linux-kernel - [PATCH 3.16 159/233] arm64: KVM: Allow unaligned accesses at EL2

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Date:   Sat, 09 Sep 2017 22:47:15 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Christoffer Dall" <cdall@...aro.org>,
        "Marc Zyngier" <marc.zyngier@....com>,
        "Alexander Graf" <agraf@...e.de>
Subject: [PATCH 3.16 159/233] arm64: KVM: Allow unaligned accesses at EL2

3.16.48-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Marc Zyngier <marc.zyngier@....com>

commit 78fd6dcf11468a5a131b8365580d0c613bcc02cb 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@...e.de>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
Signed-off-by: Christoffer Dall <cdall@...aro.org>
[bwh: Backported to 3.16: s/ELx/EL2/]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arm64/kvm/hyp-init.S | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/arch/arm64/kvm/hyp-init.S
+++ b/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