Kernel Varieties

Agenda

• Kernel Definition
• Monolithic Kernels
• Microkernels
• Nano Kernels
• Isolation Kernels
• Separation Kernels
• Unikernels
• Exokernels
• Rump Kernels
• References

Kernel Definition

Generically: Resource manager

More broadly: The thing that sits between applications and hardware that allows limited resources to be shared in a reasonable and reasonably consistent mannter. Specifically, the shared portion of code loaded into all applications that allows that resource sharing to happen.

Applications
Kernel
CPU	Memory	Devices

Monolithic Kernels

Applications
Kernel Scheduler Memory Subsystem VFS Networking Device Drivers
CPU	Memory	Devices

• All resource sharing in kernel space
• Examples:
  • Linux
  • NT
• Pros:
  • Simple
  • Performant
• Cons:
  • Huge
  • Attack Surface

Microkernels

Applications
Kernel VFS Networking Device Drivers Thread Management Memory Subsystem IPC
CPU	Memory	Devices

• Reduce the amount of code in kernel space
• Examples:
  • Mach - used by Apple *OS family and GNU HURD
  • L4
  • Minix
• Pros:
  • Small
  • Security
• Cons:
  • Complexity
  • Performance

Nano Kernels

Applications
Kernel Thread Management Memory Subsystem VFS Networking Device Drivers IPC
CPU	Memory	Devices

• Microkernel taken to the extreme
• Examples:
  • KeyKOS
• Pros:
  • Very Small
  • Security
• Cons
  • Complexity
  • Performance

Isolation Kernels

Application	Application	Application
Guest OS	Guest OS	Guest OS
Kernel Scheduler Memory Subsystem VFS Networking Device Drivers
CPU	Memory	Devices

• Separate VM per process
• Examples:
  • Denali
• Pros:
  • Security
• Cons:
  • IPC

Separation Kernels

Applications	Application	Application
Kernel
CPU	Memory	Devices

• Makes guarantees about flow of data
• Write up/read down
• Examples:
  • HiStar
  • seL4
• Pros:
  • Enables multi-domain systems
• Cons:
  • Complexity

Unikernels

Kernel and Applications Scheduler Memory Subsystem VFS Networking Device Drivers
CPU	Memory	Devices

• Everything in one address space, all kernel mode
• Examples:
  • MirageOS
  • IncludeOS
• Pros:
  • Performance
• Cons:
  • Utter lack of security

Exokernels

Applications
Libary OS	Library OS	Library OS
Kernel		Direct Hardware Access
CPU	Memory	Devices

• Applications rely on Library OSes
• Library OS may have direct hardware access
• Examples:
  • MIT Exokernel Operating System
  • Nemesis
  • Library OS (used by Microsoft SQL Server for Linux)
• Pros:
  • Enables a wide variety of OS abstractions
• Cons:
  • Security concerns of providing direct hardware access

Rump Kernels

Applications
Rump Kernel Scheduler Memory Subsystem VFS Networking Device Drivers
Kernel
CPU	Memory	Devices

• Run one kernel atop another, similar to virtualization
• Examples:
  • NetBSD Rump Kernel
• Pros:
  • Easy and fast to port subsystems and drivers to new systems
• Cons:
  • Fairly heavyweight layer

References

Examples are linked throughout. Access these slides at https://jakob.kaivo.net/talks/2020/10/30/kernels/kernels.html.