Arch arm assembly simplifications

arch/arm/core/swap_helper.S

@@ -41,6 +41,8 @@ GDATA(_kernel)
  * already been taken. In other words, when z_arm_pendsv() runs, we *know* we
  * have to swap *something*.
  *
+ * For Cortex-M, z_arm_pendsv() is invoked with no arguments.
+ *
  * For Cortex-R, PendSV exception is not supported by the architecture and this
  * function is directly called either by _IntExit in case of preemption, or
  * z_arm_svc in case of cooperative switching.
@@ -163,7 +165,9 @@ out_fp_endif:
     str v3, [v4, #0]
 #endif
 
-    /* Restore previous interrupt disable state (irq_lock key) */
+    /* Restore previous interrupt disable state (irq_lock key)
+     * (We clear the arch.basepri field after restoring state)
+     */
 #if (defined(CONFIG_CPU_CORTEX_M0PLUS) || defined(CONFIG_CPU_CORTEX_M0)) && \
 	    _thread_offset_to_basepri > 124
     /* Doing it this way since the offset to thread->arch.basepri can in
@@ -249,8 +253,7 @@ in_fp_endif:
 #if defined (CONFIG_ARM_MPU)
     /* Re-program dynamic memory map */
     push {r2,lr}
-    ldr r0, =_kernel
-    ldr r0, [r0, #_kernel_offset_to_current]
+    mov r0, r2 /* _current thread */
     bl z_arm_configure_dynamic_mpu_regions
     pop {r2,lr}
 #endif
@@ -411,25 +414,30 @@ _oops:
  * @return N/A
  */
 SECTION_FUNC(TEXT, z_arm_svc)
+  /* Use EXC_RETURN state to find out if stack frame is on the
+   * MSP or PSP
+   */
     tst lr, #0x4    /* did we come from thread mode ? */
     ite eq  /* if zero (equal), came from handler mode */
         mrseq r0, MSP   /* handler mode, stack frame is on MSP */
         mrsne r0, PSP   /* thread mode, stack frame is on PSP */
 
+
+    /* Figure out what SVC call number was invoked */
+
     ldr r1, [r0, #24]   /* grab address of PC from stack frame */
-    /* SVC is a two-byte instruction, point to it and read encoding */
-    ldrh r1, [r1, #-2]
+    /* SVC is a two-byte instruction, point to it and read the
+     * SVC number (lower byte of SCV instruction)
+     */
+    ldrb r1, [r1, #-2]
 
    /*
     * grab service call number:
     * 0: Unused
     * 1: irq_offload (if configured)
     * 2: kernel panic or oops (software generated fatal exception)
     * 3: System call (if user mode supported)
-    * Planned implementation of system calls for memory protection will
-    * expand this case.
     */
-    ands r1, #0xff
 #if defined(CONFIG_USERSPACE)
     mrs r2, CONTROL
 
@@ -443,7 +451,7 @@ SECTION_FUNC(TEXT, z_arm_svc)
     tst r2, #0x1
     bne _oops
 
-#endif
+#endif /* CONFIG_USERSPACE */
 
     cmp r1, #2
     beq _oops
@@ -495,13 +503,12 @@ _do_syscall:
     blt valid_syscall_id
 
     /* bad syscall id.  Set arg1 to bad id and set call_id to SYSCALL_BAD */
-    str r6, [r0, #0]
+    str r6, [r0]
     ldr r6, =K_SYSCALL_BAD
 
     /* Bad syscalls treated as valid syscalls with ID K_SYSCALL_BAD. */
 
 valid_syscall_id:
-    push {r0, r1}
     ldr r0, =_kernel
     ldr r0, [r0, #_kernel_offset_to_current]
     ldr r1, [r0, #_thread_offset_to_mode]
@@ -518,7 +525,6 @@ valid_syscall_id:
      * instructions with the previous privilege.
      */
     isb
-    pop {r0, r1}
 
     /* return from SVC to the modified LR - z_arm_do_syscall */
     bx lr

arch/arm/core/userspace.S

@@ -33,6 +33,9 @@ GDATA(_k_syscall_table)
  * The conversion is one way, and threads which transition to user mode do
  * not transition back later, unless they are doing system calls.
  *
+ * The function is invoked as:
+ * z_arm_userspace_enter(user_entry, p1, p2, p3,
+ *                        stack_info.start, stack_info.size);
  */
 SECTION_FUNC(TEXT,z_arm_userspace_enter)
     /* move user_entry to lr */
@@ -44,14 +47,20 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
     msr PSPLIM, r0
 #endif
 
-    /* set stack to privileged stack */
+    /* prepare to set stack to privileged stack */
     ldr r0, =_kernel
     ldr r0, [r0, #_kernel_offset_to_current]
     ldr r0, [r0, #_thread_offset_to_priv_stack_start]    /* priv stack ptr */
     ldr ip, =CONFIG_PRIVILEGED_STACK_SIZE
     add r0, r0, ip
 
+    /* store current stack pointer to ip
+     * the current stack pointer is needed to retrieve
+     * stack_info.start and stack_info.size
+     */
     mov ip, sp
+
+    /* set stack to privileged stack */
     msr PSP, r0
 
 #if defined(CONFIG_BUILTIN_STACK_GUARD)
@@ -62,7 +71,10 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
     msr PSPLIM, r0
 #endif
 
-#if defined (CONFIG_ARM_MPU)
+    /* push args to stack */
+    push {r1,r2,r3,lr}
+    push {r0,ip}
+
     /* Re-program dynamic memory map.
      *
      * Important note:
@@ -77,20 +89,17 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
      * stack, since we do not control how much stack is actually left, when
      * user invokes z_arm_userspace_enter().
      */
-    push {r0,r1,r2,r3,ip,lr}
     ldr r0, =_kernel
     ldr r0, [r0, #_kernel_offset_to_current]
     bl z_arm_configure_dynamic_mpu_regions
-    pop {r0,r1,r2,r3,ip,lr}
-#endif
+
+    pop {r0,ip}
 
     /* load up stack info from user stack */
     ldr r0, [ip]
     ldr ip, [ip, #4]
 
-
-    /* push args to stack */
-    push {r0,r1,r2,r3,ip,lr}
+    push {r0,ip}
 
     /* clear the user stack area to clean out privileged data */
     /* from right past the guard right up to the end */
@@ -102,18 +111,18 @@ SECTION_FUNC(TEXT,z_arm_userspace_enter)
 #endif
     bl memset
 
-    pop {r0,r1,r2,r3,ip,lr}
+    pop {r0,ip}
 
     /* r0 contains user stack start, ip contains user stack size */
     add r0, r0, ip   /* calculate top of stack */
 
 #if defined(CONFIG_BUILTIN_STACK_GUARD)
     /* clear stack limit (stack protection not required in user mode) */
-    push {r3}
     mov r3, #0
     msr PSPLIM, r3
-    pop {r3}
 #endif
+    /* pop remaining arguments from stack before switching stacks */
+    pop {r1,r2,r3,lr}
 
     /* set stack to user stack */
     msr PSP, r0