tcg: Return the TB pointer from the rx region from exit_tb

This produces a small pc-relative displacement within the
generated code to the TB structure that preceeds it.

Reviewed-by: Joelle van Dyne <j@getutm.app>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

accel/tcg/cpu-exec.c

@@ -154,13 +154,12 @@ static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
  * TCG is not considered a security-sensitive part of QEMU so this does not
  * affect the impact of CFI in environment with high security requirements
  */
-QEMU_DISABLE_CFI
+static inline TranslationBlock * QEMU_DISABLE_CFI
-static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
+cpu_tb_exec(CPUState *cpu, TranslationBlock *itb, int *tb_exit)
 {
     CPUArchState *env = cpu->env_ptr;
     uintptr_t ret;
     TranslationBlock *last_tb;
-    int tb_exit;
     const void *tb_ptr = itb->tc.ptr;
 
     qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
@@ -188,11 +187,20 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 
     ret = tcg_qemu_tb_exec(env, tb_ptr);
     cpu->can_do_io = 1;
-    last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
+    /*
-    tb_exit = ret & TB_EXIT_MASK;
+     * TODO: Delay swapping back to the read-write region of the TB
-    trace_exec_tb_exit(last_tb, tb_exit);
+     * until we actually need to modify the TB.  The read-only copy,
+     * coming from the rx region, shares the same host TLB entry as
+     * the code that executed the exit_tb opcode that arrived here.
+     * If we insist on touching both the RX and the RW pages, we
+     * double the host TLB pressure.
+     */
+    last_tb = tcg_splitwx_to_rw((void *)(ret & ~TB_EXIT_MASK));
+    *tb_exit = ret & TB_EXIT_MASK;
 
-    if (tb_exit > TB_EXIT_IDX1) {
+    trace_exec_tb_exit(last_tb, *tb_exit);
+
+    if (*tb_exit > TB_EXIT_IDX1) {
         /* We didn't start executing this TB (eg because the instruction
          * counter hit zero); we must restore the guest PC to the address
          * of the start of the TB.
@@ -210,7 +218,7 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
             cc->set_pc(cpu, last_tb->pc);
         }
     }
-    return ret;
+    return last_tb;
 }
 
 #ifndef CONFIG_USER_ONLY
@@ -221,6 +229,7 @@ static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 {
     TranslationBlock *tb;
     uint32_t cflags = curr_cflags() | CF_NOCACHE;
+    int tb_exit;
 
     if (ignore_icount) {
         cflags &= ~CF_USE_ICOUNT;
@@ -238,7 +247,7 @@ static void cpu_exec_nocache(CPUState *cpu, int max_cycles,
 
     /* execute the generated code */
     trace_exec_tb_nocache(tb, tb->pc);
-    cpu_tb_exec(cpu, tb);
+    cpu_tb_exec(cpu, tb, &tb_exit);
 
     mmap_lock();
     tb_phys_invalidate(tb, -1);
@@ -272,6 +281,7 @@ void cpu_exec_step_atomic(CPUState *cpu)
     uint32_t flags;
     uint32_t cflags = 1;
     uint32_t cf_mask = cflags & CF_HASH_MASK;
+    int tb_exit;
 
     if (sigsetjmp(cpu->jmp_env, 0) == 0) {
         start_exclusive();
@@ -288,7 +298,7 @@ void cpu_exec_step_atomic(CPUState *cpu)
         cpu_exec_enter(cpu);
         /* execute the generated code */
         trace_exec_tb(tb, pc);
-        cpu_tb_exec(cpu, tb);
+        cpu_tb_exec(cpu, tb, &tb_exit);
         cpu_exec_exit(cpu);
     } else {
         /*
@@ -684,13 +694,10 @@ static inline bool cpu_handle_interrupt(CPUState *cpu,
 static inline void cpu_loop_exec_tb(CPUState *cpu, TranslationBlock *tb,
                                     TranslationBlock **last_tb, int *tb_exit)
 {
-    uintptr_t ret;
     int32_t insns_left;
 
     trace_exec_tb(tb, tb->pc);
-    ret = cpu_tb_exec(cpu, tb);
+    tb = cpu_tb_exec(cpu, tb, tb_exit);
-    tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
-    *tb_exit = ret & TB_EXIT_MASK;
     if (*tb_exit != TB_EXIT_REQUESTED) {
         *last_tb = tb;
         return;

tcg/tcg-op.c

@@ -2666,7 +2666,18 @@ void tcg_gen_extr32_i64(TCGv_i64 lo, TCGv_i64 hi, TCGv_i64 arg)
 
 void tcg_gen_exit_tb(const TranslationBlock *tb, unsigned idx)
 {
-    uintptr_t val = (uintptr_t)tb + idx;
+    /*
+     * Let the jit code return the read-only version of the
+     * TranslationBlock, so that we minimize the pc-relative
+     * distance of the address of the exit_tb code to TB.
+     * This will improve utilization of pc-relative address loads.
+     *
+     * TODO: Move this to translator_loop, so that all const
+     * TranslationBlock pointers refer to read-only memory.
+     * This requires coordination with targets that do not use
+     * the translator_loop.
+     */
+    uintptr_t val = (uintptr_t)tcg_splitwx_to_rx((void *)tb) + idx;
 
     if (tb == NULL) {
         tcg_debug_assert(idx == 0);