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The description in the Arm ARM of the requirements of FEAT_BBM is admirably clear on the guarantees it provides software, but slightly more obscure on what that means for implementations. The description of the equivalent SMMU feature in the SMMU specification (IHI0070D.b section 3.21.1) is perhaps a bit more detailed and includes some example valid implementation choices. (The SMMU version of this feature is slightly tighter than the CPU version: the CPU is permitted to raise TLB Conflict aborts in some situations that the SMMU may not. This doesn't matter for QEMU because we don't want to do TLB Conflict aborts anyway.) The informal summary of FEAT_BBM is that it is about permitting an OS to switch a range of memory between "covered by a huge page" and "covered by a sequence of normal pages" without having to engage in the 'break-before-make' dance that has traditionally been necessary. The 'break-before-make' sequence is: * replace the old translation table entry with an invalid entry * execute a DSB insn * execute a broadcast TLB invalidate insn * execute a DSB insn * write the new translation table entry * execute a DSB insn The point of this is to ensure that no TLB can simultaneously contain TLB entries for the old and the new entry, which would traditionally be UNPREDICTABLE (allowing the CPU to generate a TLB Conflict fault or to use a random mishmash of values from the old and the new entry). FEAT_BBM level 2 says "for the specific case where the only thing that changed is the size of the block, the TLB is guaranteed not to do weird things even if there are multiple entries for an address", which means that software can now do: * replace old translation table entry with new entry * DSB * broadcast TLB invalidate * DSB As the SMMU spec notes, valid ways to do this include: * if there are multiple entries in the TLB for an address, choose one of them and use it, ignoring the others * if there are multiple entries in the TLB for an address, throw them all out and do a page table walk to get a new one QEMU's page table walk implementation for Arm CPUs already meets the requirements for FEAT_BBM level 2. When we cache an entry in our TCG TLB, we do so only for the specific (non-huge) page that the address is in, and there is no way for the TLB data structure to ever have more than one TLB entry for that page. (We handle huge pages only in that we track what part of the address space is covered by huge pages so that a TLB invalidate operation for an address in a huge page results in an invalidation of the whole TLB.) We ignore the Contiguous bit in page table entries, so we don't have to do anything for the parts of FEAT_BBM that deal with changis to the Contiguous bit. FEAT_BBM level 2 also requires that the nT bit in block descriptors must be ignored; since commit 39a1fd25287f5dece5 we do this. It's therefore safe for QEMU to advertise FEAT_BBM level 2 by setting ID_AA64MMFR2_EL1.BBM to 2. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> Message-id: 20220426160422.2353158-3-peter.maydell@linaro.org
=========== 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>`_