accel/tcg: Allow the second page of an instruction to be MMIO

If an instruction straddles a page boundary, and the first page
was ram, but the second page was MMIO, we would abort.  Handle
this as if both pages are MMIO, by setting the ram_addr_t for
the first page to -1.

Reported-by: Sid Manning <sidneym@quicinc.com>
Reported-by: Jørgen Hansen <Jorgen.Hansen@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/translator.c

@@ -176,8 +176,16 @@ static void *translator_access(CPUArchState *env, DisasContextBase *db,
         if (host == NULL) {
             tb_page_addr_t phys_page =
                 get_page_addr_code_hostp(env, base, &db->host_addr[1]);
-            /* We cannot handle MMIO as second page. */
-            assert(phys_page != -1);
+
+            /*
+             * If the second page is MMIO, treat as if the first page
+             * was MMIO as well, so that we do not cache the TB.
+             */
+            if (unlikely(phys_page == -1)) {
+                tb_set_page_addr0(tb, -1);
+                return NULL;
+            }
+
             tb_set_page_addr1(tb, phys_page);
 #ifdef CONFIG_USER_ONLY
             page_protect(end);