powerpc: Add a proper syscall for switching endianness

We currently have a "special" syscall for switching endianness. This is
syscall number 0x1ebe, which is handled explicitly in the 64-bit syscall
exception entry.

That has a few problems, firstly the syscall number is outside of the
usual range, which confuses various tools. For example strace doesn't
recognise the syscall at all.

Secondly it's handled explicitly as a special case in the syscall
exception entry, which is complicated enough without it.

As a first step toward removing the special syscall, we need to add a
regular syscall that implements the same functionality.

The logic is simple, it simply toggles the MSR_LE bit in the userspace
MSR. This is the same as the special syscall, with the caveat that the
special syscall clobbers fewer registers.

This version clobbers r9-r12, XER, CTR, and CR0-1,5-7.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/syscalls.c

@@ -121,3 +121,20 @@ long ppc_fadvise64_64(int fd, int advice, u32 offset_high, u32 offset_low,
 	return sys_fadvise64(fd, (u64)offset_high << 32 | offset_low,
 			     (u64)len_high << 32 | len_low, advice);
 }
+
+long sys_switch_endian(void)
+{
+	struct thread_info *ti;
+
+	current->thread.regs->msr ^= MSR_LE;
+
+	/*
+	 * Set TIF_RESTOREALL so that r3 isn't clobbered on return to
+	 * userspace. That also has the effect of restoring the non-volatile
+	 * GPRs, so we saved them on the way in here.
+	 */
+	ti = current_thread_info();
+	ti->flags |= _TIF_RESTOREALL;
+
+	return 0;
+}