Fabiano Rosas <farosas@suse.de> Link: https://lore.kernel.org/r/20250613141217.474825-3-peterx@redhat.com Signed-off-by: Peter Xu <peterx@redhat.com> Signed-off-by: Fabiano Rosas <farosas@suse.de> |
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| accel | ||
| audio | ||
| authz | ||
| backends | ||
| block | ||
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| dump | ||
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| fsdev | ||
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| host/include | ||
| hw | ||
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| libdecnumber | ||
| linux-headers | ||
| linux-user | ||
| migration | ||
| monitor | ||
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| blockjob.c | ||
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| configure | ||
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| LICENSE | ||
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| pythondeps.toml | ||
| qemu-bridge-helper.c | ||
| qemu-edid.c | ||
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=========== 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>`_