feat(hcx): add build option to enable FEAT_HCX

FEAT_HCX adds the extended hypervisor configuration register (HCRX_EL2)
and access to this register must be explicitly enabled through the
SCR_EL3.HXEn bit.  This patch adds a new build flag ENABLE_FEAT_HCX to
allow the register to be accessed from EL2.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ibb36ad90622f1dc857adab4b0d4d7a89456a522b
diff --git a/lib/el3_runtime/aarch64/context.S b/lib/el3_runtime/aarch64/context.S
index 40e7ddf..e270ad0 100644
--- a/lib/el3_runtime/aarch64/context.S
+++ b/lib/el3_runtime/aarch64/context.S
@@ -193,6 +193,11 @@
 	str	x13, [x0, #CTX_SCXTNUM_EL2]
 #endif
 
+#if ENABLE_FEAT_HCX
+	mrs	x14, hcrx_el2
+	str	x14, [x0, #CTX_HCRX_EL2]
+#endif
+
 	ret
 endfunc el2_sysregs_context_save
 
@@ -362,6 +367,11 @@
 	msr	scxtnum_el2, x13
 #endif
 
+#if ENABLE_FEAT_HCX
+	ldr	x14, [x0, #CTX_HCRX_EL2]
+	msr	hcrx_el2, x14
+#endif
+
 	ret
 endfunc el2_sysregs_context_restore
 
diff --git a/lib/el3_runtime/aarch64/context_mgmt.c b/lib/el3_runtime/aarch64/context_mgmt.c
index 52102dd..08022d4 100644
--- a/lib/el3_runtime/aarch64/context_mgmt.c
+++ b/lib/el3_runtime/aarch64/context_mgmt.c
@@ -112,6 +112,14 @@
 	if (EP_GET_ST(ep->h.attr) != 0U)
 		scr_el3 |= SCR_ST_BIT;
 
+	/*
+	 * If FEAT_HCX is enabled, enable access to HCRX_EL2 by setting
+	 * SCR_EL3.HXEn.
+	 */
+#if ENABLE_FEAT_HCX
+	scr_el3 |= SCR_HXEn_BIT;
+#endif
+
 #if RAS_TRAP_LOWER_EL_ERR_ACCESS
 	/*
 	 * SCR_EL3.TERR: Trap Error record accesses. Accesses to the RAS ERR