arm64: KVM: Allow unaligned accesses at EL2

commit 78fd6dcf upstream.

We currently have the SCTLR_EL2.A bit set, trapping unaligned accesses
at EL2, but we're not really prepared to deal with it. So far, this
has been unnoticed, until GCC 7 started emitting those (in particular
64bit writes on a 32bit boundary).

Since the rest of the kernel is pretty happy about that, let's follow
its example and set SCTLR_EL2.A to zero. Modern CPUs don't really
care.
Reported-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>
[bwh: Backported to 3.16: s/ELx/EL2/]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/arm64/kvm/hyp-init.S

@@ -86,9 +86,10 @@ __do_hyp_init:
 
 	/*
 	 * Preserve all the RES1 bits while setting the default flags,
-	 * as well as the EE bit on BE.
+	 * as well as the EE bit on BE. Drop the A flag since the compiler
+	 * is allowed to generate unaligned accesses.
 	 */
-	ldr	x4, =(SCTLR_EL2_RES1 | SCTLR_EL2_FLAGS)
+	ldr	x4, =(SCTLR_EL2_RES1 | (SCTLR_EL2_FLAGS & ~SCTLR_EL2_A))
 CPU_BE(	orr	x4, x4, #SCTLR_EL2_EE)
 	msr	sctlr_el2, x4
 	isb