Exokernel (Aegis + ExOS).md

Exokernel (Aegis + ExOS).md

aliases	tags	gists
		id url createdAt updatedAt filename isPublic d49e215ce6a31f0dd2a7057f25b9e833 https://gist.github.com/d49e215ce6a31f0dd2a7057f25b9e833 2022-10-02T03:32:35Z 2022-10-02T03:58:51Z Exokernel (Aegis + ExOS).md false

Exokernel (Aegis + ExOS)

Motivation — Interface between applications and hardware resources to allow applications with specific needs to use resources effectively by user-level applications.

Objective: Separate Protection of Resources From Management

• Exokernel ensures protection of resources
• “library operating systems” manage them

End-to-end Design

• E2E like networking
• Exokernel is simple; Ensure safe multiplexing of resources
• Most complex functionality is found in the library OS

Exokernel Design

• Secure bindings
  • fine-grained access to all hardware;
  • manage authorizations to use resource, not control;
  • use a Software TLB to cache secure bindings
• Visible revocation
  • library OS is notified (and takes part) in resource revocation; slower, done even for CPU time; uses exported physical names to speed up process and avoid ambiguity
• Abort protocol
  • revoke resource, use a “repossession vector” to notify library OS of lost resources (small number of resources is protected from revocation)

Downloadable Code

• VCODE:
  • create executable code at runtime,
  • run inside the Exokernel without requiring a context switch
• Fast Networking:
  • Dynamic Packet Filter (DPF) – packets can begin to be processed in the same buffer where they are received
• Application Specific Handlers:
  • untrusted code checked at time of download; high-speed messaging possible in Exokernel, allowing

Pros and Cons

• Advantages

• Different library OS’s can coexist easily
• IPC primitives coexist in the same library OS; very fast communication between processes since no trip to the kernel code is necessary
• Benchmarks comparing Aegis/ExOS to UNIX usually favor the former by considerable margins

• Weaknesses of This Solution

• Both the Exokernel and the Library OS are architecture dependent! Portability of applications is no longer straightforward
• Even within the same architecture, changes to the hardware require rewrite of Exokernel and Library OS to take advantage of new features or just to guarantee basic compatibility