Specifying circuit properties in PSL Formal methods Mathematical - PDF document

Specifying circuit properties in PSL Formal methods Mathematical and logical methods used in system development Aim to increase confidence in riktighet of system Apply to both hardware and software 1

Formal methods Complement other analysis methods Are good at finding bugs Reduce development (and test) time (Verification time is often 70% of total time in hardware design projects) Some fundamental facts Low level of abstraction, Finite state systems => automatic proofs possible High level of abstraction, Fancy data types, general programs => automatic proofs IMPOSSIBLE 2

Two main approaches • Squeeze the problem down into one that can be handled automatically – industrial success of model checkers – automatic proof-based methods very hot • Use powerful interactive theorem provers and highly trained staff – for example Harrison’s work at Intel on floating point algorithms (http://www.cl.cam.ac.uk/users/jrh/) Model Checking G(p -> F q) yes property MC algorithm no p p q q counterexample finite-state model (Ken McMillan) 3

Again two main approaches • Linear-time Temporal Logic (LTL) – must properties, safety and liveness – Pnueli, 1977 • Computation Tree Logic (CTL) – branching time, may properties, safety and liveness – Clarke and Emerson, Queille and Sifakis, 1981 Linear time conceptually simplier (words vs trees) Branching time computationally more efficient We will return to this in a later lecture But temporal logics hard to read and write! 4

Computation Tree Logic A sequence beginning with the assertion of signal strt, and containing two not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing two assertions of signal put before signal end can be asserted AG~(strt & EX E[~get & ~kill U get & ~kill & EX E[~get & ~kill U get & ~kill & E[~put U end] | E[~put & ~end U (put & ~end & EX E[~put U end])]]]) Basis of PSL was Sugar (IBM, Haifa) Grew out of CTL (I believe) Added lots of syntactic sugar Engineer friendly, used in many projects Used in the industrial strength MC RuleBase 5

Assertion Based Verification (ABV) can be done in two ways During simulation – (dynamic, at runtime, called semi-formal verification, checks only those runs) As a static check – (formal verification, covers all possible runs, more comprehensive, harder to do, restricted to a subset of the property language ) (Note: this duality has been important for PSL’s practical success, but it also complicates the semantics!) Properties always (p) states that p (a boolean expression made from signal names, constants and operators) is true on every cycle always (! (gr1 & gr2)) 6

Properties always (a -> b) If a is true, then b is true a implies b a -> b not a or b always ((rd=’0’) -> next (prev(dout)=dout) ) Safety Properties always (p) ”Nothing bad will ever happen” Most common type of property checked in practice Easy to check (more later) Disproved by a finite run of the system 7

Observer: a second approach Observer written in same language as circuit Safety properties only Used in verification of control programs (and in Lava later) F ok Prop Back to PSL always (p) Talks about one cycle at a time Sequential Extended Regular Expressions (SEREs) allow us to talk about spans of time A SERE describes a set of traces It is a building block for a property http://www.eda.org/vfv/docs/PSL-v1.1.pdf 8

SERE examples {req; busy; grnt} All sequences of states, or traces, in which req is high on the first cycle, busy on the second, and grnt on the third. (source Sugar 2.0 presentation from IBM’s Dana Fisman and Cindy Eisner, with thanks) SERE examples {req; busy; grnt} req is in the set of traces busy grnt 9

SERE examples {req; busy; grnt} req This too busy grnt SERE examples {req; busy; grnt} req busy and this grnt 10

SERE examples {req; busy; grnt} req busy but not this Why? grnt SERE examples How can we specify ONLY those traces that start like this? req busy grnt 11

SERE examples {req & !busy & !grnt; req !req & busy & !grnt; !req & !busy & grnt} busy grnt SERE examples How do we say that the {req,busy,grnt} sequence can start anywhere? req busy grnt 12

SERE examples {[*]; req; busy; grnt} req [*] means skip zero or more busy cycles grnt SERE examples {[*]; req; busy; grnt} req so our original trace is still in the set busy described grnt 13

