Scaling Formal Methods Toward Hierarchical Protocols in Shared Memory Processors Presenters: Ganesh Gopalakrishnan and Xiaofang Chen School of Computing,

Scaling Formal Methods Toward Hierarchical Protocols

in Shared Memory Processors

Presenters: Ganesh Gopalakrishnan and Xiaofang ChenSchool of Computing , University of Utah, Salt Lake City, UT 84112

{ganesh, xiachen}@cs.utah.edu

http://www.cs.utah.edu/formal_verification

GRC CADTS Review, Berkeley, March 18, 2008

Supported by SRC Contract TJ-1318 (Intel Customization)

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Multicores are the future!Their caches are visibly central…

(photo courtesy of

Intel Corporation.)

> 80% of chipsshipped will bemulti-core

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Hierarchical Cache Coherence Protocols will play a major role in multi-core processors

Chip-level protocols

Inter-cluster protocols

Intra-cluster protocols

dirmem dirmem

…

State Space grows multiplicatively across the hierarchy!

Verification will become harder

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Protocol design happens in “the thick of things” (many interfaces, constraints of performance, power, testability).

From “High-throughput coherence control and hardware messaging in Everest,” by Nanda et.al., IBM J.R&D 45(2), 2001.

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Future Coherence Protocols

Cache coherence protocols that are tuned for the contexts in which they are operating can significantly increase performance and reduce power consumption [Liqun Cheng]

Producer-consumer sharing pattern-aware protocol [Cheng et.al, HPCA07] 21% speedup and 15% reduction in network traffic

Interconnect-aware coherence protocols [Cheng et.al., ISCA06] Heterogeneous Interconnect Improve performance AND reduce power 11% speedup and 22% wire power savings

Bottom-line: Protocols are going to get more complex!

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Main Result #1 : Hierarchical

RAC

L2 Cache+Local Dir

L1 Cache

Main Mem

Home ClusterRemote Cluster 1

Remote Cluster 2

L1 Cache

Global Dir

RAC

L2 Cache+Local Dir

L1 Cache

L1 Cache

RAC

L2 Cache+Local Dir

L1 Cache

L1 Cache

Intra-cluster

Inter-cluster

Developed way to reduce verification complexity

of hierarchical (CMP) protocols using A/G

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Main Result #2 : Refinement

Developed way to Verify a Proposed Refinement of

ONE unit into its low level (RTL) implementation

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Murphi

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Murphi

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Murphi

HMurphi

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Differences in Modeling: Specs vs. Impls

home remote

One step in high-level

Multiple steps in low-level

an atomic guarded/command

home

router

buf

remote

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Our Refinement Check

Spec(I)

I

Spec(I’)Spec

transition

Multi-step Impl

transactionI’

Guard for Spec transition must

hold

I is a reachable Impl state

Observable vars changed

by either must match

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Workflow of Our Refinement Check

Hardware Murphi

Impl model

Product model in

Hardware Murphi

Product model in VHDL

MurphiSpec model

Property check

Muv

Check implementation meets specification

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Anticipated Future Result


the ENTIRE hierarchy

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Deal with pipelining

Sequential InteractionPipelined Interaction

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Develop ways to “tease apart” protocols that are “blended in”

e.g. for power-down or post-si observability enhancement

More protocols…

.. do they interfere?

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Basics

PI : Ganesh Gopalakrishnan Industrial Liaisons : Ching Tsun Chou (Intel), Steven M. Geman (IBM), John

W. O’Leary (Intel), Jayanta Bhadra (Freescale), Alper Sen (Freescale), Aseem Maheshwari (TI)

Primary Student : Xiaofang Chen Graduation Date : Writing PhD Dissertation; in the market Other Students: Yu Yang (PhD), Guodong Li (PhD), Michael DeLisi (BS/MS) Anticipated Results:

Hierarchical : Methodology for Hierarchical (Cache Coherence) Protocol Verification, with Emphasis on Complexity Reduction (was in original SRC proposal)

Refinement : Methodology for Expressing and Verifying Refinement of Higher Level Protocol Descriptions (not in original SRC proposal)

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Basics

Deliverables (Papers, Software, Xiaofang’s Dissertation) Hierarchical:

Methodology for Applying A/G Reasoning for Complexity Reduction Verified Protocol Benchmarks – Inclusive, Non-Inclusive, Snoopy

(Large Benchmarks) Automatic Abstraction Tool in support of A/G Reasoning

Refinement: Muv Language Design (for expressing Designs) Refinement Checking Theory and Methodology Complete Muv tool implementation

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What’s Going On

Accomplishments during the past year Hierarchical:

Finishing Non-inclusive Hierarchical Protocol Verif

Developing and Verifying a Hier. Protocol with a

Snoopy First Level

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Insert Table of Hier + Snoopy Here

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What’s Going On

Accomplishments during the past year (contd.) Refinement:

