ARMv7-A: Modify up_fullcontextrestore() for CONFIG_BUILD_KERNEL. It changed CPSR while in kernel. That will crash is the new CPSR is user mode while executing in kernel space. Fixed by adding a SYS_context_restore system call. There is an alternative, simpler modification to up_fullcontextrestore() that could have been done: It might have been possible to use the SPSR instead of the CPRSR and then do an exception return from up_fullcontextrestore(). That would be more efficient, but I never tried it.

arch/arm/src/armv7-a/arm_fullcontextrestore.S

@@ -37,8 +37,10 @@
  * Included Files
  ****************************************************************************/
 
+#include <nuttx/config.h>
 #include <nuttx/irq.h>
 #include "up_internal.h"
+#include "svcall.h"
 
 	.file	"arm_fullcontextrestore.S"
 
@@ -112,6 +114,28 @@ up_fullcontextrestore:
 	vmsr	fpscr, r2				/* Restore the FPCSR */
 #endif
 
+#ifdef CONFIG_BUILD_KERNEL
+	/* For the kernel build, we need to be able to transition gracefully
+	 * between kernel- and user-mode tasks.  We have to do that with a system
+	 * call; the system call will execute in kernel mode and but can return
+	 * to either user or kernel mode.
+	 */
+
+	/* Perform the System call with R0=SYS_context_restore, R1=restoreregs */
+
+	mov		r1, r0					/* R1: restoreregs */
+	mov		r0, #SYS_context_restore	/* R0: SYS_context_restore syscall */
+	svc		#0x900001				/* Perform the system call */
+
+	/* This call should not return */
+
+	bx		lr						/* Unnecessary ... will not return */
+
+#else
+	/* For a flat build, we can do all of this here... Just think of this as
+	 * a longjmp() all on steriods.
+	 */
+
 	/* Recover all registers except for r0, r1, R15, and CPSR */
 
 	add		r1, r0, #(4*REG_R2)		/* Offset to REG_R2 storage */
@@ -148,4 +172,7 @@ up_fullcontextrestore:
 	 */
 
 	ldr		pc, [sp], #4
+
+#endif
+
 	.size up_fullcontextrestore, . - up_fullcontextrestore

arch/arm/src/armv7-a/arm_syscall.c

@@ -58,16 +58,16 @@
  ****************************************************************************/
 /* Debug ********************************************************************/
 
-/* Output debug info if stack dump is selected -- even if
+#if defined(CONFIG_DEBUG_SYSCALL)
- * debug is not selected.
- */
-
-#if defined(CONFIG_DEBUG_SYSCALL) || defined(CONFIG_DEBUG_SVCALL)
 # define svcdbg(format, ...) lldbg(format, ##__VA_ARGS__)
 #else
 # define svcdbg(x...)
 #endif
 
+/* Output debug info if stack dump is selected -- even if debug is not
+ * selected.
+ */
+
 #ifdef CONFIG_ARCH_STACKDUMP
 # undef  lldbg
 # define lldbg lowsyslog
@@ -229,6 +229,30 @@ uint32_t *arm_syscall(uint32_t *regs)
         }
         break;
 
+      /* R0=SYS_context_restore:  Restore task context
+       *
+       * void up_fullcontextrestore(uint32_t *restoreregs) noreturn_function;
+       *
+       * At this point, the following values are saved in context:
+       *
+       *   R0 = SYS_context_restore
+       *   R1 = restoreregs
+       */
+
+#ifdef CONFIG_BUILD_KERNEL
+      case SYS_context_restore:
+        {
+          /* Replace 'regs' with the pointer to the register set in
+           * regs[REG_R1].  On return from the system call, that register
+           * set will determine the restored context.
+           */
+
+          regs = (uint32_t *)regs[REG_R1];
+          DEBUGASSERT(regs);
+        }
+        break;
+#endif
+
       /* R0=SYS_task_start:  This a user task start
        *
        *   void up_task_start(main_t taskentry, int argc, FAR char *argv[]) noreturn_function;
@@ -427,9 +451,9 @@ uint32_t *arm_syscall(uint32_t *regs)
   svcdbg("CPSR: %08x\n", regs[REG_CPSR]);
 #endif
 
