arm/gic: Detect broken configurations

Some virtualization platforms provide broken configurations. There
is a GIC interrupt controller, however accessing the CPU interface
registers leads to an external data abort. As these are needed to
handle interrupts we are unable to boot further.

Detect this misconfiguration and panic to tell the user the issue.

Reviewed by:	emaste
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D54832

sys/arm/arm/gic.c

@@ -151,6 +151,8 @@ static struct arm_gic_softc *gic_sc = NULL;
 /* CPU Interface */
 #define	gic_c_read_4(_sc, _reg)		\
     bus_read_4((_sc)->gic_res[GIC_RES_CPU], (_reg))
+#define	gic_c_peek_4(_sc, _reg, _val)	\
+    bus_peek_4((_sc)->gic_res[GIC_RES_CPU], (_reg), (_val))
 #define	gic_c_write_4(_sc, _reg, _val)		\
     bus_write_4((_sc)->gic_res[GIC_RES_CPU], (_reg), (_val))
 /* Distributor Interface */
@@ -347,7 +349,18 @@ arm_gic_attach(device_t dev)
 		goto cleanup;
 	}
 
-	icciidr = gic_c_read_4(sc, GICC_IIDR);
+	/*
+	 * Try accessing a CPU interface register. On some broken
+	 * virtualization environments this will raise an external
+	 * data abort. When this happens we can detect it using
+	 * by peeking at the register & checking for the fault.
+	 * As there is no way to continue with a normal boot we
+	 * panic.
+	 */
+	if (gic_c_peek_4(sc, GICC_IIDR, &icciidr) != 0)
+		panic("Unable to access %s CPU registers, "
+		    "broken hardware or hypervisor configuration",
+		    device_get_nameunit(dev));
 	device_printf(dev,
 	    "pn 0x%x, arch 0x%x, rev 0x%x, implementer 0x%x irqs %u\n",
 	    GICD_IIDR_PROD(icciidr), GICD_IIDR_VAR(icciidr),