HMurphi was fleshed out in great detail

Most of Muv was implemented (a large portion during

IBM T.J. Watson Internship) – joint work with Steven

German and Geert Janssen

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What’s Going On

Future directions Hierarchical + Refinement

Develop ways to verify hierarchies of HMurphi modules interacting Pipelining Teasing out protocols supporting non-functional aspects

Power-down protocols Protocols to enhance Post-si Observability

Architectural Characterization How do we describe the “ISA” of future multi-core

machines? How do we make sure that this ISA has no hidden

inconsistencies

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What’s Going On

Technology Transfer & Industrial Interactions With Liaisons

Publications FMCAD 06, 07, HLDVT 07, TECHCON 07 (best

session paper award), Journal paper (under prep),

Dissertation (under prep)

Request to IBM for Open-sourcing Muv has been

placed

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Overview of “Hierarchical”

Given a protocol to verify, create a

verification model that models a small

number of clusters acting on a single

cache line Verification Model

Inv P

Home

Remote

Global directory

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2. Exploit Symmetries

Model “home” and the two “remote”s

(one remote, in case of symmetry)

Verification Model

Inv P

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4. Initial abstraction will be extreme; slowly back-off from this extreme…

Inv P1 Inv P2

Inv P3

P1 fails

Diagnose failure

Bug

report to user

False Alarm

Diagnose where guard

is overly weak

Add Strengthening Guard

Introduce Lemma to ensure

Soundness of Strengthening

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Overview of Theory Involved

rule g1 ==> a1;

rule g2 ==> a2;

invariant P;rule g1 ==> a1;

rule g2 /\ cond2 ==> a2;

invariant P /\ (g1 => cond1);

rule g1 /\ cond1 ==> a1;

rule g2 ==> a2;

invariant P /\ (g2 => cond2);

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3. Create Abstract Models (three models in this example)

Inv P

Inv P1 Inv P2

Inv P3

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Step 1 of Refinement

Inv P1 Inv P2

Inv P3

Inv P1 Inv P2

Inv P3’

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Step 2 of Refinement

Inv P1 Inv P2

Inv P3

Inv P1 Inv P2

Inv P3’

Inv P1 Inv P2’

Inv P3’

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Final Step of Refinement

Inv P1 Inv P2

Inv P3

Inv P1 Inv P2

Inv P3’

Inv P1’ Inv P2’

Inv P3’

Inv P1 Inv P2’

Inv P3’’

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Detailed Presentation of Refinement

Note: Three examples have been presented in full detail at

http://www.cs.utah.edu/formal_verification/muv

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Here, arrange the rest of the slides + the new ones you are making as you feel best. Most of the remaining slides are quite good, so your work need not include any “clean-up” but just delete those already covered…

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Project Summary: Year 2

Verification of hierarchical cache coherence protocols Non-inclusive multicore benchmark Compositional approach one level a time Can reduce >95% explicit state space

Refinement check: protocol RTL Impls vs. Specs Refinement theory and methodology Compositional approach theory

Publications FMCAD 2007, HLDVT 2007 TECHCON 2007 (best session paper award)

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Yearly Summary: 2007 - 2008

Refinement check: protocol RTL Impls vs. Specs A comprehensive tool path

Can find bugs on RTL protocols with realistic features

A simple pipelined stack example Verification of hierarchical cache coherence protocols

A snoop multicore protocol benchmark

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A Simple Snoop Multicore Protocol

Motivation: Snoop protocols commonly used in 1st level of caches

Have applied our approach on directory protocols

How about snoop protocols?

L1 Cache

L2 Cache

RAC

Global Dir

Main Mem

Cluster 1

L1 Cache L1 Cache

L2 Cache

RAC

Cluster 2

L1 Cache

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Applying Our Approach

L1 Cache

L2 Cache

Global Dir

Main Mem

Cluster 1

L1 Cache L2 Cache

RAC

Cluster 2

L2 Cache

RAC

Cluster 1

Abstracted protocols

Model checking results

Model checkpassed

Use mem (GB)

1.8

1.8

1.8

Model check time (sec)

86

6

7

# of states

552,375

474

15,371

Original model

Abs. intra

Abs. inter

Monolithicapproach

Our approach

Yes

Yes

Yes

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Cycle accurate RTL level

Refinement Check Spec vs. Impl

Specification

Abstraction level

Model size

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Differences in Execution: Specs vs. Impls

Interleaving in HL

Concurrency in LL

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Our Approach of Refinement Check

Modeling

Specification: Murphi

Implementation: Hardware Murphi

Use transactions in Impl to relate to Spec

Verification

Muv: Hardware Murphi synthesizable VHDL

Tool: IBM SixthSense and RuleBase

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What Are Transactions?

