Verifying architecture
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Verifying Architecture

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Verifying architecture

  • This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation

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Verifying Architecture

Jaein Jeong

Johnathon Jamison


Introduction

Introduction

  • Processors are more vulnerable to transient errors due to small feature size.

  • Can detect transient errors with more stable processors and execute instructions again if an error occurs.

  • Overhead won't be high for errors occurring rarely.


Introduction cont

Introduction (Cont.)

  • DIVA: verifies execution each individual instruction with a second, slower.

  • Our idea: a dual-processor verification system.

  • Proof-carrying code: A proof of safety accompanies executable code.

  • Our idea: executable code is annotated with invariants.


Assumptions

Assumptions

  • We assume there are no permanent errors.

  • Thus we need not worry about invariants failing always.

  • So, processor can work correctly if it is verified by a more stable processor.


Assumptions cont

Assumptions (Cont.)

  • We assume the processor operates correctly most of the time.

  • Therefore it is reasonable to check for errors rarely.

  • The overhead is not problematic, for errors occur rarely.


System structure

System Structure

  • Implemented as two communicating processors.

  • The main processor executes instructions and sends the verifier all its registers.

  • If the verifier confirms the execution, the main continues to execute instructions.

  • Otherwise, the main processor loads the old register values and re-executes its instructions.


System structure cont

System Structure (Cont.)


Programming for simplescalar

Programming for SimpleScalar

  • Since gcc can not handle everything, we intervene at the assembly code level.

  • After changing the assembly code, we compile it to object code.

  • The message passing system calls qread and qwrite are not implemented in gcc.

  • So, we insert the syscall instruction and pass arguments by explicitly filling registers.


Programming for simplescalar cont

Programming for SimpleScalar (Cont.)

  • addiu $2,$0,258 la $4,MQO subu $5,$16,4 move $6,$0 syscallWriting a message to a queue

  • $L2: addiu $2,$0,259 la $4,MQI addu $5,$sp,16 move $6,$0 syscall bne $7,$0,$L2Reading a message from a queue.


Programming interface for c

Programming interface for C

  • Assembly language programming is error prone and unproductive.

  • We wrote a interface for C with macros and inline assembly.

  • Since syscall is not accessible in C, we generate a “jal syscall” in assembly.

  • A Perl script replaces it with “syscall”.

  • Now we can compile the assembly code without further modification.


Multiprocessor program example

Multiprocessor Program Example

long regs[32];char msg[]="\006\000\000\000cool\n";long nullmsg[]={0};char MQI[]="\003min";char MQO[]="\004mout";… qwrite(MQO,msg,0,error); do { qread(length,MQI,regs,0,error); } while(error);…


Passing invariants 1 st method

Passing Invariants (1st method)

  • The main program sends the invariant instructions as a message.

  • We enclosed the invariant instructions with .rdata and .text directives and insert the length of the message after .rdata.

  • Then the main can send the instructions as a message.

  • The verifying processor then can load its registers with it, and do a jal to the message that was sent.


Passing invariants 2 nd method

Passing Invariants (2nd method)

  • Generate a verifying program specific to the main program.

  • When running the main program, just send the the contents of registers and the invariant number.

  • The verifying processor takes the invariant number, calculates the value of the invariant, and replies.


Passing invariants cont

Passing Invariants (Cont.)

  • A bit of a problem for the first method.

  • The verifying program receives invariant instructions as data.

  • Execution of those instructions would bring up the same issues as self-modifying code.

  • Due to pitfalls of first method, we chose the second method.


Using invariants

Using Invariants

  • We maintain two sets of registers in the verifier for roll back purposes.

  • Not all registers must be sent to the verifier, just those needed for the invariant or possible rollback.

  • Currently, creating the verifier requires careful inspection of the main program

  • We hope to automate some of this.


Performance

Performance

  • For best performance, the main processor should not check for the invariant reply immediately.

  • Rather, check when the next invariant is reached, so to give time for verification.

  • Then the read is done, and execution is rolled back or continued as appropriate.


Tidbits

Tidbits

  • The message passing mechanism took time to understand.

  • We found we could use the asm directive in gcc so hand modification of assembly was minimized.

  • We encountered a couple bugs in SimpleScalar.


Future work

Future Work

  • Additional logic for floating point registers, easily extended from what we have now.

  • Memory rollback logic; this is more substantial, for we need to retire memory writes only on invariant confirmation.

  • A program to generate the verifying program automatically.


Thoughts

Thoughts

  • Seems like this is an energy intensive method of verification.

  • Invariants are not easy to generate, and must be done by hand.

  • There is a large amount of processing overhead.


Summary

Summary

  • Decreasing feature size makes verification necessary.

  • DIVA is on attempt to address the problem.

  • We wrote programs for SimpleScalar.

  • This showed that we can have one processor verify another with invariants.


Acknowledgement

Acknowledgement

  • Mark Whitney:

    • Our work is based on the SimpleScalar multiprocessing extension, written by him.

    • He also helped us configure SimpleScalar and fixed bugs.


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