signal: add support for restartable system calls on RISC-V64

Author: eatvector

components/lwp/arch/risc-v/rv64/lwp_arch.c

@@ -258,14 +258,48 @@ struct signal_ucontext
     struct rt_hw_stack_frame frame;
 };
 
-void *arch_signal_ucontext_restore(rt_base_t user_sp)
+void arch_syscall_restart(void *sp, void *ksp);
+
+void arch_signal_check_erestart(void *eframe, void *ksp)
+{
+    struct rt_hw_stack_frame *exp_frame = eframe;
+    long rc = exp_frame->a0;
+    long sys_id = exp_frame->a7;
+
+    (void)sys_id;
+
+    if (rc == -ERESTART)
+    {
+        LOG_D("%s(rc=%ld,sys_id=%ld,pid=%d)", __func__, rc, sys_id, lwp_self()->pid);
+        LOG_D("%s: restart rc = %ld", lwp_get_syscall_name(sys_id), rc);
+
+        /* t0 stores the copy of user's first syscall argument */
+        exp_frame->a0 = exp_frame->t0;
+        /* adjust for epc auto-increment in syscall_handler */
+        exp_frame->epc -= 4;
+        /* copy exception frame from user stack to kernel stack */
+        lwp_memcpy(ksp - sizeof(*exp_frame),  exp_frame, sizeof(*exp_frame));
+
+        arch_syscall_restart(exp_frame, ksp);
+    }
+
+    return;
+}
+
+static void arch_signal_post_action(struct signal_ucontext *new_sp, rt_base_t kernel_sp)
+{
+    arch_signal_check_erestart(&new_sp->frame, (void *)kernel_sp);
+}
+
+void *arch_signal_ucontext_restore(rt_base_t user_sp, rt_base_t kernel_sp)
 {
     struct signal_ucontext *new_sp;
     new_sp = (void *)user_sp;
 
     if (lwp_user_accessable(new_sp, sizeof(*new_sp)))
     {
         lwp_thread_signal_mask(rt_thread_self(), LWP_SIG_MASK_CMD_SET_MASK, &new_sp->save_sigmask, RT_NULL);
+        arch_signal_post_action(new_sp, kernel_sp);
     }
     else
     {
@@ -320,8 +354,12 @@ void *arch_signal_ucontext_save(int signo, siginfo_t *psiginfo,
 
 void arch_syscall_set_errno(void *eframe, int expected, int code)
 {
-    /* NO support */
-    return ;
+    struct rt_hw_stack_frame *exp_frame = eframe;
+
+    if (exp_frame->a0 == -expected)
+    {
+        exp_frame->a0 = -code;
+    }
 }
 
 /**
@@ -354,4 +392,4 @@ int arch_backtrace_uthread(rt_thread_t thread)
             return -1;
     }
     return -1;
-}
+}

components/lwp/arch/risc-v/rv64/lwp_arch.h

@@ -69,7 +69,7 @@ rt_inline void icache_invalid_all(void)
 }
 
 struct rt_hw_stack_frame;
-void *arch_signal_ucontext_restore(rt_base_t user_sp);
+void *arch_signal_ucontext_restore(rt_base_t user_sp, rt_base_t kernel_sp);
 void *arch_signal_ucontext_save(int signo, siginfo_t *psiginfo,
                                 struct rt_hw_stack_frame *exp_frame, rt_base_t user_sp,
                                 lwp_sigset_t *save_sig_mask);
@@ -79,4 +79,4 @@ void *arch_signal_ucontext_save(int signo, siginfo_t *psiginfo,
 
 #endif
 
-#endif  /*LWP_ARCH_H__*/
+#endif  /*LWP_ARCH_H__*/

components/lwp/arch/risc-v/rv64/lwp_gcc.S

@@ -98,12 +98,28 @@ ret_to_user_exit:
     // `RESTORE_ALL` also reset sp to user sp, and setup sscratch
     sret
 
+/*
+ * void void arch_syscall_restart(void *sp, void *ksp);
+ */
+.global arch_syscall_restart
+arch_syscall_restart:
+    addi a1, a1, -CTX_REG_NR * REGBYTES
+
+    /* set stack pointer to saved user exception frame on kernel stack */
+    mv sp, a1
+
+    /* restore all user registers, and setup sscratch to point to ksp */
+    RESTORE_ALL
+
+    j trap_entry
+
 /**
  * Restore user context from exception frame stroraged in ustack
  * And handle pending signals;
  */
 arch_signal_quit:
     LOAD a0, FRAME_OFF_SP(sp)
+    addi a1, sp, CTX_REG_NR * REGBYTES
     call arch_signal_ucontext_restore
 
     /* reset kernel sp to the stack */

libcpu/risc-v/common64/syscall_c.c

@@ -54,9 +54,13 @@ void syscall_handler(struct rt_hw_stack_frame *regs)
 
     LOG_I("[0x%lx] %s(0x%lx, 0x%lx, 0x%lx, 0x%lx, 0x%lx, 0x%lx, 0x%lx)", rt_thread_self(), syscall_name,
         regs->a0, regs->a1, regs->a2, regs->a3, regs->a4, regs->a5, regs->a6);
+    /* Save original a0 (1st argument) in kernel-stored t0 context:
+     * - a0 serves as both system call's 1st argument (input) and return value (output)
+     * - t0 is caller-saved, user code must preserve it before ecall if needed
+     */
+    regs->t0 = regs->a0;
     regs->a0 = syscallfunc(regs->a0, regs->a1, regs->a2, regs->a3, regs->a4, regs->a5, regs->a6);
-    regs->a7 = 0;
     regs->epc += 4; // skip ecall instruction
     LOG_I("[0x%lx] %s ret: 0x%lx", rt_thread_self(), syscall_name, regs->a0);
 }
-#endif /* RT_USING_SMART */
+#endif /* RT_USING_SMART */