hw debug api: modify single stepping checks

On aarch32, single stepping is configured by setting an instruction
breakpoint to mismatch mode. The way that it is checked whether a
given breakpoint is being used as a normal breakpoint or for
single-stepping is by checking if it has been configured to mismatch.
This works because the HW debug API does not currently provide a way
to otherwise configure mismatch breakpoints, but seems like an
unsatisfactory solution. Whether single-stepping is enabled, and
the breakpoint that is being used for it is already stored in the
TCB of a thread, and this commit changes checks related to
single-stepping to use this information instead.

Signed-off-by: Alwin Joshy <[email protected]>

include/arch/arm/arch/machine/debug.h

@@ -303,14 +303,12 @@ static inline syscall_error_t Arch_decodeUnsetBreakpoint(tcb_t *t, uint16_t bp_n
     }
 
     word_t type;
-    dbg_bcr_t bcr;
 
     type = getTypeFromBpNum(bp_num);
     bp_num = convertBpNumToArch(bp_num);
 
-    bcr.words[0] = t->tcbArch.tcbContext.breakpointState.breakpoint[bp_num].cr;
     if (type == seL4_InstructionBreakpoint) {
-        if (Arch_breakpointIsMismatch(bcr) == true && dbg_bcr_get_enabled(bcr)) {
+        if (Arch_breakpointIsSingleStepping(t, bp_num)) {
             userError("Rejecting call to unsetBreakpoint on breakpoint configured "
                       "for single-stepping (hwid %u).", bp_num);
             ret.type = seL4_IllegalOperation;

include/arch/arm/armv/armv7-a/armv/debug.h

@@ -161,13 +161,11 @@ static inline dbg_wcr_t Arch_setupWcr(dbg_wcr_t in_val)
     return wcr;
 }
 
-static inline bool_t Arch_breakpointIsMismatch(dbg_bcr_t in_val)
-{
-    /* Detect if the register is set up for mismatch (single-step). */
-    if (dbg_bcr_get_breakpointType(in_val) == DBGBCR_TYPE_UNLINKED_INSTRUCTION_MISMATCH) {
-        return true;
-    }
-    return false;
+static inline bool_t Arch_breakpointIsSingleStepping(tcb_t *t, uint16_t bp_num) {
+    /* Detect if the bp is set up for single stepping */
+
+    return (t->tcbArch.tcbContext.breakpointState.single_step_enabled &&
+            t->tcbArch.tcbContext.breakpointState.single_step_hw_bp_num == bp_num);
 }
 
 #endif /* CONFIG_HARDWARE_DEBUG_API */

src/arch/arm/machine/debug.c

@@ -344,7 +344,7 @@ getBreakpoint_t getBreakpoint(tcb_t *t, uint16_t bp_num)
         dbg_bcr_t bcr;
 
         bcr.words[0] = readBcrContext(t, bp_num);
-        if (Arch_breakpointIsMismatch(bcr) == true) {
+        if (Arch_breakpointIsSingleStepping(t, bp_num)) {
             ret.type = seL4_SingleStep;
         };
         ret.size = 0;