hw/intc/arm_gicv3_its: Provide read accessor for translation_ops

The MemoryRegionOps gicv3_its_translation_ops currently provides only
a .write_with_attrs function, because the only register in this
region is the write-only GITS_TRANSLATER.  However, if you don't
provide a read function and the guest tries reading from this memory
region, QEMU will crash because
memory_region_read_with_attrs_accessor() calls a NULL pointer.

Add a read function which always returns 0, to cover both bogus
attempts to read GITS_TRANSLATER and also reads from the rest of the
region, which is documented to be reserved, RES0.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220122182444.724087-11-peter.maydell@linaro.org

hw/intc/arm_gicv3_its.c

@@ -813,6 +813,18 @@ static void extract_cmdq_params(GICv3ITSState *s)
     }
 }
 
+static MemTxResult gicv3_its_translation_read(void *opaque, hwaddr offset,
+                                              uint64_t *data, unsigned size,
+                                              MemTxAttrs attrs)
+{
+    /*
+     * GITS_TRANSLATER is write-only, and all other addresses
+     * in the interrupt translation space frame are RES0.
+     */
+    *data = 0;
+    return MEMTX_OK;
+}
+
 static MemTxResult gicv3_its_translation_write(void *opaque, hwaddr offset,
                                                uint64_t data, unsigned size,
                                                MemTxAttrs attrs)
@@ -1168,6 +1180,7 @@ static const MemoryRegionOps gicv3_its_control_ops = {
 };
 
 static const MemoryRegionOps gicv3_its_translation_ops = {
+    .read_with_attrs = gicv3_its_translation_read,
     .write_with_attrs = gicv3_its_translation_write,
     .valid.min_access_size = 2,
     .valid.max_access_size = 4,