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

Hi,

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

The KASLR processing in __enable_mmu() is only used by the primary boot
path, and complements the processing that takes place in __primary_switch().
Move the two parts together, to make the code easier to understand.

As a heads-up, while reviewing this I spotted an existing issue [1]. I'd meant
to comment so when posting that patch, but in my hubris from making
git-send-email work I forgot to do so. :/

[...]

Unrelated change? Perhaps it's worth putting all the whitespace fixup in a
preparatory patch?

[...]

+__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

[...]

+	/*
+	 * If we return here, we have a KASLR displacement in x23 which we need
+	 * to take into account by discarding the current kernel mapping and
+	 * creating a new one.
+	 */
+	msr	sctlr_el1, x20			// disable the MMU
+	isb
+	bl	__create_page_tables		// recreate kernel mapping

As per the issue I mentioned above [1], here we also need:

	tlbi	vmalle1
	dsb	nsh

... in order to avoid TLB conflicts and other issues resulting from BBM
violations.

+
+	msr	sctlr_el1, x19			// re-enable the MMU
+	isb
+	ic	iallu				// flush instructions fetched
+	dsb	nsh				// via old mapping
+	isb

Thanks,
Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/451294.html