[PATCH v17 12/14] KVM: selftests: arm64: Only restore SPSR_EL1 and ELR_EL1 if they change

[Mark Brown]

When EXLOCK is enabled for ELx writes to SPSR_ELx and ELR_ELx generate
GCS exceptions. Currently the selftest helpers unconditionally reload the
saved values for these registers regardless of if the handler didn't change
them. This gets in the way of EXLOCK testing, making it hard to generate
contexts which will generate an invalid ERET since we never get as far as
doing an exception return while EXLOCK is enabled.

In order to facilitate testing of nested virtualisation emulation of EXLOCK
change our reloads into read/modify/write cycles, suppressing the writes
when they would not change the value in the register.

Signed-off-by: Mark Brown <broonie@xxxxxxxxxx>
---
 tools/testing/selftests/kvm/lib/arm64/handlers.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/testing/selftests/kvm/lib/arm64/handlers.S b/tools/testing/selftests/kvm/lib/arm64/handlers.S
index 0e443eadfac6..367c22d61cba 100644
--- a/tools/testing/selftests/kvm/lib/arm64/handlers.S
+++ b/tools/testing/selftests/kvm/lib/arm64/handlers.S
@@ -33,8 +33,18 @@
 
 .macro restore_registers
 	ldp	x1, x2, [sp, #16 * 16] /* PC, PSTATE */
+
+	/* Suppress redundant writes to support GCS exception lock testing */
+	mrs	x3, elr_el1
+	cmp	x1, x3
+	beq	.Lskip_elr_\@
 	msr	elr_el1, x1
+.Lskip_elr_\@:
+	mrs	x3, spsr_el1
+	cmp	x2, x3
+	beq	.Lskip_spsr_\@
 	msr	spsr_el1, x2
+.Lskip_spsr_\@:
 
 	/* sp is not restored */
 	ldp	x30, xzr, [sp, #16 * 15] /* x30, SP */

-- 
2.47.3