[PATCH v2 4/9] arm64: head.S: move KASLR processing out of __enable_mmu()

On 24 August 2016 at 21:46, Mark Rutland [off-list ref] wrote:

On Wed, Aug 24, 2016 at 09:36:10PM +0100, Mark Rutland wrote:

quoted

On Wed, Aug 24, 2016 at 04:36:01PM +0200, Ard Biesheuvel wrote:

quoted

+__primary_switch:
+#ifdef CONFIG_RANDOMIZE_BASE
+   mov     x19, x0                         // preserve new SCTLR_EL1 value
+   mrs     x20, sctlr_el1                  // preserve old SCTLR_EL1 value
+#endif
+
+   adr     x27, 0f
+   b       __enable_mmu

As we do elsewhere, it's probably worth a comment on the line with the ADR into
x27, mentioning that __enable_mmu will branch there.

... or perhaps we should just have __enable_mmu return to the LR like a normal
AAPCS function, place the switch routines in the idmap, and use the idiomatic
sequence:

__thing_switch:
      bl      __enable_mmu
      ldr     xN, =__thing
      blr     xN

... and now I see that this is what subsequent patches do ;)

Is it possible to first AAPCS-ify __enable_mmu (with shuffling of callers as
above) in one patch, prior to this?

Yes, but that would result in an __enable_mmu() that needs to stash
the link register value, and essentially returns twice in the KASLR
case. As an intermediate step working towards the result after the
series, I think the adr + label above is the lesser evil

That would avoid introducing the unusual 0f
label above, and the temporary x30 usage in a subsequent patch.

Thanks,
Mark.