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Microprocessor system architectures– IA32 debugging and performance monitoring Jakub Yaghob
Debug exception#DB Nearly all debug events Breakpoint exception#BP Exception invoked by INT3 Debug registers DR0-DR3, DR6, DR7 Tflag in TSS During attempt to switch task EFLAGS[RF] Resume Flag – disables multiple exceptions to the instruction EFLAGS[TF] Trap Flag – generates #DB after every execution of an instruction Breakpoint instruction INT3 Last branch recording Only P6+ Overview of debugging support
Conditions for invoking the debugger • Task switch • Execution of breakpoint instruction • Execution of any instruction • Execution of an instruction at a specified address • Read or write of B, W, D at a specified memory address • Write to B, W, D at a specified memory address • Input of B, W, D at a specified I/O address (Pentium+) • Output to B, W, D at a specified I/O address (Pentium+) • Attempt to change the contents of a debug register
Program tracing • Tracing enabled byEFLAGS[TF]=1 • Exception#DBinvoked after every instruction execution withTFset before the instruction execution • It is a TRAP • An instruction, which sets TF, does not invoke the exception • An instruction, which clearsTF, invokes the exception • TFcleared before calling the exception/interrupt handler • TFdoes not change its state during CPL changes • Concurrent occurrence of single-step and an external interrupt • #DB invoked • TF cleared • The external interrupt handler invoked before the first instruction of #DB handler
Task switching • TSS[T] set for a target task • Invoked after the task is switched before the first instruction of the target task is executed • #DB should not be handled by a task with T flag set • CPU in loop
Breakpoint instruction • InstructionINT3 • Encoded as 1B instruction • Suitable for inserting into the code to any address • Using • Write theINT3instruction to the breakpoint and remember previous contents • #BPinvokes debugger • Before returning from#BPthe EFLAGS[TF] is set on the stack and original contents is written back to the breakpoint • Return from the exception • #DB for single-step invoked after the instruction on the breakpoint is executed • Clear the TFand write back INT3to the breakpoint
Debug registers– II • Status registerDR6 • Bn – condition n detected • BD – debug registers access detected withDR7[GD] • BS – singlestep • BT – task switch • Control registerDR7 • Ln – local breakpoint enabled • Gn – global breakpoint enabled • GD – general detect enable • R/Wn – breakpoint condition • LENn – size of memory location
Breakpoint recognition • Address • The addressmust be aligned to the size • Data and I/O breakpoint • Any accessed byte inside the range • Instruction breakpoint • Must have size 1B • Only the first byte of an instruction
Instruction breakpoint • It is FAULT • We need to avoid endless loop on the breakpoint • High priority • Invoked before other exceptions • Not invoked on an instruction after the POP SS/MOV SS instruction • How to avoid the endless loop • The flagEFLAGS[RF] • Automatically set before any FAULT exception is invoked, except of instruction breakpoint • Cleared after successful execution of an instruction, except of task switch (JMP, CALL, INT, IRET) • It can be explicitly set only by reading EFLAGS from the stack/TSS returning from interrupt handler • Using as a breakpoint • Set the RF in the EFLAGS image on the stack/v TSS
Last branch recording • Introduced in the Pentium Pro • Depends tightly on model and MSRs • Branches, interrupts, exceptions • Branches: JMP, Jcc, LOOP, CALL/RET
Performance monitoring • Introduced in the Pentium • Time-stamp counter • Architectural performance monitoring • Behaves consistently across microarchitectures • Performance monitoring depending on a microarchitecture • MSRs
Time-stamp counter • Support • TSC flag • Availability check in CPUID.1:EDX.TSC • IA32_TIME_STAMP_COUNTER MSR • MSR used as a counter • 64-bit • RDTSC instruction • TSD flag • Enable/disable TSC (CR4[TSD]) • Behavior • Older CPUs • Increments with every internal clock cycle • Newer CPUs • Increments at a constant rate