riscv/syscall: Optimize user service call performance

This patch changes how user service calls are executed:
Instead of using the common interrupt logic, execute the user service
call directly.

Why? When a user makes a service call request, all of the service call
parameters are already loaded into the correct registers, thus it makes
no sense to first clobber them and then reload them, which is what the
old logic does. It is much more effective to run the system call directly.

During a user system call the interrupts must be re-enabled, which the
new logic does as soon as we know the exception is a user service call
request.

This patch does NOT change the behavior of reserved system calls (like
switch_context), only the user service call request is affected.

arch/risc-v/include/irq.h

@@ -96,14 +96,6 @@
 
 /* Configuration ************************************************************/
 
-/* If this is a kernel build, how many nested system calls should we
- * support?
- */
-
-#ifndef CONFIG_SYS_NNEST
-#  define CONFIG_SYS_NNEST  2
-#endif
-
 /* Processor PC */
 
 #define REG_EPC_NDX         0
@@ -539,18 +531,6 @@
 
 #ifndef __ASSEMBLY__
 
-/* This structure represents the return state from a system call */
-
-#ifdef CONFIG_LIB_SYSCALL
-struct xcpt_syscall_s
-{
-  uintptr_t sysreturn;   /* The return PC */
-#ifndef CONFIG_BUILD_FLAT
-  uintptr_t int_ctx;     /* Interrupt context (i.e. m-/sstatus) */
-#endif
-};
-#endif
-
 /* The following structure is included in the TCB and defines the complete
  * state of the thread.
  */
@@ -582,16 +562,6 @@ struct xcptcontext
   uintptr_t sigreturn;
 #endif
 
-#ifdef CONFIG_LIB_SYSCALL
-  /* The following array holds information needed to return from each nested
-   * system call.
-   */
-
-  uint8_t nsyscalls;
-  struct xcpt_syscall_s syscall[CONFIG_SYS_NNEST];
-
-#endif
-
 #ifdef CONFIG_ARCH_ADDRENV
 #ifdef CONFIG_ARCH_KERNEL_STACK
   /* In this configuration, all syscalls execute from an internal kernel

arch/risc-v/include/syscall.h

@@ -77,15 +77,6 @@
 
 #define SYS_switch_context        (2)
 
-#ifdef CONFIG_LIB_SYSCALL
-/* SYS call 3:
- *
- * void riscv_syscall_return(void);
- */
-
-#define SYS_syscall_return        (3)
-#endif /* CONFIG_LIB_SYSCALL */
-
 #ifndef CONFIG_BUILD_FLAT
 /* SYS call 4:
  *

arch/risc-v/src/common/riscv_exception_common.S

@@ -30,6 +30,10 @@
 
 #include <sys/types.h>
 
+#ifdef CONFIG_LIB_SYSCALL
+#  include <syscall.h>
+#endif
+
 #include "chip.h"
 
 #include "riscv_percpu.h"
@@ -73,6 +77,7 @@
   .section EXCEPTION_SECTION
   .global exception_common
   .global return_from_exception
+  .global return_from_syscall
   .align  8
 
 exception_common:
@@ -99,24 +104,69 @@ exception_common:
   addi       sp, sp, -XCPTCONTEXT_SIZE
   save_ctx   sp
 
-  csrr       s0, CSR_STATUS
-  REGSTORE   s0, REG_INT_CTX(sp)  /* status */
+  csrr       s0, CSR_STATUS       /* s0=status */
+  csrr       s1, CSR_EPC          /* s1=exception PC */
+  csrr       s2, CSR_CAUSE        /* s2=cause */
 
 #ifdef CONFIG_ARCH_KERNEL_STACK
-  csrr       s0, CSR_SCRATCH
-  REGLOAD    s0, RISCV_PERCPU_USP(s0)
+  csrr       s3, CSR_SCRATCH
+  REGLOAD    s3, RISCV_PERCPU_USP(s3)
 #else
-  addi       s0, sp, XCPTCONTEXT_SIZE
+  addi       s3, sp, XCPTCONTEXT_SIZE
 #endif
 
