[PATCH 1/3] arm64: adjust el0_sync so that a function can be called

[+ Mark R. since we looked at this together a little at ELC]

Catalin Marinas [off-list ref] writes:

On Sun, Apr 27, 2014 at 08:44:12PM +0100, Larry Bassel wrote:

quoted

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 39ac630..eda7755 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S

[...]

quoted

@@ -421,28 +421,30 @@ el0_da:
 	/*
 	 * Data abort handling
 	 */
-	mrs	x0, far_el1
-	bic	x0, x0, #(0xff << 56)
 	disable_step x1
 	isb
 	enable_dbg
 	// enable interrupts before calling the main handler
 	enable_irq
+	mrs	x0, far_el1
+	bic	x0, x0, #(0xff << 56)
 	mov	x1, x25
 	mov	x2, sp

quoted

 	b	do_mem_abort

Reading the far_el1 after enable_dbg and enable_irq is racy, we can no
longer guarantee its value in the original data abort context.

Catalin, can you confirm x26 would be a safe place to stash far_el1 and
then restore it after ct_user_exit.  Something like this (which seems to
work for me):

el0_da:
        /*                                                                                                                                                    * Data abort handling                                                                                                                                */
        mrs     x26, far_el1
        disable_step x1
        isb
        enable_dbg
        // enable interrupts before calling the main handler                                                                                          
        enable_irq
        ct_user_exit
        mov     x0, x26
        bic     x0, x0, #(0xff << 56)
        mov     x1, x25
        mov     x2, sp
        adr     lr, ret_from_exception
        b       do_mem_abort

Kevin