accel/tcg: introduce CF_NOIRQ

Here we introduce a new compiler flag to disable the checking of exit
request (icount_decr.u32). This is useful when we want to ensure the
next block cannot be preempted by an asynchronous event.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20211129140932.4115115-2-alex.bennee@linaro.org>

include/exec/exec-all.h

@@ -503,6 +503,7 @@ struct TranslationBlock {
 #define CF_USE_ICOUNT    0x00020000
 #define CF_INVALID       0x00040000 /* TB is stale. Set with @jmp_lock held */
 #define CF_PARALLEL      0x00080000 /* Generate code for a parallel context */
+#define CF_NOIRQ         0x00100000 /* Generate an uninterruptible TB */
 #define CF_CLUSTER_MASK  0xff000000 /* Top 8 bits are cluster ID */
 #define CF_CLUSTER_SHIFT 24
 

include/exec/gen-icount.h

@@ -21,7 +21,6 @@ static inline void gen_tb_start(const TranslationBlock *tb)
 {
     TCGv_i32 count;
 
-    tcg_ctx->exitreq_label = gen_new_label();
     if (tb_cflags(tb) & CF_USE_ICOUNT) {
         count = tcg_temp_local_new_i32();
     } else {
@@ -42,7 +41,19 @@ static inline void gen_tb_start(const TranslationBlock *tb)
         icount_start_insn = tcg_last_op();
     }
 
-    tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label);
+    /*
+     * Emit the check against icount_decr.u32 to see if we should exit
+     * unless we suppress the check with CF_NOIRQ. If we are using
+     * icount and have suppressed interruption the higher level code
+     * should have ensured we don't run more instructions than the
+     * budget.
+     */
+    if (tb_cflags(tb) & CF_NOIRQ) {
+        tcg_ctx->exitreq_label = NULL;
+    } else {
+        tcg_ctx->exitreq_label = gen_new_label();
+        tcg_gen_brcondi_i32(TCG_COND_LT, count, 0, tcg_ctx->exitreq_label);
+    }
 
     if (tb_cflags(tb) & CF_USE_ICOUNT) {
         tcg_gen_st16_i32(count, cpu_env,
@@ -74,8 +85,10 @@ static inline void gen_tb_end(const TranslationBlock *tb, int num_insns)
                            tcgv_i32_arg(tcg_constant_i32(num_insns)));
     }
 
-    gen_set_label(tcg_ctx->exitreq_label);
-    tcg_gen_exit_tb(tb, TB_EXIT_REQUESTED);
+    if (tcg_ctx->exitreq_label) {
+        gen_set_label(tcg_ctx->exitreq_label);
+        tcg_gen_exit_tb(tb, TB_EXIT_REQUESTED);
+    }
 }
 
 #endif