-  /* Return the last value of curent_regs.  This supports context switchs
+  /* Return the last value of curent_regs.  This supports context switches
-   * on return from the exception.  That capability is not used here,
+   * on return from the exception.  That capability is only used with the
-   * however.
+   * SYS_context_switch system call.
    */
 
   return regs;

arch/arm/src/armv7-a/crt0.c

@@ -101,7 +101,7 @@ static void sig_trampoline(void)
     " blx  ip\n"         /* Call the signal handler */
     " pop  {r2}\n"       /* Recover LR in R2 */
     " mov  lr, r2\n"     /* Restore LR */
-    " mov  r0, #4\n"     /* SYS_signal_handler_return */
+    " mov  r0, #5\n"     /* SYS_signal_handler_return */
     " svc #0x900001\n"   /* Return from the signal handler */
   );
 }

arch/arm/src/armv7-a/svcall.h

@@ -60,9 +60,9 @@
 
 #ifdef CONFIG_BUILD_KERNEL
 #  ifndef CONFIG_SYS_RESERVED
-#    error "CONFIG_SYS_RESERVED must be defined to have the value 5"
+#    error "CONFIG_SYS_RESERVED must be defined to have the value 6"
-#  elif CONFIG_SYS_RESERVED != 5
+#  elif CONFIG_SYS_RESERVED != 6
-#    error "CONFIG_SYS_RESERVED must have the value 5"
+#    error "CONFIG_SYS_RESERVED must have the value 6"
 #  endif
 #else
 #  ifndef CONFIG_SYS_RESERVED
@@ -83,35 +83,42 @@
 
 #ifdef CONFIG_BUILD_KERNEL
 /* SYS call 1:
+ *
+ * void up_fullcontextrestore(uint32_t *restoreregs) noreturn_function;
+ */
+
+#define SYS_context_restore       (1)
+
+/* SYS call 2:
  *
  * void up_task_start(main_t taskentry, int argc, FAR char *argv[])
  *        noreturn_function;
  */
 
-#define SYS_task_start            (1)
+#define SYS_task_start            (2)
 
-/* SYS call 2:
+/* SYS call 3:
  *
  * void up_pthread_start(pthread_startroutine_t entrypt, pthread_addr_t arg)
  *        noreturn_function
  */
 
-#define SYS_pthread_start         (2)
+#define SYS_pthread_start         (3)
 
-/* SYS call 3:
+/* SYS call 4:
  *
  * void signal_handler(_sa_sigaction_t sighand, int signo, FAR siginfo_t *info,
  *                     FAR void *ucontext);
  */
 
-#define SYS_signal_handler        (3)
+#define SYS_signal_handler        (4)
 
-/* SYS call 4:
+/* SYS call 5:
  *
  * void signal_handler_return(void);
  */
 
-#define SYS_signal_handler_return (4)
+#define SYS_signal_handler_return (5)
 
 #endif /* CONFIG_BUILD_KERNEL */
 

configs/sama5d4-ek/knsh/defconfig

@@ -455,7 +455,7 @@ CONFIG_USERMAIN_STACKSIZE=2048
 CONFIG_PTHREAD_STACK_MIN=256
 CONFIG_PTHREAD_STACK_DEFAULT=2048
 CONFIG_LIB_SYSCALL=y
-CONFIG_SYS_RESERVED=5
+CONFIG_SYS_RESERVED=6
 CONFIG_SYS_NNEST=2
 
 #

configs/sama5d4-ek/knsh/defconfig.ROMFS

@@ -438,7 +438,7 @@ CONFIG_USERMAIN_STACKSIZE=2048
 CONFIG_PTHREAD_STACK_MIN=256
 CONFIG_PTHREAD_STACK_DEFAULT=2048
 CONFIG_LIB_SYSCALL=y
-CONFIG_SYS_RESERVED=5
+CONFIG_SYS_RESERVED=6
 CONFIG_SYS_NNEST=2
 
 #