QEMU TCG plugins provide a way for users to run experiments taking advantage of the total system control emulation can have over a guest. It provides a mechanism for plugins to subscribe to events during translation and execution and optionally callback into the plugin during these events. TCG plugins are unable to change the system state only monitor it passively. However they can do this down to an individual instruction granularity including potentially subscribing to all load and store operations.
However the project reserves the right to change or break the API should it need to do so. All plugins need to declare a symbol which exports the plugin API version they were built against.
This can be done simply by:. While there are conceptions such as translation time and translation blocks the details are opaque to plugins. Each callback provides an opaque anonymous information handle which can usually be further queried to find out information about a translation, instruction or operation. The handles themselves are only valid during the lifetime of the callback so it is important that any information that is needed is extracted during the callback and saved by the plugin.
Arguments are plugin specific and can be used to modify their behaviour. In this case the howvec plugin is being asked to use inline ops to count and break down the hint instructions by type. The plugin will then register callbacks for various plugin events. When a registered event occurs the plugin callback is invoked. The callbacks may provide additional information. In the case of a translation event the plugin has an option to enumerate the instructions in a block of instructions and optionally register callbacks to some or all instructions when they are executed.
There is also a facility to add an inline event where code to increment a counter can be directly inlined with the translation. Currently only a simple increment is supported. This is not atomic so can miss counts. If you want absolute precision you should use a callback which can then ensure atomicity itself.
For this we acquire a lock when called from plugin code. We also keep the list of callbacks under RCU so that we do not have to hold the lock when calling the callbacks. This is also for performance, since some callbacks e.
But this is very infrequent; we want performance when calling or not calling callbacks, not when registering them. Using RCU is great for this.
We support the uninstallation of a plugin at any time e. This allows plugins to remove themselves if they no longer want to instrument the code. This operation is asynchronous which means callbacks may still occur after the uninstall operation is requested. Finally when QEMU exits all the registered atexit callbacks are invoked.
Use a recursive lock, since we can get registration calls from callbacks.Not really.
The best source of internal documentation are the various KVM Forum talks and potentially talks at other conferences. These can be a bit out dated but often are good overviews of various subsystems.
First, make sure that you followed SubmitAPatch especially ensuring that you've CC'd the appropriate maintainer. Are you submitting a patch during the SoftFreeze or HardFreeze? If so, the maintainers may be busy preparing the next release. You may need to wait until the next development window opens up and resubmit your patch.
Even during the development windows, most QEMU maintainers are very busy. It's normal for a patch to not get a response for a week or possibly two. If your patch hasn't gotten a response after two weeks, you can reply to the patch with a simple "ping" message. This will raise the thread to the top of most people's inboxes and give your patch another chance to be reviewed. You can repeat this process two or three times.
It's extremely unusual for a patch to not get feedback after being pinged a few times. In order for your patch to be merged, it must either 1 receive a Reviewed-by by a trusted reviewer on qemu-devel or 2 be reviewed by a maintainer and accepted into their tree.
If you received feedback, you should correct the issues pointed out and resubmit the patch indicating that this is a new version of the patch by saying v2 in the subject. QEMU is a community of developers that sometimes have conflicting opinions. Figuring out the right way to resolve conflicting feedback is a skill developed over time and requires understanding whose feedback is the most relevant for a given subsystem.
When submitting the patch, it's sufficient to CC qemu-stable to queue the patch for consideration in a stable release.
Ultimately, the stable maintainer will decide which patches to backport to stable releases. If you do plan to contribute a feature to QEMU, you should make that clear up front.
The best way to encourage people to review your patches is by reviewing other people's patches. Reviewing other people's patches helps in two ways.Introduction 1. Full system emulation. It can be used to launch an different Operating System without rebooting the PC or to debug system code. The virtual x86 CPU supports 16 bit and 32 bit addressing with segmentation.
QEMU Internals 2. But QEMU is much faster than bochs as it uses dynamic compilation. Valgrind is mainly a memory debugger while QEMU has no support for it QEMU could be used to detect out of bound memory accesses as Valgrind, but it has no support to track uninitialised data as Valgrind does. The Valgrind dynamic translator generates better code than QEMU in particular it does register allocation but it is closely tied to an x86 host and target and has no support for precise exceptions and system emulation.
EM86 was limited to an alpha host and used a proprietary and slow interpreter the interpreter part of the FX! It is less accurate than Wine but includes a protected mode x86 interpreter to launch x86 Windows executables. Such an approach has greater potential because most of the Windows API is executed natively but it is far more difficult to develop because all the data structures and function parameters exchanged between the API and the x86 code must be converted.
