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ent to receive the response explicitly 22 servers edit microkernel servers are essentially daemon programs like any others except that the kernel grants some of them privileges to interact with parts of physical memory that are otherwise off limits to most programs this allows some servers particularly device drivers to interact directly with hardware a basic set of servers for a general purpose microkernel includes file system servers device driver servers networking servers display servers and user interface device servers this set of servers drawn from qnx provides roughly the set of services offered by a unix monolithic kernel the necessary servers are started at system startup and provide services such as file network and device access to ordinary application programs with such servers running in the environment of a user application server development is similar to ordinary application development rather than the build and boot process needed for kernel development additionally many crashes can be corrected by simply stopping and restarting the server which would not be feasible if the entire kernel had to reboot however part of the system state is lost with the failing server hence this approach requires applications to cope with failure a good example is a server responsible for tcp ip connections if this server is restarted applications will experience a lost connection a normal occurrence in a networked system for other services failure is less expected and may require changes to application code for qnx restart capability is offered as the qnx high availability toolkit 23 device drivers edit device drivers frequently perform direct memory access dma and therefore can write to arbitrary locations of physical memory including various kernel data structures such drivers must therefore be trusted it is a common misconception that this means that they must be part of the kernel in fact a driver is not inherently more or less trustworthy by being part of the kernel while running a device driver in user space does not necessarily reduce the damage a misbehaving driver can cause in practice it is beneficial for system stability in the presence of buggy rather than malicious drivers memory access violations by the driver code as opposed to the device may still be caught by the memory management hardware furthermore many devices are not dma capable their drivers can be made untrusted by running them in user space recently an increasing number of computers feature iommus many of which can be used to restrict a device s access to physical memory 24 this also allows user mode drivers to become untrusted user mode drivers actually predate microkernels the michigan terminal system mts in 1967 supported user space drivers including its file system support the first operating system to be designed with that capability 25 historically drivers were less of a problem as the number of devices was small and trusted anyway so having them in the kernel simplified the design and avoided potential performance problems this led to the traditional driver in the kernel style of unix 26 linux and windows nt with the proliferation of various kinds of peripherals the amount of driver code escalated and in modern operating systems dominates the kernel in code size essential components and minimality edit as a microkernel must allow building arbitrary operating system services on top it must provide some core functionality at a minimum this includes some mechanisms for dealing with address spaces required for managing memory protection some execution abstraction to manage cpu allocation typically threads or scheduler activations inter process communication required to invoke servers running in their own address spaces this minimal design was pioneered by brinch hansen s nucleus and the hypervisor of ibm s vm it has since been formalised in liedtke s minimality principle a concept is tolerated inside the microkernel only if moving it outside the kernel i e permitting competing implementations would prevent the implementation of the system s required functionality 20 everything else can be done in a user mode program although device drivers implemented as user programs may on some processor architectures require special privileges to access i o hardware related to the minimality principle and equally important for microkernel design is the separation of mechanism and policy which enables the construction of arbitrary systems on top of a minimal kernel any policy built into the kernel cannot be overwritten at user level and therefore limits the generality of the microkernel 16 policy implemented in user level servers can be changed by replacing the servers or letting the application choose between competing servers offering similar services for efficiency most microkernels contain schedulers and manage timers in violation of the minimality principle and the principle of policy mechanism separation startup booting of a microkernel based system requires device drivers which are not part of the kernel typically this means that they are packaged with the kernel in the boot image and the kernel supports a bootstrap protocol that defines how the drivers are located and started this is the traditional bootstrap procedure of l4 microkernels some microkernels such as lynxos and the original minix simplify this by placing some key drivers inside the kernel in violation of the minimality principle some even include a file system in the kernel to simplify booting a microkernel based system may boot via multiboot compatible boot loader such systems usually load statically linked servers to make an initial bootstrap or mount an os image to continue bootstrapping a key component of a microkernel is a good ipc system and virtual memory manager design that allows implementing page fault handling and swapping in user mode servers in a safe way since all services are performed by user mode programs efficient means of communication between programs are essential far more so than in monolithic kernels the design of the ipc system makes or breaks a microkernel to be effective the ipc system must not only have low overhead but also interact well with cpu scheduling performance edit on most mainstream processors obtaining a service is inherently more expensive in a microkernel based system than a monolithic system 16 in the monolithic system the service is obtained by a single system call which requires two mode switches changes of the processor s ring or cpu mode in the microkernel based system the service is obtained by sending an ipc message to a server and obtaining the result in another ipc message from the server this requires a context switch if the drivers are implemented as processes or a function call if they are implemented as procedures in addition passing actual data to the server and back may incur extra copying overhead while in a monolithic system the kernel can directly access the data in the client s buffers performance is therefore a potential issue in microkernel systems and first generation microkernels such as mach and chorusos indeed performed poorly 19 however jochen liedtke showed that mach s performance problems were the result of poor design and implementation specifically mach s excessive cache footprint 20 liedtke demonstrated with his own l4 microkernel that through careful design and implementation and especially by following the minimality principle ipc costs could