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(1) The virtio-1.2 specification <http://docs.oasis-open.org/virtio/virtio/v1.2/virtio-v1.2.html> writes: > 3 General Initialization And Device Operation > 3.1 Device Initialization > 3.1.1 Driver Requirements: Device Initialization > > [...] > > 7. Perform device-specific setup, including discovery of virtqueues for > the device, optional per-bus setup, reading and possibly writing the > device’s virtio configuration space, and population of virtqueues. > > 8. Set the DRIVER_OK status bit. At this point the device is “live”. and > 4 Virtio Transport Options > 4.1 Virtio Over PCI Bus > 4.1.4 Virtio Structure PCI Capabilities > 4.1.4.3 Common configuration structure layout > 4.1.4.3.2 Driver Requirements: Common configuration structure layout > > [...] > > The driver MUST configure the other virtqueue fields before enabling the > virtqueue with queue_enable. > > [...] (The same statements are present in virtio-1.0 identically, at <http://docs.oasis-open.org/virtio/virtio/v1.0/virtio-v1.0.html>.) These together mean that the following sub-sequence of steps is valid for a virtio-1.0 guest driver: (1.1) set "queue_enable" for the needed queues as the final part of device initialization step (7), (1.2) set DRIVER_OK in step (8), (1.3) immediately start sending virtio requests to the device. (2) When vhost-user is enabled, and the VHOST_USER_F_PROTOCOL_FEATURES special virtio feature is negotiated, then virtio rings start in disabled state, according to <https://qemu-project.gitlab.io/qemu/interop/vhost-user.html#ring-states>. In this case, explicit VHOST_USER_SET_VRING_ENABLE messages are needed for enabling vrings. Therefore setting "queue_enable" from the guest (1.1) -- which is technically "buffered" on the QEMU side until the guest sets DRIVER_OK (1.2) -- is a *control plane* operation, which -- after (1.2) -- travels from the guest through QEMU to the vhost-user backend, using a unix domain socket. Whereas sending a virtio request (1.3) is a *data plane* operation, which evades QEMU -- it travels from guest to the vhost-user backend via eventfd. This means that operations ((1.1) + (1.2)) and (1.3) travel through different channels, and their relative order can be reversed, as perceived by the vhost-user backend. That's exactly what happens when OVMF's virtiofs driver (VirtioFsDxe) runs against the Rust-language virtiofsd version 1.7.2. (Which uses version 0.10.1 of the vhost-user-backend crate, and version 0.8.1 of the vhost crate.) Namely, when VirtioFsDxe binds a virtiofs device, it goes through the device initialization steps (i.e., control plane operations), and immediately sends a FUSE_INIT request too (i.e., performs a data plane operation). In the Rust-language virtiofsd, this creates a race between two components that run *concurrently*, i.e., in different threads or processes: - Control plane, handling vhost-user protocol messages: The "VhostUserSlaveReqHandlerMut::set_vring_enable" method [crates/vhost-user-backend/src/handler.rs] handles VHOST_USER_SET_VRING_ENABLE messages, and updates each vring's "enabled" flag according to the message processed. - Data plane, handling virtio / FUSE requests: The "VringEpollHandler::handle_event" method [crates/vhost-user-backend/src/event_loop.rs] handles the incoming virtio / FUSE request, consuming the virtio kick at the same time. If the vring's "enabled" flag is set, the virtio / FUSE request is processed genuinely. If the vring's "enabled" flag is clear, then the virtio / FUSE request is discarded. Note that OVMF enables the queue *first*, and sends FUSE_INIT *second*. However, if the data plane processor in virtiofsd wins the race, then it sees the FUSE_INIT *before* the control plane processor took notice of VHOST_USER_SET_VRING_ENABLE and green-lit the queue for the data plane processor. Therefore the latter drops FUSE_INIT on the floor, and goes back to waiting for further virtio / FUSE requests with epoll_wait. Meanwhile OVMF is stuck waiting for the FUSET_INIT response -- a deadlock. The deadlock is not deterministic. OVMF hangs infrequently during first boot. However, OVMF hangs almost certainly during reboots from the UEFI shell. The race can be "reliably masked" by inserting a very small delay -- a single debug message -- at the top of "VringEpollHandler::handle_event", i.e., just before the data plane processor checks the "enabled" field of the vring. That delay suffices for the control plane processor to act upon VHOST_USER_SET_VRING_ENABLE. We can deterministically prevent the race in QEMU, by blocking OVMF inside step (1.2) -- i.e., in the write to the device status register that "unleashes" queue enablement -- until VHOST_USER_SET_VRING_ENABLE actually *completes*. That way OVMF's VCPU cannot advance to the FUSE_INIT submission before virtiofsd's control plane processor takes notice of the queue being enabled. Wait for VHOST_USER_SET_VRING_ENABLE completion by: - setting the NEED_REPLY flag on VHOST_USER_SET_VRING_ENABLE, and waiting for the reply, if the VHOST_USER_PROTOCOL_F_REPLY_ACK vhost-user feature has been negotiated, or - performing a separate VHOST_USER_GET_FEATURES *exchange*, which requires a backend response regardless of VHOST_USER_PROTOCOL_F_REPLY_ACK. Cc: "Michael S. Tsirkin" <mst@redhat.com> (supporter:vhost) Cc: Eugenio Perez Martin <eperezma@redhat.com> Cc: German Maglione <gmaglione@redhat.com> Cc: Liu Jiang <gerry@linux.alibaba.com> Cc: Sergio Lopez Pascual <slp@redhat.com> Cc: Stefano Garzarella <sgarzare@redhat.com> Signed-off-by: Laszlo Ersek <lersek@redhat.com> Reviewed-by: Stefano Garzarella <sgarzare@redhat.com> Tested-by: Albert Esteve <aesteve@redhat.com> [lersek@redhat.com: work Eugenio's explanation into the commit message, about QEMU containing step (1.1) until step (1.2)] Reviewed-by: Eugenio Pérez <eperezma@redhat.com> Message-Id: <20231002203221.17241-8-lersek@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
=========== 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>`_