[Qemu-devel] [PATCH v5 09/10] tcg: Clean up direct block chaining safety checks

[Sergey Fedorov]

From: Sergey Fedorov <address@hidden>

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 <address@hidden>
Signed-off-by: Sergey Fedorov <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
---

Changes in v4:
 * Documented tcg_gen_goto_tb() declaration
 * Destination PC check explanatory comments moved to tcg_gen_goto_tb()
   declaration comment
 * Updated commit message

 cpu-exec.c               |  7 ++-----
 target-arm/translate.c   |  3 ++-
 target-cris/translate.c  |  4 +++-
 target-i386/translate.c  |  2 +-
 target-m68k/translate.c  |  2 +-
 target-s390x/translate.c |  2 +-
 tcg/tcg-op.h             | 10 ++++++++++
 7 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/cpu-exec.c b/cpu-exec.c
index debc65ca691a..f984dc71cbf1 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -508,11 +508,8 @@ int cpu_exec(CPUState *cpu)
                     next_tb = 0;
                     tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
                 }
-                /* see if we can patch the calling TB. When the TB
-                   spans two pages, we cannot safely do a direct
-                   jump. */
-                if (next_tb != 0 && tb->page_addr[1] == -1
-                    && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+                /* See if we can patch the calling TB. */
+                if (next_tb != 0 && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
                     tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
                                 next_tb & TB_EXIT_MASK, tb);
                 }
diff --git a/target-arm/translate.c b/target-arm/translate.c
index 940ec8d981d1..34196a821772 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -4054,7 +4054,8 @@ static inline void gen_goto_tb(DisasContext *s, int n, 
target_ulong dest)
     TranslationBlock *tb;
 
     tb = s->tb;
-    if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
+    if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK) ||
+        ((s->pc - 1) & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
         tcg_gen_goto_tb(n);
         gen_set_pc_im(s, dest);
         tcg_gen_exit_tb((uintptr_t)tb + n);
diff --git a/target-cris/translate.c b/target-cris/translate.c
index a73176c118b0..9c8ff8f2308a 100644
--- a/target-cris/translate.c
+++ b/target-cris/translate.c
@@ -524,7 +524,9 @@ static void gen_goto_tb(DisasContext *dc, int n, 
target_ulong dest)
 {
     TranslationBlock *tb;
     tb = dc->tb;
-    if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
+
+    if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK) ||
+        (dc->ppc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
         tcg_gen_goto_tb(n);
         tcg_gen_movi_tl(env_pc, dest);
                 tcg_gen_exit_tb((uintptr_t)tb + n);
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 3a32f65f8de8..058d85a1abf7 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -2094,7 +2094,7 @@ static inline void gen_goto_tb(DisasContext *s, int 
tb_num, target_ulong eip)
     tb = s->tb;
     /* NOTE: we handle the case where the TB spans two pages here */
     if ((pc & TARGET_PAGE_MASK) == (tb->pc & TARGET_PAGE_MASK) ||
-        (pc & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK))  {
+        (pc & TARGET_PAGE_MASK) == (s->pc_start & TARGET_PAGE_MASK))  {
         /* jump to same page: we can use a direct jump */
         tcg_gen_goto_tb(tb_num);
         gen_jmp_im(eip);
diff --git a/target-m68k/translate.c b/target-m68k/translate.c
index 7560c3a80841..e2ce6c615e07 100644
--- a/target-m68k/translate.c
+++ b/target-m68k/translate.c
@@ -861,7 +861,7 @@ static void gen_jmp_tb(DisasContext *s, int n, uint32_t 
dest)
     if (unlikely(s->singlestep_enabled)) {
         gen_exception(s, dest, EXCP_DEBUG);
     } else if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK) ||
-               (s->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
+               (s->insn_pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
         tcg_gen_goto_tb(n);
         tcg_gen_movi_i32(QREG_PC, dest);
         tcg_gen_exit_tb((uintptr_t)tb + n);
diff --git a/target-s390x/translate.c b/target-s390x/translate.c
index c871ef2bb302..c5179fe05d7e 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));
diff --git a/tcg/tcg-op.h b/tcg/tcg-op.h
index c446d3dc7293..ace39619ef89 100644
--- a/tcg/tcg-op.h
+++ b/tcg/tcg-op.h
@@ -753,6 +753,16 @@ static inline void tcg_gen_exit_tb(uintptr_t val)
     tcg_gen_op1i(INDEX_op_exit_tb, val);
 }
 
+/**
+ * tcg_gen_goto_tb() - output goto_tb TCG operation
+ * @idx: Direct jump slot index (0 or 1)
+ *
+ * See tcg/README for more info about this TCG operation.
+ *
+ * NOTE: Direct jumps with goto_tb are only safe within the pages this TB
+ * resides in because we don't take care of direct jumps when address mapping
+ * changes, e.g. in tlb_flush().
+ */
 void tcg_gen_goto_tb(unsigned idx);
 
 #if TARGET_LONG_BITS == 32
-- 
2.8.1