accel/tcg: Document the faulting lookup in tb_lookup_cmp

It was non-obvious to me why we can raise an exception in
the middle of a comparison function, but it works.
While nearby, use TARGET_PAGE_ALIGN instead of open-coding.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 7887af6f4578b80c0fa26903a842c92375ff3d60..5f43b9769ac71cfb1f58a20b8eb2fac193442723 100644 (file)
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -198,7 +198,16 @@ static bool tb_lookup_cmp(const void *p, const void *d)
             tb_page_addr_t phys_page2;
             target_ulong virt_page2;
 
-            virt_page2 = (desc->pc & TARGET_PAGE_MASK) + TARGET_PAGE_SIZE;
+            /*
+             * We know that the first page matched, and an otherwise valid TB
+             * encountered an incomplete instruction at the end of that page,
+             * therefore we know that generating a new TB from the current PC
+             * must also require reading from the next page -- even if the
+             * second pages do not match, and therefore the resulting insn
+             * is different for the new TB.  Therefore any exception raised
+             * here by the faulting lookup is not premature.
+             */
+            virt_page2 = TARGET_PAGE_ALIGN(desc->pc);
             phys_page2 = get_page_addr_code(desc->env, virt_page2);
             if (tb->page_addr[1] == phys_page2) {
                 return true;