Does the 'softmmu' mean that the virtual machine has a single linear address space available to machine and user mode? Or does it have some virtual memory capabilities that are implemented via software and not the underlying processor? Or maybe it means something different entirely?
-softmmu as a suffix in QEMU target names means "complete system emulation including an emulated MMU, for running entire guest OSes or bare metal programs". It is opposed to QEMU's -linux-user mode, which means "emulates a single Linux binary only, translating syscalls it makes into syscalls on the host". Building the foo-softmmu target will give you a qemu-system-foo executable; building foo-linux-user will give you a qemu-foo executable.
So a CPU emulated by -softmmu should provide all the facilities that the real guest CPU's hardware MMU provides, which usually means multiple address spaces which can be configured via the guest code setting up page tables and enabling the MMU.
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As far as I understand, ...
virtualization, although commonly used to refer to server virtualization, refers to creating virtual versions of any IT component, such as networking and storage
although containerization is commonly contrasted to virtualization, it is technically a form of server virtualization that takes place on the OS level
although virtual machines (VMs) commonly refer to the output of hardware-level server virtualization (system VMs), they can also refer to the output of application virtualization (process VMs), such as JVM
Bearing the above in mind, I am trying to wrap my head around the difference between containers and process VMs (NOT system VMs). In other words, what is the difference between OS-level server virtualization and application virtualization?
Don't both technically refer to one and the same thing: a platform-independent software execution environment that is created using software that abstract the environment’s underlying OS?
Although some say that the isolation achieved by container is a key difference, it is also stated that a system VM "is limited to the resources and abstractions provided by the virtual machine"
I have created a graphic representation for you, it is easier (for me) to explain the differences like this, I hope it helps.
OS-level virtualization aims to run unmodified application for a particular OS. Application can communicate with external world only through OS API, therefore a virtualization component put on that API allows to present different image of external world (e.g. amount of memory, network configuration, process list) to applications running in different virtualization context (container). Generally application runs on "real" CPU (if not already virtualized) and does not need (and sometimes have) to know that environment presented by OS is somehow filtered. It is not platform-independent software execution environment.
On the other hand, application VM aims to run applications that are prepared specially for that VM. For example, a Java VM interpretes a bytecode compiled for a "processor" which has little common with a real CPU. There are CPUs which can run some Java byte code natively, but the general concept is to provide a bytecode effective for software interpretation on different "real" OS platforms. For it to work, JVM has to provide some so called native code to interface with OS API calls it is run on. You can run your program on Sparc, ARM, Intel etc. provided that you have OS-specific intepreter application and your bytecode is conformant to specification.
I am searching real difference between firmware and embedded software.
On the internet it is written for firmware is firmware is a type of embedded software but not vice versa. In addition to that a classic BIOS example it is very old.
They both run in non-volatile memory. One difference is Embedded software like an application programming that has an rtos and file system and can be run on RAM.
If i dont use rtos and RAM and only uses flash memory it means my embedded software is a firmware, it is true?
What actually makes real difference its memory layout.
The answers on the internet are lack of technical explanations and not satisfied.
Thank you very much.
They are not distinctly separate things, or even well defined. Firmware is a subset of software; the term typically implies that it is in read-only memory:
Software refers to any machine executable code - including "firmware".
Firmware refers to software in read-only memory
Read-only memory in this context includes re-writable memory such as flash or EPROM that requires a specific erase/write operation and is not simply random-access writable.
The distinction between RAM and ROM execution is not really a distinction between firmware and software. Many embedded systems load executable code from ROM and execute from RAM for performance reasons, while others execute directly from ROM. Rather if the end-user cannot easily modify or replace the software without special tools or a bootloader, then it might be regarded as "firm". If on the other hand a normal end-user can modify, update or replace the software using facilities on the system itself (by copying a file from removable media or network for example), then it is not firmware. Consider the difference in operation for example in updating your PC's BIOS and updating Microsoft Office - the former requires a special procedure distinct from normal operating system services for loading and running software.
For example, the operating system, bootloader and BIOS of a smart phone might be considered firmware. The apps a user loads from an app-store are certainly not firmware.
In other contexts "firmware" might refer to the configuration of a programmable logic device such as an FPGA as opposed to sequentially executed processor instructions. But that is rather a niche distinction, but useful in systems employing both programmable logic and software execution.
