tcg: Clean up direct block chaining safety checks

We don't take care of direct jumps when address mapping changes. Thus we
must be sure to generate direct jumps so that they always keep valid
even if address mapping changes. Luckily, we can only allow to execute a
TB if it was generated from the pages which match with current mapping.

Document tcg_gen_goto_tb() declaration and note the reason for
destination PC limitations.

Some targets with variable length instructions allow TB to straddle a
page boundary. However, we make sure that both of TB pages match the
current address mapping when looking up TBs. So it is safe to do direct
jumps into the both pages. Correct the checks for some of those targets.

Given that, we can safely patch a TB which spans two pages. Remove the
unnecessary check in cpu_exec() and allow such TBs to be patched.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

1 files changed, 1 insertions, 1 deletions

diff --git a/target-s390x/translate.c b/target-s390x/translate.c
index c871ef2bb3..c5179fe05d 100644
--- a/target-s390x/translate.c
+++ b/target-s390x/translate.c
@@ -610,7 +610,7 @@ static int use_goto_tb(DisasContext *s, uint64_t dest)
 {
     /* NOTE: we handle the case where the TB spans two pages here */
     return (((dest & TARGET_PAGE_MASK) == (s->tb->pc & TARGET_PAGE_MASK)
-             || (dest & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK))
+             || (dest & TARGET_PAGE_MASK) == (s->pc & TARGET_PAGE_MASK))
             && !s->singlestep_enabled
             && !(s->tb->cflags & CF_LAST_IO)
             && !(s->tb->flags & FLAG_MASK_PER));