arm64: kvm: Move lr save/restore from do_el2_call into EL1

Today the 'hvc' calling KVM or the hyp-stub is expected to preserve all
registers. KVM saves/restores the registers it needs on the EL2 stack using
do_el2_call(). The hyp-stub has no stack, later patches need to be able to
be able to clobber the link register.

Move the link register save/restore to the the call sites.

Signed-off-by: James Morse <[e-mail]>
Acked-by: Marc Zyngier <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/kernel/hyp-stub.S

···
  */
 
 ENTRY(__hyp_get_vectors)
+	str	lr, [sp, #-16]!
 	mov	x0, xzr
-	// fall through
-ENTRY(__hyp_set_vectors)
 	hvc	#0
+	ldr	lr, [sp], #16
 	ret
 ENDPROC(__hyp_get_vectors)
+
+ENTRY(__hyp_set_vectors)
+	str	lr, [sp, #-16]!
+	hvc	#0
+	ldr	lr, [sp], #16
+	ret
 ENDPROC(__hyp_set_vectors)

arch/arm64/kvm/hyp.S

···
  * A function pointer with a value of 0 has a special meaning, and is
  * used to implement __hyp_get_vectors in the same way as in
  * arch/arm64/kernel/hyp_stub.S.
+ * HVC behaves as a 'bl' call and will clobber lr.
  */
 ENTRY(__kvm_call_hyp)
-alternative_if_not ARM64_HAS_VIRT_HOST_EXTN	
+alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
+	str     lr, [sp, #-16]!
 	hvc	#0
+	ldr     lr, [sp], #16
 	ret
 alternative_else
 	b	__vhe_hyp_call
+	nop
+	nop
 	nop
 alternative_endif
 ENDPROC(__kvm_call_hyp)

arch/arm64/kvm/hyp/hyp-entry.S

···
 	 * Shuffle the parameters before calling the function
 	 * pointed to in x0. Assumes parameters in x[1,2,3].
 	 */
-	sub	sp, sp, #16
-	str	lr, [sp]
 	mov	lr, x0
 	mov	x0, x1
 	mov	x1, x2
 	mov	x2, x3
 	blr	lr
-	ldr	lr, [sp]
-	add	sp, sp, #16
 .endm
 
 ENTRY(__vhe_hyp_call)
+	str	lr, [sp, #-16]!
 	do_el2_call
+	ldr	lr, [sp], #16
 	/*
 	 * We used to rely on having an exception return to get
 	 * an implicit isb. In the E2H case, we don't have it anymore.