-  REGSTORE   s0, REG_X2(sp)       /* original SP */
+  REGSTORE   s0, REG_INT_CTX(sp)
+  REGSTORE   s1, REG_EPC(sp)
+  REGSTORE   s3, REG_SP(sp)
+
+#ifdef CONFIG_LIB_SYSCALL
+  /* Check whether it is an exception or interrupt */
+
+  blt        s2, x0, handle_irq   /* If cause < 0 it is interrupt */
+
+  /* Is it a system call ? */
+
+  li         s3, RISCV_IRQ_ECALLU /* Is it a system call ? */
+  bne        s2, s3, handle_irq
+
+  /* Is it one of the reserved system calls ? */
+
+  li         s3, CONFIG_SYS_RESERVED
+  blt        a0, s3, handle_irq   /* If a0 < CONFIG_SYS_RESERVED */
+
+  /* It is a system call, re-enable interrupts if they were enabled */
+
+  andi       s3, s0, STATUS_PIE
+  beqz       s3, 1f
+  csrs       CSR_STATUS, STATUS_IE
+
+1:
+  addi       s1, s1, 0x4          /* Must move EPC forward by +4 */
+  REGSTORE   s1, REG_EPC(sp)      /* Updated EPC to user context */
+
+  csrr       tp, CSR_SCRATCH      /* Load kernel TP */
+  REGLOAD    tp, RISCV_PERCPU_TCB(tp)
+
+  call       x1, dispatch_syscall /* Dispatch the system call */
+
+return_from_syscall:
+
+  /* System call is done, disable interrupts */
+
+  csrc       CSR_STATUS, STATUS_IE
 
-  csrr       s0, CSR_EPC
-  REGSTORE   s0, REG_EPC(sp)      /* exception PC */
+  /* Clean up after system call */
+
+  REGSTORE   a0, REG_A0(sp)       /* Syscall return value to user context */
+  mv         a0, sp               /* Return to same context */
+  tail       return_from_exception
+
+handle_irq:
+#endif
 
   /* Setup arg0(exception cause), arg1(context) */
 
-  csrr       a0, CSR_CAUSE        /* exception cause */
+  mv         a0, s2               /* exception cause */
   mv         a1, sp               /* context = sp */
 
 #if CONFIG_ARCH_INTERRUPTSTACK > 15
@@ -152,9 +202,17 @@ return_from_exception:
   REGLOAD    s0, REG_EPC(sp)      /* restore sepc */
   csrw       CSR_EPC, s0
 
-  REGLOAD    s0, REG_INT_CTX(sp)  /* restore sstatus */
+  REGLOAD    s0, REG_INT_CTX(sp)  /* restore status */
   csrw       CSR_STATUS, s0
 
+#ifdef CONFIG_LIB_SYSCALL
+  /* Store tcb to scratch register */
+
+  call       x1, nxsched_self
+  csrr       s1, CSR_SCRATCH
+  REGSTORE   a0, RISCV_PERCPU_TCB(s1)
+#endif
+
 #ifdef CONFIG_ARCH_KERNEL_STACK
   /* Returning to userspace ? */
 

arch/risc-v/src/common/riscv_fork.c

@@ -246,29 +246,6 @@ pid_t riscv_fork(const struct fork_s *context)
   fregs[REG_FS11]               = context->fs11; /* Saved register fs11 */
 #endif
 
-#ifdef CONFIG_LIB_SYSCALL
-  /* If we got here via a syscall, then we are going to have to setup some
-   * syscall return information as well.
-   */
-
-  if (parent->xcp.nsyscalls > 0)
-    {
-      int index;
-      for (index = 0; index < parent->xcp.nsyscalls; index++)
-        {
-          child->cmn.xcp.syscall[index].sysreturn =
-            parent->xcp.syscall[index].sysreturn;
-
-#ifndef CONFIG_BUILD_FLAT
-          child->cmn.xcp.syscall[index].int_ctx =
-            parent->xcp.syscall[index].int_ctx;
-#endif
-        }
-
-      child->cmn.xcp.nsyscalls = parent->xcp.nsyscalls;
-    }
-#endif /* CONFIG_LIB_SYSCALL */
-
   /* And, finally, start the child task.  On a failure, nxtask_start_fork()
    * will discard the TCB by calling nxtask_abort_fork().
    */

arch/risc-v/src/common/riscv_internal.h

@@ -438,16 +438,6 @@ void *riscv_perform_syscall(uintptr_t *regs);
 #define riscv_switchcontext(prev, next) \
   sys_call2(SYS_switch_context, (uintptr_t)prev, (uintptr_t)next)
 
-#ifdef CONFIG_BUILD_KERNEL
-/* SYS call 3:
- *
- * void riscv_syscall_return(void);
- */
-
-#define riscv_syscall_return() sys_call0(SYS_syscall_return)
-
-#endif /* CONFIG_BUILD_KERNEL */
-
 #undef EXTERN
 #ifdef __cplusplus
 }