APPLICATION PERFORMANCE AND FLEXIBILITY ON

EXOKERNEL SYSTEMS

M. F. Kaashoek, D. R. Engler, G. R. GangerH. M. Briceño, R. Hunt, D. Mazières, T. Pinckney, R. Grimm, J. Jannotti

and K. Mackenzie

M.I.T. Laboratory for Computer Science

Introduction

• In most operating systems only privileged servers and the kernel can manage system resources– User interface must anticipate all application

needs• The exokernel architecture delegates resource

management to user applications.• Applications that do not want this responsibility

communicate with the exokernel through libOSes.

Conventional OS user interface

User process

Kernelprotects and manages

all the system resources

User process

System calls

Exokernels (I)

Exokernelprotects but

does not managesystem resources

User process User process

Exokernels (II)

• User processes manage their own resources– Take over tasks previously done by the kernel

(file system buffering, virtual memory, …)• Kernel still responsible for protection• Requires a more complex and more powerful

user-kernel interface

Advantages and Disadvantages

• User applications can often manage their resources better than the kernel– They know—or should know—better how they

will use each individual resource• Most user applications would still prefer to let the

kernel handle resource allocation for them– Do you want to do your own paging in all your

programs?

LibOSes (I)

• User-level library of functions emulating conventional system call interface– Manages resources for applications that do

not want to do it themselves• Can have different libOSes coexisting on the top

of same exokernel– Allows system to emulate behaviors of

several conventional operating systems

LibOSes (II)

Exokernel

User process

libOS

User process

libOS

LibOSes (III)

• Same interface between application and libOS as between application and a conventional kernel

• libOS runs as part of application– Cannot be trusted by the kernel or other user

processes

Previous Work

• Other techniques to provide extensible systems or to give applications more control over their resources include:– Some newer microkernels (SPACE)– Virtual machines – Allowing applications to download code into

the kernel (SPIN, Vino)– User-level networking – Application-controlled virtual memory.

Five Principles

Separate protection and management. Letting applications allocate resources explicitly. Using physical names whenever possible. Expose revocation: let applications choose

which instance of a resource to give up. Expose all kernel information.

Three Design Techniques

• Xok performs access control on all resources in a uniform manner.

• Software abstractions bind hardware resources together, like, disk blocks and the memory pages caching them

• Some Xok abstractions let applications download code into the kernel to achieve a finer grain of protection:– For validating file update times in a file system

Protected Sharing (I)

• Four mechanisms:– software regions: areas of memory that can only

be accessed through system calls– on-the-fly_creation of hierarchically named

capabilities– wake-up predicates: wake up processes when

arbitrary conditions become true– robust critical sections: implemented by disabling

software interrupts

Protected Sharing (II)

• Three level of trust:– mutual trust (common case)– unidirectional trust– mutual distrust (very infrequent)

• Several library files systems can safely share the same disk

An Example: The File System

• Most file system functions are left to untrusted library file systems (libFSes)– Will share access to the stable storage (disk)– Can define new file types with arbitrary

metadata formats• Problem is to give maximum of flexibility to these

libFses while protecting files from unauthorized accesses

Four requirements

• Creating new file formats should not require any special privilege

• libFSes should be able to safely share blocks at the raw disk block level

• Storage system should be efficient• Storage system should facilitate cache sharing

among distinct libFses

The Solution: XN (I)

• Provides access to stable storage at the level of disk blocks

• Exports a buffer cache registry (contains only metadata)

• Main problem is to decide when to allow or disallow access to a specific block– Difficult problem because each libFS may use

different metadata

The Solution: XN (II)

• XN uses UDF(untrusted deterministic functions) – Specific to a user-defined metadata type– own-udfT(m) returns set of blocks to which

instance m of metadata type T point to– Stored in templates (one per file type)– Cannot be changed after they are specified

XN Security Issues

• XN uses secure bindings:– Access checks are done once at bind time

not at each access time• Individual disk blocks are protected through

UDFs and libFS’s own metadata– Keeps exokernel simple

XN Consistency Issues

• XN has an in-kernel system-wide cache registry – Maps cached disk blocks to the physical pages

holding them– Guarantees that same block cannot be cached

in two different physical pages by two different libFSes

• XN also ensures safe ordering of disk updates(more about it later)

Xok• Runs on Inter x86-based computers

– Since TLB misses are handled by hardware, cannot change the page table structure.

• Xok exposes– The capabilities of the hardware (like all MMU

protection bits)– Many kernel data structures (like free lists and inverse

page mappings)• Paging is handled by applications.

ExOS 1.0

• libOS that supports most 4.4 BSD abstractions• Runs many unmodified UNIX applications:

– Most shells and utilities, many networking applications.

– see page 10

Some Lessons

• It is a good idea to expose kernel data structures– Leads to much better performance

• Libraries are simpler than kernels• Exokernel interface design is not simple• Self-paging is difficult to implement, especially

in libOSes• Downloading is powerful