User mode Linux  was the only solution before QEMU to launch a Linux kernel as a process while not needing any host kernel patches.
The price to pay is that QEMU is slower. The new Plex86  PC virtualizer is done in the same spirit as the qemu-fast system emulator. It requires a patched Linux kernel to work you cannot launch the same kernel on your PCbut the patches are really small. As it is a PC virtualizer no emulation is done except for some priveledged instructionsit has the potential of being faster than QEMU.
The downside is that a complicated and potentially unsafe host kernel patch is needed. Moreover, they are unable to provide cycle exact simulation as an emulator can. When it first encounters a piece of code, it converts it to the host instruction set. Usually dynamic translators are very complicated and highly CPU dependent.
QEMU uses some tricks which make it relatively easily portable and simple while achieving good performances.
The basic idea is to split every x86 instruction into fewer simpler instructions. In essence, the process is similar to but more work is done at compile time. A key idea to get optimal performances is that constant parameters can be passed to the simple operations. For that purpose, dummy ELF relocations are generated with gcc for each constant parameter. When it can be proved that the condition codes are not needed by the next instructions, no condition codes are computed at all.
In order to achieve a good speed, the translation phase considers that some state information of the virtual x86 CPU cannot change in it. For example, if the SS, DS and ES segments have a zero base, then the translator does not even generate an addition for the segment base. For simplicity, it is completely flushed when it is full.Currently libvirt supports 2 kind of virtualization, and its internal structure is based on a driver model which simplifies adding new engines:.
When running in a Xen environment, programs using libvirt have to execute in "Domain 0", which is the primary Linux OS loaded on the machine. That OS kernel provides most if not all of the actual drivers used by the set of domains. It also runs the Xen Store, a database of information shared by the hypervisor, the backend drivers, any running domains, and libxl aka libxenlight. The hypervisor, drivers, kernels and daemons communicate though a shared system bus implemented in the hypervisor.
The figure below tries to provide a view of this environment:. Libvirt tries to expose all the emulations models of QEMU, the selection is done when creating the new domain, by specifying the architecture and machine type targeted. As the previous section explains, libvirt can communicate using different channels with the current hypervisor, and should also be able to use different kind of hypervisor.
To simplify the internal design, code, ease maintenance and simplify the support of other virtualization engine the internals have been structured as one core component, the libvirt. That way the Xen Daemon access, the Xen Store one, the Hypervisor hypercall are all isolated in separate C modules implementing at least a subset of the common operations defined by the drivers present in driver.
Note that a given driver may only implement a subset of those functions, for example saving a Xen domain state to disk and restoring it is only possible though the Xen Daemonin that case the driver entry points for unsupported functions are initialized to NULL. The figure below tries to provide a view of this environment: The library will interact with libxl for all management operations on a Xen system. Note that the libvirt libxl driver only supports root access. Download Contribute Docs.
Website Wiki Developers list Users list. Contact email irc. Community twitter stackoverflow serverfault. Contribute edit this page. Participants in the libvirt project agree to abide by the project code of conduct.Memory is one of the key aspects of emulating computer systems.
After reading this post you will know enough to dig into the QEMU source code yourself. Note that guest virtual memory is not covered here since it deserves its own post.
The reason for the maximum size and slots is that QEMU emulates DIMM hotplug so the guest operating system can detect when new memory is added and removed using the same mechanism as on real hardware. This involves plugging or unplugging a DIMM into a slot, just like on a physical machine.
In other words, changing the amount of memory available isn't done in byte units, it's done by changing the set of DIMMs plugged into the emulated machine. Memory is hotplugged by creating a new "pc-dimm" device. Although the name includes "pc" this device is also used with ppc and s machine types. As a side-note, the initial RAM that the guest started with might not be modelled with a "pc-dimm" device and it can't be unplugged.
The guest RAM itself isn't contained inside the "pc-dimm" object. Instead the "pc-dimm" must be associated with a "memory-backend" object. This can either be anonymous mmapped memory or file-backed mmapped memory. File-backed guest RAM allows Linux hugetlbfs usage for huge pages on the host and also shared-memory so other host applications can access to guest RAM.
This is just the tip of the iceberg though because there are still several aspects of guest RAM internal to QEMU that will be covered next.