SERE examples {true; req; busy; grnt} says that the req, busy, grnt req sequence starts exactly in the second cycle. It constrains only cycles 2,3,4 busy grnt {true[*4]; req; busy; grnt} rbg sequence must start at cycle 5 {true[+]; req; busy; grnt} true[+] = [+] one or more trues true[*] = [*] 14

{[*]; req; busy[*3..5]; grnt} at least 3 and at most 5 busys {[*]; req; {b1,b2}[*]; grnt} {[*]; req; {b1,b2,b3}[*7]; grnt} subsequences can also be repeated && Simultaneous subsequences Same length, start and end together {start; a[*]; end} && {!abort[*]} 15

|| One of the subsequences should be matched Don’t need to be the same length {request; {rd; !cncl_r; !dne[*]} || {wr;!cncl_w;!dne[*]}; dne} Fancier properties at last! SEREs are themselves properties (in the latest version of PSL). Properties are also built from subproperties. {SERE1} |=> {SERE2} is a property If a trace matches SERE1, then its continuation should match SERE2 16

{true[*]; req; ack} |=> {start; busy[*]; end} then if Not just the first req, ack {true[*]; req; ack} => {start; busy[*]; end} then if then if 17

Overlap also possible! {true[*]; req; ack} => {start; busy[*]; end} then if then if {true[*]; req; ack} => {start; data[*]; end} then if 18

Can check for data in non-consecutive cycles {true[*]; req; ack} => {start; data[=8]; end} 2 3 4 5 6 7 1 8 then if A form of implication {SERE1} |=> {SERE2} If a trace matches SERE1, then its continuation should match SERE2 19

Another form of implication {SERE1} |-> {SERE2} If a trace matches SERE1, then SERE2 should be matched, starting from the last element of the trace matching SERE1 So there is one cycle of overlap in the middle Example {[*]; start; busy[*]; end} |-> {success; done} If signal start is asserted, signal end is asserted at the next cycle or later, and in the meantime signal busy holds, then success is asserted at the same time as end is, and in the next cycle done is asserted 20

Example {[*]; {start; c[*]; end}&&{!abort[*]}} |-> {success} If there is no abort during {start,c[*],end}, success will be asserted with end {SERE1} |=> {SERE2} = {SERE1} |-> {true, SERE2} Both are formulas of the linear fragment (which is based on LTL) In Jasper Gold, we use this linear part. There is also an optional branching extension (which is where CTL comes back in) 21

PSL has a small core and the rest is syntactic sugar, for example b[=i] = {not b[*]; b}[*i] ; not b[*] See formal semantics in LRM PSL Regular expressions (plus some operators) + Linear temporal logic (LTL) + Lots of syntactic sugar + (optional) Computation tree logic (CTL) 22

Example revisited A sequence beginning with the assertion of signal strt, and containing two not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing two assertions of signal put before signal end can be asserted AG~(strt & EX E[~get & ~kill U get & ~kill & EX E[~get & ~kill U get & ~kill & E[~put U end] | E[~put & ~end U (put & ~end & EX E[~put U end])]]]) In PSL (with 8 for 2) A sequence beginning with the assertion of signal strt, and containing eight not necessarily consecutive assertions of signal get, during which signal kill is not asserted, must be followed by a sequence containing eight assertions of signal put before signal end can be asserted always({strt; {get[=8]}&&{kill[=0]}} |=> {{put[=8]}&&{end[=0]}}) 23

From PSL to CTL? always {a;b[*];c} -> {d;e[*];f} (source, lecture notes by Mike Gordon, question asked by PSL/Sugar designers) PSL Seems to be reasonably simple, elegant and concise! Jasper’s Göteborg based team have helped to define and simplify the formal semantics. See the LRM and also the paper in FMCAD 2004 (on course home page) 24

Friday’s lecture By Jiri Gaisler on his two process method of using VHDL Do NOT miss this lecture. It is brilliant! Next week, I will return to LTL, CTL and how to model check them 25