Group a multi-step execution in implementations

Spec

Impl

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Outline

Project background

Extensions to the tool path

Experimental results

Future work

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Tool Path

Initial efforts from IBM By German and Janssen Hardware Murphi language

Muv: Hardware Murphi Synthesizable VHDL

Our extensions -- enable refinement check Language extensions

Muv extensions

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Basic Features of Hardware Murphi vs Murphi

…

signal s1, s2 …

s1 <= …

chooserule rules; end; …

firstrule rules; end; …

transaction

rule-1; rule-2; …

end; …

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Language Extensions to Hardware Murphi (I)

--include spec.m

correspondence

u1[0..7] :: v1[1..8]; u1 :: v2; end;

Directives

Joint variables correspondence

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Language Extensions to Hardware Murphi (II)

transactionset p1:T1; p2:T2 do

transaction …

end;

Transactionset

rule:id guard ==> action;

ruleset p1:T1; p2:T2 do

rule:id …

end;

Rules with IDs

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Language Extensions to Hardware Murphi (III)

<< id.guard() >>;

<< id.action() >>;

<< id[v1][v2].guard() >>; …

Execute a rule by ID

var[i] <:= data;

Fine-grained assignments for write-write conflicts

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How to Annotate an Impl Model with Spec?

…

transaction

rule-1 g1 a1;

rule-2 g2 a2;

end;

…

…

rule:id g a;

…

impl.m

spec.m

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How to Annotate an Impl Model with Spec?

--include spec.m

correspondence

u1 :: v1; …

end; …

transaction

rule-1 g1 a1;

<< id.guard() >>;

<< id.action() >>;

rule-2 g2 a2;

end; …

…

rule:id g a;

…

impl.m

spec.m

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The Framework of Muv

Hardware Murphi model AST

parserAST’

pre-processor

AST’’

refinement

check analysis

VHDL modeltranslator

Constant propagation

rule:id, <<id.guard()>>

ruleset, transactionset …

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Our Extensions to Muv

Language extension support

Refinement check assertions generation Ensure exclusive write to a variable

Serializability for Spec rules

Enableness for Spec rules

Joint variables equivalence when inactive

Mostly done with static analysis

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Refinement Extensions to Muv (I)

v := d;

for i: s1..s2 do

assert (update_bits[i] = false);

end;

v := d;

for i: s1..s2 do

update_bits[i] := true;

end;

No write-write conflicts

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Refinement Extensions to Muv (II)

Serializability for specification rules

S0 S1 S0 S1

t1

t2

t3S’1 S’2

t1 t2 t3

Obtain read and write sets of variables of each rule

Analyze read-write dependency

Check for cycles

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Check for Dependency Cycles

S0 S1 S0 S1

t1

t2

t3S’1 S’2

t1 t2 t3

t3 write v2

t3 read v1

r(v1)w(v3)

t2 write v1

t2 read v2

t1

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Refinement Extensions to Muv (III)

rule:id

guard action;

bool function id_guard() {…}

void procedure id_action(…) {…}

Enableness of specification rules

<< id.guard() >>;

<< id.action() >>;

assert id_guard();

id_action();

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Refinement Extensions to Muv (IV)

Joint variables equivalence when inactive

For each joint variable v When all transactions that write to v are inactive

v must be equivalent in Impl and Spec

…

Transaction T1 …

Transaction T2 …

…

Assert

inactive(T1) & inactive(T2)

=>

v = v’;

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Outline

Project background



Future work

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A Driving Protocol Benchmark

S. German and G. Janssen, IBM Research Tech Report 2006

Buf

Buf

Buf Remote

Dir Cache

Mem

Router

Buf

Buf

Buf

Local

Home

Remote

Dir Cache

Mem

Local

Home

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More Detail of the Cache Example

Hardware Murphi model ~2500 LOC

15 transactionsets

Generated VHDL ~1800 assertions, of which ~1600 are write-write

conflicts check assertions

Took ~16min with SixthSense for all assertions

Took ~13min w/o write-write conflicts check

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Bugs Found with Refinement Check

Benchmark satisfies cache coherence already

Bugs still found Bug 1: router unit loses messages

Bug 2: home unit replies twice for one request

Bug 3: cache unit gets updated twice from 1 reply

Refinement check is an automatic way of

constructing such checks

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Model Checking Approaches

Monolithic Straightforward property check

Compositional Divide and conquer

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Compositional Refinement Check

Reduce the verification complexity

Basic Techniques Abstraction

Removing details to make verification easier

Assume guarantee A simple form of induction which introduces

assumptions and justifies them

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Experimental Results

Verification Time

1-bit 10-bit

1-day

Datapath

Configurations 2 nodes, 2 addresses, SixthSense

30 min

Monolithic approach

Compositional approach

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A Simple 2-Stage Pipelined Stack

pipelined pushes pipelined pops

overlapped pop & push

Push: push data + increase counter

Pop: decrease counter + pop data

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Outline

Project background



Future work

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Future Work

Muv-like refinement check for interaction modules RTL modules interaction via communication

protocols

Interfaces involving buffers and pipelining

Refinement of initial RTL protocols Power-down issues

Post-silicon validation support

Runtime verification support

Safe augmentation of verified protocols

Cheap re-verification

Documents

Scaling Formal Methods Toward Hierarchical Protocols in Shared Memory Processors Presenters: Ganesh Gopalakrishnan and Xiaofang Chen School of Computing,