When the guest CPU or device DMA stores to guest RAM this needs to be noticed by several users: The live migration feature relies on tracking dirty memory pages so they can be resent if they change during live migration. TCG relies on tracking self-modifying code so it can recompile changed instructions. Graphics card emulation relies on tracking dirty video memory to redraw only scanlines that have changed.
This is how hardware register accesses from a guest CPU are dispatched to emulated devices. There are a few different layers involved in managing guest physical memory.
The "pc-dimm" and "memory-backend" objects are the user-visible configuration objects for DIMMs and memory.QEMU is a dynamic translator.
When it first encounters a piece of code, it converts it to the host instruction set. Usually dynamic translators are very complicated and highly CPU dependent.
QEMU uses some tricks which make it relatively easily portable and simple while achieving good performances. After each translated basic block is executed, QEMU uses the simulated Program Counter PC and other cpu state information such as the CS segment base value to find the next basic block.
In order to accelerate the most common cases where the new simulated PC is known, QEMU can patch a basic block so that it jumps directly to the next one. The most portable code uses an indirect jump. An indirect jump makes it easier to make the jump target modification atomic. Self-modifying code is a special challenge in x86 emulation because no instruction cache invalidation is signaled by the application when code is modified.
User-mode emulation marks a host page as write-protected if it is not already read-only every time translated code is generated for a basic block. QEMU then invalidates all the translated code in the page and enables write accesses to the page.
For system emulation, write protection is achieved through the software MMU. Correct translated code invalidation is done efficiently by maintaining a linked list of every translated block contained in a given page. Other linked lists are also maintained to undo direct block chaining.
On RISC targets, correctly written software uses memory barriers and cache flushes, so some of the protection above would not be necessary. However, QEMU still requires that the generated code always matches the target instructions in memory in order to handle exceptions correctly.
QEMU keeps a map from host program counter to target program counter, and looks up where the exception happened based on the host program counter at the exception point.
This state is stored for each target instruction, and looked up on exceptions. In that mode, the MMU virtual to physical address translation is done at every memory access.
This means that each basic block is indexed with its physical address. In order to avoid invalidating the basic block chain when MMU mappings change, chaining is only performed when the destination of the jump shares a page with the basic block that is performing the jump.
Access is faster for RAM and ROM because the translation cache also hosts the offset between guest address and host memory. Finally, the MMU helps tracking dirty pages and pages pointed to by translation blocks. QEMU 2. In order to achieve a good speed, the translation phase considers that some state information of the virtual CPU cannot change in it. The state is recorded in the Translation Block TB.The Qemu/KVM ecosystem: Navigating the code! - Bandan Das - FOSSASIA Summit 2017
If the state changes e. The same idea can be applied to other aspects of the CPU state. For example, on x86, if the SS, DS and ES segments have a zero base, then the translator does not even generate an addition for the segment base.ICMP Ping is not allowed. Also connections from host to guest are not allowed unless using port forwarding.
Create the user mode network backend having id mynet0. Redirect incoming tcp connections on host port to guest port Create a NIC model e and connect to mynet0 backend created by the previous parameter.
TAP network overcomes all of the limitations of user mode networking, but requires a tap to be setup before running qemu. Also qemu must be run with root privileges. Create a tap network backend with id mynet0. This will connect to a tap interface tap0 which must be already setup.
Do not use any network configuration scripts.
Also specify a mac address for the NIC. Hey there, I'm having issues implementing a connection type to my virtual machine. I'm not very familiar with Qemu I usually use vmware. What are the equivalent to NAT, bridged and host-only on qemu? How do you implemented them? Skip to content. Instantly share code, notes, and snippets. Code Revisions 4 Stars 51 Forks Embed What would you like to do? Embed Embed this gist in your website. Share Copy sharable link for this gist. Learn more about clone URLs.
Download ZIP. Setting up Qemu with a tap interface. Setting up Qemu with a tap interface There are two parts to networking within QEMU: The virtual network device that is provided to the guest e. The network backend that interacts with the emulated NIC e. QEMU emulator version 2. Linux kernel 3. It is a nearly-universal standard RFC Note - this implementation uses static pre-configured tunnels same as the Linux kernel.
Note that the 'id' property must be set. During emulation, the following keys are useful: ctrl-alt-f toggle full screen ctrl-alt-n switch to virtual console 'n' ctrl-alt toggle mouse and keyboard grab When using -nographic, press 'ctrl-a h' to get some help.
This comment has been minimized. Sign in to view. Copy link Quote reply. Chris You can see the virt-manager libvirt tool which usage is likes the vmware. Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment. You signed in with another tab or window.