+/*
+ * Check how the syscall number gets loaded into %r20 within
+ * the delay branch in userspace and adjust as needed.
+ */
+
+static void check_syscallno_in_delay_branch(struct pt_regs *regs)
+{
+ u32 opcode, source_reg;
+ u32 __user *uaddr;
+ int err;
+
+ /* Usually we don't have to restore %r20 (the system call number)
+ * because it gets loaded in the delay slot of the branch external
+ * instruction via the ldi instruction.
+ * In some cases a register-to-register copy instruction might have
+ * been used instead, in which case we need to copy the syscall
+ * number into the source register before returning to userspace.
+ */
+
+ /* A syscall is just a branch, so all we have to do is fiddle the
+ * return pointer so that the ble instruction gets executed again.
+ */
+ regs->gr[31] -= 8; /* delayed branching */
+
+ /* Get assembler opcode of code in delay branch */
+ uaddr = (unsigned int *) ((regs->gr[31] & ~3) + 4);
+ err = get_user(opcode, uaddr);
+ if (err)
+ return;
+
+ /* Check if delay branch uses "ldi int,%r20" */
+ if ((opcode & 0xffff0000) == 0x34140000)
+ return; /* everything ok, just return */
+
+ /* Check if delay branch uses "nop" */
+ if (opcode == INSN_NOP)
+ return;
+
+ /* Check if delay branch uses "copy %rX,%r20" */
+ if ((opcode & 0xffe0ffff) == 0x08000254) {
+ source_reg = (opcode >> 16) & 31;
+ regs->gr[source_reg] = regs->gr[20];
+ return;
+ }
+
+ pr_warn("syscall restart: %s (pid %d): unexpected opcode 0x%08x\n",
+ current->comm, task_pid_nr(current), opcode);
+}
+