xtensa: userspace: swap page tables at context restore

Swap page tables at exit of exception handler if we are going to
be restored to another thread context. Or else we would be using
the outgoing thread's page tables which is not going to work
correctly due to mapping and permissions.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

arch/xtensa/include/xtensa-asm2-s.h

@@ -507,6 +507,8 @@ _do_call_\@:
 	 * spills to the right place.
 	 */
 	beq a6, a1, _restore_\@
+
+#ifndef CONFIG_USERSPACE
 	l32i a1, a1, 0
 	l32i a0, a1, ___xtensa_irq_bsa_t_a0_OFFSET
 	addi a1, a1, ___xtensa_irq_bsa_t_SIZEOF
@@ -516,7 +518,37 @@ _do_call_\@:
 	 */
 	SPILL_ALL_WINDOWS
 #endif
+
+	/* Restore A1 stack pointer from "next" handle. */
 	mov a1, a6
+#else
+	/* With userspace, we cannot simply restore A1 stack pointer
+	 * at this pointer because we need to swap page tables to
+	 * the incoming thread, and we do not want to call that
+	 * function with thread's stack. So we stash the new stack
+	 * pointer into A2 first, then move it to A1 after we have
+	 * swapped the page table.
+	 */
+	mov a2, a6
+
+	/* Need to switch page tables because the "next" handle
+	 * returned above is not the same handle as we started
+	 * with. This means we are being restored to another
+	 * thread.
+	 */
+	rsr a6, ZSR_CPU
+	l32i a6, a6, ___cpu_t_current_OFFSET
+
+	call4 z_xtensa_swap_update_page_tables
+	l32i a1, a1, 0
+	l32i a0, a1, ___xtensa_irq_bsa_t_a0_OFFSET
+	addi a1, a1, ___xtensa_irq_bsa_t_SIZEOF
+
+	SPILL_ALL_WINDOWS
+
+	/* Moved stashed stack pointer to A1 to restore stack. */
+	mov a1, a2
+#endif
 
 _restore_\@:
 	j _restore_context