Exploring the t-Kernel: Interrupt Handling and Aspect-Oriented Programming

Tutorial of the t-kernel(Session B)July 7, 2006 Lin Gu Dept. of Computer Science University of Virginia lingu@cs.virginia.edu http://www.cs.virginia.edu/~lg6e

Resources • Web page: http://www.cs.virginia.edu/~lg6e/t-kernel/ • Source code in the vert repository: www.sf.net/projects/vert • Please let me know your sourceforge accounts, or use the anonymous account to check out the code • Directory: /cvsroot/vert/t-kernel

Outline • Interrupt handling • Code review • Coding example • t-kernel and Aspect Oriented Programming

Interrupt Handling • The kernel monitors interrupts • The application handles interrupts • The application can use any instructions, including disabling/enabling interrupts • It is possible for the application and kernel to cooperate to handle interrupts • SPI interrupts • Byte level cooperation • Frame level cooperation

Source code tree townLogic() clockLogic() translate() translateBranch() pleaseBridge() leadOut()

Code review – Data structure typedef uint16_t * Code; typedef struct { Code codeEntry; uint8_t cWlen, /*length of the code chunk*/ cCur /* count of the current word */; } CodeChunk; CodeChunk ccRaw;

Code review – Data structure typedef struct { uint32_t lVPWCStart; uint8_t cWlen; /* length of the code chunk */ Code codeEntry; IFPMapEntry *pifpmeStanza; uint8_t cMaxStanza; uint8_t cFlag; /* TKNL_NCFLAG_* flags */ uint16_t nPgmPageNo; uint16_t nMemAreaStart; uint16_t nVoidPage[TKNL_NC_LINKIN_NUM]; } NaturalizedChunk; NaturalizedChunk ncNat;

Code review – Functions uint16_t translate (uint32_t lVPWCWanted, uint16_t nPageNo, uint16_t nSrcPage, uint8_t bRevert); • Copy/modify application code to construct a natin page. • Multiple-use • Create new natin page for a new VPC • Add a new entry point to an existing natin page • Invalidate bridging jumps in a natin page

Code review – Functions uint8_t translateBranch(); uint8_t pleaseBridge(uint16_t nDest); • Translate conventional conditional branches • Differentiate forward and backward branches

Bridging – Backward branch • The naturalizer patches the natin page • SystemCounter is 8-bit • One trap into kernel per 256 backward branches • “headmaster” does sanity check in the dispatcher ... add r0, r1 ... push r31 in r31, 0x3f push r31 lds SystemCounter inc SystemCounter sts SystemCounter brne go cli call headmaster go: pop r31 Out 0x3f, r31 pop r31 jmp HPC ... naturalizer ... Prev: add r0, r1 ... rcall branch_stub Prev (VPC) ... ... HPC dispatcher

Code review – Functions void leadOut(uint16_t nSrcPage, char bCopyLinkin); • Write or copy the link-in records • Write finis

Link-in and finis • Bridging makes direct jumps between natin pages • The incoming pages are recorded in the link-in record of the natin page • Needed for invalidating direct jumps when a natin page is changed (new entry point inserted, or swapped out) ... ... last town transition incoming page 0 incoming page 1 ... incoming page 2 branch_stub ... page 109

Code review – Functions uint16_t townLogic(); • Handle town transitions • Rewrite “rcall branch_stub” to bridging jumps

Bridging – Forward branch • The dispatcher looks up HPC for “Next” • The naturalizer patches the natin page • Handled by “townLogic” (town transition) naturalizer rcall branch_stub Next (VPC) ... ... add r0, r1 ... jmp HPC ... ... add r0, r1 ... Stack frame Stack frame HPC ret1 branch_stub: push r31 in r31, 0x3f cli call townGate branch_stub: push r31 in r31, 0x3f cli call townGate r31 ret0 dispatcher

Code review – Interrupt uint16_t clockLogic(){ uint16_t nStackHead = getAppStackHead(); IntRoom irT; ncNat.lVPWCStart = nVector[TKNL_SIG_OUTPUT_COMPARE0]; cMind |= 1; lTick++; if (checkNestInt()) return -5; irT.c1 = *((uint8_t *)(nStackHead+4)); irT.c0 = *((uint8_t *)(nStackHead+5)); irT.n0 <<= 1; pinInterruptedNatin(irT.c1); <run ncNat.lVPWCStart>; } // clockLogic

Code review – Flags commtknl.h contains system-wide flags • TKNL_PRODUCTION – Disables all debug flags • TKNL_MERGE_NATIN_PAGE – Enables merging of natin pages (add entry points to a natin page) • TKNL_GREEN_LANE – Enables VPC look-aside buffer • TKNL_VISIT_TOWN – Enables bridging • TKNL_INTERRUPT_NUM_NEST – Interrupt nesting level

t-kernel & Aspect Oriented Programming • Aspect oriented programming (AOP) • Pointcut • Advice • XPI • AspectJ, AspectC, etc. • Works on source code or compiled (existing) code • Techniques: aspect compilers, virtual machines, reverse-engineering

t-kernel & Aspect Oriented Programming • A basic Aspect framework • Pointcut, Advice, XPI • Supporting compiled (existing) code • Aspects can be propagated in the network • How to implement such a framework in the t-kernel?

t-kernel & AOP • A basic Aspect framework • Pointcut, Advice, XPI • Supporting compiled (existing) code • Aspects can be propagated in the network • How to implement such a framework in the t-kernel?

t-kernel & AOP • Program+XPI  tagged program • Tags can be as simple as a number, or contain semantic information • Not limited to function calls • Advice = new code • E.g., advice1 to be attached before pointcut1 Add r0, r1 Call func1 Next: Sub r1, r2 Add r0, r1 <jp_1_start> Call func1 <jp_1_end> Next:Sub r1, r2 Pointcut: function func*

t-kernel & AOP “before” a join point • Insert the new code before the “start” tag Add r0, r1 <jp_1_start> Call func1 <jp_1_end> Next:Sub r1, r2 Add r0, r1 <jp_1_start> Call adv1 Call func1 <jp_1_end> Next: Sub r1, r2 Code modification Advice1: Call adv1

t-kernel & AOP “after” a join point • Insert the new code after the “end” tag Add r0, r1 <jp_1_start> Call adv1 Call func1 <jp_1_end> Next: Sub r1, r2 Add r0, r1 <jp_1_start> Call adv1 Call func1 Call adv2 <jp_1_end> Next: Sub r1, r2 Code modification Advice2: Call adv2

t-kernel & AOP “around” a join point • Insert the new code after the “start” tag and jump to after the “end” tag Add r0, r1 <jp_1_start> Call adv1 Call func1 Call adv2 <jp_1_end> Next: Sub r1, r2 Add r0, r1 <jp_1_start> Call adv3 jmp jp_1_end Call adv1 Call func1 Call adv2 <jp_1_end> Next: Sub r1, r2 Code modification Advice3: Call adv3

t-kernel & AOP Key function needed • Online code weaving • Invalidate old code • Generate new code based on the old code and the new advices • May be implemented by the “entry point” inserting and natin page swapping mechanism in the t-kernel • Aspect compiler • modify nesC or gcc? New compiler? Preprocessor?

t-kernel & AOP Supporting mechanisms • XPI definition language • Examples exist • Advice delivery mechanism • The t-kernel support wireless communication

Exploring the t-Kernel: Interrupt Handling and Aspect-Oriented Programming