Ultimately you would use the term "firmware" to imply some level of "permanence" of software in a system, but there is a spectrum, so you would use the term in whatever manner is useful in the context of your particular system. For example, I am working on a system where all the code runs from flash, so only ever use the term software to refer to it because there is no need to distinguish it from any other kind of software in the system.
As per wikipedia, google compute engine uses KVM as hypervisor. I can see mention about vcpu while creating an instance.
Why KVM? Why not VMware OR Xen?
I mean what is the specific reason to choose KVM as a Hypervisor choice?
PS:
Even Xen is a Open source product.
There were a number of factors in the decision, you might not be surprised to learn. :-)
One important factor was compatibility between KVM and existing isolation/scaling processes at Google. (cgroups aka "containers") This allows Google to reuse the same mechanisms that it uses to ensure performance of applications like websearch and gmail to provide consistent performance between VMs scheduled on the machine. This helps GCE avoid noisy neighbor problems.
As you're probably aware, Google has had a long history of Linux kernel development; using KVM allows Google to leverage that talent for GCE. In addition, the hypervisor/hardware emulation split in KVM (where the hypervisor implemented by KVM only emulates a few low-level devices/features, and defers the remaining emulation the the process that opens /dev/kvm) allows for development of virtual devices that have access to the full range of user-space software, including infrastructure like Colossus and BigTable where needed.
Xen, VMware, and HyperV are also great hypervisors and machine emulators, but hopefully that gives you a glimpse into some of the reasons that KVM was a good fit for Google.
Let's say I have a piece of code that runs fine on an OS. Now, if I install that OS on a virtual machine (server virtualization), and run that code on that, is it possible that the code behaves differently?
If so, what are the prerequisites for that? For example, does it have to be compiled machine code (in other words, are interpreted languages safe?)? Does it have to be certain OS instructions? Specific virtualization technology (Xen, KVM, VMware..)?
Also, what are the possible different behaviors?
Yes. Like any machine, the virtual machine is just another computer (implemented in software instead of hardware).
For one, lots of commercial apps will blow up when you run them on a VM due to:
copy protection detecting the VM
copy protection rigging your hardware, using undocumented features of BIOS/Kernel/hardware
Secondly, a VM is just another computer consisting of hardware implemented in assembly instead of circuits/dye/microcode/magic. This means the VM must provide the emulated hardware either through pass-through or emulation. The fact that hardware is very diverse can cause all kinds of different behavior. Also note the possible lack of drivers for or acceleration of the emulated hardware.
But of course a typical business application for example isn't nearly as likely to rely on any hardware details as all it does is call some GUI API.
Interpreted languages are only safe from this to the extent that they are "interpreted", if the interpreted language calls out to some native code, all this is possible again.
For an example of something detecting that it's running under a VM, check this, it's just one of the literally thousands of ways to detect the VM.
In theory the program should run exactly the same as on a physical machine.
In practice however, there may be differences due to
Machine\OS configuration and drivers
Load of the virtual machine host.
Differences in machine configuration are similar to difference you would see between any difference physical machine. Depending on how critical you application is to the end user, you should run the same set of tests that you would a physical box to determine whether the environment is acceptable for use.
Depending on the virtualisation technology, the host may not have the ability to guarantee the client resources at specific times. This can lead to weird behavior on the client. Potentially you would see more occurrences of application errors due to IO timeouts an starvation of memory.
To successfully virtualise an application for production use you need to do a bit of work to understand the resource profile of the application\client and virtual host.
Do they exist on linux platforms?
Rings are x86 processor architecture terminology, in which the processor can execute in one of four different operating modes called "priority levels, numbered zero to three. Priority level zero is allowed to perform any operation on the CPU, while priority level three is the most restricted - there are some instructions that cannot be executed at priority level three. Ref.
DLL injection is not specific to any operating system.
Well, DLL injection is not a Windows-specific concept, Linux can do it too, and it might be slightly simpler. (See http://en.wikipedia.org/wiki/DLL_injection). Also, IIRC the three "rings" are an x86 specific concept (not OS dependent). So to answer your question, no, none of these things is Windows specific.
The ring concept is a very general one, as the wikipedia entry explains. Re Linux specifically, it says:
Linux and Windows are two operating
systems that use supervisor/user-mode.
To perform specialized functions,
user-mode code must perform a system
call into supervisor mode or even to
the kernel space where trusted code of
the operating system will perform the
needed task and return it back to user
space.
Other operating systems (as, again, the article mentions, pointing to other articles for more details) can use different security architectures (esp. capability-based ones).