be reduced by more than an order of magnitude compared to mach l4 s ipc performance is still unbeaten across a range of architectures 27 28 29 while these results demonstrate that the poor performance of systems based on first generation microkernels is not representative for second generation kernels such as l4 this constitutes no proof that microkernel based systems can be built with good performance it has been shown that a monolithic linux server ported to l4 exhibits only a few percent overhead over native linux 30 however such a single server system exhibits few if any of the advantages microkernels are supposed to provide by structuring operating system functionality into separate servers a number of commercial multi server systems exist in particular the real time systems qnx and integrity no comprehensive comparison of performance relative to monolithic systems has been published for those multiserver systems furthermore performance does not seem to be the overriding concern for those commercial systems which instead emphasize reliably quick interrupt handling response times qnx and simplicity for the sake of robustness an attempt to build a high performance multiserver operating system was the ibm sawmill linux project 31 however this project was never completed it has been shown in the meantime that user level device drivers can come close to the performance of in kernel drivers even for such high throughput high interrupt devices as gigabit ethernet 32 this seems to imply that high performance multi server systems are possible security edit the security benefits of microkernels have been frequently discussed 33 34 in the context of security the minimality principle of microkernels is some have argued a direct consequence of the principle of least privilege according to which all code should have only the privileges needed to provide required functionality minimality requires that a system s trusted computing base tcb should be kept minimal as the kernel the code that executes in the privileged mode of the hardware has unvetted access to any data and can thus violate its integrity or confidentiality the kernel is always part of the tcb minimizing it is natural in a security driven design consequently microkernel designs have been used for systems designed for high security applications including keykos eros and military systems in fact common criteria cc at the highest assurance level evaluation assurance level eal 7 has an explicit requirement that the target of evaluation be simple an acknowledgment of the practical impossibility of establishing true trustworthiness for a complex system again the term simple is misleading and ill defined at least the department of defense trusted computer system evaluation criteria introduced somewhat more precise verbiage at the b3 a1 classes the tcb shall implement complete conceptually simple protection mechanisms with precisely defined semantics significant system engineering shall be directed toward minimizing the complexity of the tcb as well as excluding from the tcb those modules that are not protection critical department of defense trusted computer system evaluation criteria in 2018 a paper presented at the asia pacific systems conference claimed that microkernels were demonstrably safer than monolithic kernels by investigating all published critical cves for the linux kernel at the time the study concluded that 40 of the issues could not occur at all in a formally verified microkernel and only 4 of the issues would remain entirely unmitigated in such a system 35 third generation edit more recent work on microkernels has been focusing on formal specifications of the kernel api and formal proofs of the api s security properties and implementation correctness the first example of this is a mathematical proof of the confinement mechanisms in eros based on a simplified model of the eros api 36 more recently in 2007 a comprehensive set of machine checked proofs was performed of the properties of the protection model of sel4 a version of l4 37 this has led to what is referred to as third generation microkernels 38 characterised by a security oriented api with resource access controlled by capabilities virtualization as a first class concern novel approaches to kernel resource management 39 and a design goal of suitability for formal analysis besides the usual goal of high performance examples are coyotos sel4 nova 40 41 redox and fiasco oc 40 42 in the case of sel4 complete formal verification of the implementation has been achieved 38 i e a mathematical proof that the kernel s implementation is consistent with its formal specification this provides a guarantee that the properties proved about the api actually hold for the real kernel a degree of assurance which goes beyond even cc eal7 it was followed by proofs of security enforcement properties of the api and a proof demonstrating that the executable binary code is a correct translation of the c implementation taking the compiler out of the tcb taken together these proofs establish an end to end proof of security properties of the kernel 43 examples edit some examples of microkernels are amigaos amoeba os blackberry qnx genode harmonyos harmonyos 5 liteos discontinued helenos horizon nintendo 3ds and nintendo switch system software the l4 microkernel family mach mac os 7 to mac os 9 mac os nanokernel minix abandoned midori discontinued phantom os qubes os redox symbian discontinued symbos threadx zircon nanokernel edit look up nanokernal in wiktionary the free dictionary the term nanokernel or picokernel historically referred to a kernel where the total amount of kernel code i e code executing in the privileged mode of the hardware is very small the term picokernel was sometimes used to further emphasize small size the term nanokernel was coined by jonathan s shapiro in the paper the keykos nanokernel architecture it was a sardonic response to mach which claimed to be a microkernel while shapiro considered it monolithic essentially unstructured and slower than the systems it sought to replace subsequent reuse of and response to the term including the picokernel coinage suggest that the point was largely missed both nanokernel and picokernel have subsequently come to mean the same thing as the term microkernel a virtualization layer underneath an operating system which is more correctly referred to as a hypervisor a hardware abstraction layer that forms the lowest level part of a kernel sometimes used to provide real time functionality to normal operating systems like adeos 44 there is also at least one case where the term nanokernel refers not to a small kernel but one that supports a nanosecond clock resolution 45 see also edit loadable kernel module microservices tanenbaum torvalds debate unikernel multi environment real time references edit herder jorrit n 23 february 2005 toward a true microkernel operating system pdf minix3 org retrieved 22 june 2015 read more retrieved 20 december 2016 per brinch hansen ieee computer society retrieved 13 september 2016 brinch hansen per 2004 a programmer s story the life of a computer pioneer retrieved 13 september 2016 brinch hansen per april 1969 rc 4000 software multiprogramming system pdf technical report regnecentralen retrieved 13 september 2016 brinch hansen per 1970 the nucleus of a multiprogramming operating system pdf communications of the acm 13 4 238 250 citeseerx 10 1 1 105 4204 doi 10 1145 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