KVM: arm64: Avoid repetitive stack access on host EL1 to EL2 exception

Registers x0/x1 get repeateadly pushed and poped during a host
HVC call. Instead, leave the registers on the stack, trading
a store instruction on the fast path for an add on the slow path.
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/hyp/nvhe/host.S

@@ -13,8 +13,6 @@
 	.text
 
 SYM_FUNC_START(__host_exit)
-	stp	x0, x1, [sp, #-16]!
-
 	get_host_ctxt	x0, x1
 
 	/* Store the host regs x2 and x3 */
@@ -99,13 +97,15 @@ SYM_FUNC_END(__hyp_do_panic)
 	mrs	x0, esr_el2
 	lsr	x0, x0, #ESR_ELx_EC_SHIFT
 	cmp	x0, #ESR_ELx_EC_HVC64
-	ldp	x0, x1, [sp], #16
 	b.ne	__host_exit
 
+	ldp	x0, x1, [sp]		// Don't fixup the stack yet
+
 	/* Check for a stub HVC call */
 	cmp	x0, #HVC_STUB_HCALL_NR
 	b.hs	__host_exit
 
+	add	sp, sp, #16
 	/*
 	 * Compute the idmap address of __kvm_handle_stub_hvc and
 	 * jump there. Since we use kimage_voffset, do not use the