[Qemu-devel] [PATCH] target-arm/translate.c: Handle non-executable page-straddling Thumb insns

[Peter Maydell]

When the memory we're trying to translate code from is not executable we have
to turn this into a guest fault. In order to report the correct PC for this
fault, and to make sure it is not reported until after any other possible
faults for instructions earlier in execution, we must terminate TBs at
the end of a page, in case the next instruction is in a non-executable page.
This is simple for T16, A32 and A64 instructions, which are always aligned
to their size. However T32 instructions may be 32-bits but only 16-aligned,
so they can straddle a page boundary.

Correct the condition that checks whether the next instruction will touch
the following page, to ensure that if we're 2 bytes before the boundary
and this insn is T32 then we end the TB.

Reported-by: Pavel Dovgalyuk <address@hidden>

Signed-off-by: Peter Maydell <address@hidden>
---
The other way you could do this would be to check before each 'read halfword'
in the decoder whether you were going to go off the end of the page, and if
so roll back anything you'd already generated, but that sounds really painful.
I'm glad I don't have to fix this bug for x86 :-)


 target-arm/translate.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/target-arm/translate.c b/target-arm/translate.c
index 84a21ac..d5cfe84 100644
--- a/target-arm/translate.c
+++ b/target-arm/translate.c
@@ -11167,6 +11167,38 @@ undef:
                        default_exception_el(s));
 }
 
+static bool insn_crosses_page(CPUARMState *env, DisasContext *s)
+{
+    /* Return true if the insn at dc->pc might cross a page boundary.
+     * (False positives are OK, false negatives are not.)
+     */
+    uint16_t insn;
+
+    if ((s->pc & 3) == 0) {
+        /* At a 4-aligned address we can't be crossing a page */
+        return false;
+    }
+
+    /* This must be a Thumb insn */
+    insn = arm_lduw_code(env, s->pc, s->bswap_code);
+
+    switch (insn >> 11) {
+    case 0x1d: /* 0b11101 */
+    case 0x1e: /* 0b11110 */
+    case 0x1f: /* 0b11111 */
+        /* First half of a 32-bit Thumb insn. Thumb-1 cores might
+         * end up actually treating this as two 16-bit insns (see the
+         * code at the start of disas_thumb2_insn()) but we don't bother
+         * to check for that as it is unlikely, and false positives here
+         * are harmless.
+         */
+        return true;
+    default:
+        /* 16-bit Thumb insn */
+        return false;
+    }
+}
+
 /* generate intermediate code in gen_opc_buf and gen_opparam_buf for
    basic block 'tb'. If search_pc is TRUE, also generate PC
    information for each intermediate instruction. */
@@ -11183,6 +11215,7 @@ static inline void 
gen_intermediate_code_internal(ARMCPU *cpu,
     target_ulong next_page_start;
     int num_insns;
     int max_insns;
+    bool end_of_page;
 
     /* generate intermediate code */
 
@@ -11404,11 +11437,24 @@ static inline void 
gen_intermediate_code_internal(ARMCPU *cpu,
          * Also stop translation when a page boundary is reached.  This
          * ensures prefetch aborts occur at the right place.  */
         num_insns ++;
+
+        /* We want to stop the TB if the next insn starts in a new page,
+         * or if it spans between this page and the next. This means that
+         * if we're looking at the last halfword in the page we need to
+         * see if it's a 16-bit Thumb insn (which will fit in this TB)
+         * or a 32-bit Thumb insn (which won't).
+         * This is to avoid generating a silly TB with a single 16-bit insn
+         * in it at the end of this page (which would execute correctly
+         * but isn't very efficient).
+         */
+        end_of_page = (dc->pc >= next_page_start) ||
+            ((dc->pc >= next_page_start - 3) && insn_crosses_page(env, dc));
+
     } while (!dc->is_jmp && !tcg_op_buf_full() &&
              !cs->singlestep_enabled &&
              !singlestep &&
              !dc->ss_active &&
-             dc->pc < next_page_start &&
+             !end_of_page &&
              num_insns < max_insns);
 
     if (tb->cflags & CF_LAST_IO) {
-- 
1.9.1