tcg: Emit prologue to the beginning of code_gen_buffer

By putting the prologue at the end, we risk overwriting the
prologue should our estimate of maximum TB size.  Given the
two different placements of the call to tcg_prologue_init,
move the high water mark computation into tcg_prologue_init.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

2 files changed, 37 insertions, 26 deletions

diff --git a/tcg/tcg.c b/tcg/tcg.c
index d3693b17f2..56091087a4 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -363,17 +363,38 @@ void tcg_context_init(TCGContext *s)
 
 void tcg_prologue_init(TCGContext *s)
 {
-    /* init global prologue and epilogue */
-    s->code_buf = s->code_gen_prologue;
-    s->code_ptr = s->code_buf;
+    size_t prologue_size, total_size;
+    void *buf0, *buf1;
+
+    /* Put the prologue at the beginning of code_gen_buffer.  */
+    buf0 = s->code_gen_buffer;
+    s->code_ptr = buf0;
+    s->code_buf = buf0;
+    s->code_gen_prologue = buf0;
+
+    /* Generate the prologue.  */
     tcg_target_qemu_prologue(s);
-    flush_icache_range((uintptr_t)s->code_buf, (uintptr_t)s->code_ptr);
+    buf1 = s->code_ptr;
+    flush_icache_range((uintptr_t)buf0, (uintptr_t)buf1);
+
+    /* Deduct the prologue from the buffer.  */
+    prologue_size = tcg_current_code_size(s);
+    s->code_gen_ptr = buf1;
+    s->code_gen_buffer = buf1;
+    s->code_buf = buf1;
+    total_size = s->code_gen_buffer_size - prologue_size;
+    s->code_gen_buffer_size = total_size;
+
+    /* Compute a high-water mark, at which we voluntarily flush the
+       buffer and start over.  */
+    s->code_gen_buffer_max_size = total_size - TCG_MAX_OP_SIZE * OPC_BUF_SIZE;
+
+    tcg_register_jit(s->code_gen_buffer, total_size);
 
 #ifdef DEBUG_DISAS
     if (qemu_loglevel_mask(CPU_LOG_TB_OUT_ASM)) {
-        size_t size = tcg_current_code_size(s);
-        qemu_log("PROLOGUE: [size=%zu]\n", size);
-        log_disas(s->code_buf, size);
+        qemu_log("PROLOGUE: [size=%zu]\n", prologue_size);
+        log_disas(buf0, prologue_size);
         qemu_log("\n");
         qemu_log_flush();
     }
diff --git a/translate-all.c b/translate-all.c
index 3454f4ed45..0e8d176de0 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -690,23 +690,15 @@ static inline void code_gen_alloc(size_t tb_size)
     }
 
     qemu_madvise(tcg_ctx.code_gen_buffer, tcg_ctx.code_gen_buffer_size,
-            QEMU_MADV_HUGEPAGE);
-
-    /* Steal room for the prologue at the end of the buffer.  This ensures
-       (via the MAX_CODE_GEN_BUFFER_SIZE limits above) that direct branches
-       from TB's to the prologue are going to be in range.  It also means
-       that we don't need to mark (additional) portions of the data segment
-       as executable.  */
-    tcg_ctx.code_gen_prologue = tcg_ctx.code_gen_buffer +
-            tcg_ctx.code_gen_buffer_size - 1024;
-    tcg_ctx.code_gen_buffer_size -= 1024;
-
-    tcg_ctx.code_gen_buffer_max_size = tcg_ctx.code_gen_buffer_size -
-        (TCG_MAX_OP_SIZE * OPC_BUF_SIZE);
-    tcg_ctx.code_gen_max_blocks = tcg_ctx.code_gen_buffer_size /
-            CODE_GEN_AVG_BLOCK_SIZE;
-    tcg_ctx.tb_ctx.tbs =
-            g_malloc(tcg_ctx.code_gen_max_blocks * sizeof(TranslationBlock));
+                 QEMU_MADV_HUGEPAGE);
+
+    /* Estimate a good size for the number of TBs we can support.  We
+       still haven't deducted the prologue from the buffer size here,
+       but that's minimal and won't affect the estimate much.  */
+    tcg_ctx.code_gen_max_blocks
+        = tcg_ctx.code_gen_buffer_size / CODE_GEN_AVG_BLOCK_SIZE;
+    tcg_ctx.tb_ctx.tbs = g_new(TranslationBlock, tcg_ctx.code_gen_max_blocks);
+
     qemu_mutex_init(&tcg_ctx.tb_ctx.tb_lock);
 }
 
@@ -717,8 +709,6 @@ void tcg_exec_init(unsigned long tb_size)
 {
     cpu_gen_init();
     code_gen_alloc(tb_size);
-    tcg_ctx.code_gen_ptr = tcg_ctx.code_gen_buffer;
-    tcg_register_jit(tcg_ctx.code_gen_buffer, tcg_ctx.code_gen_buffer_size);
     page_init();
 #if defined(CONFIG_SOFTMMU)
     /* There's no guest base to take into account, so go ahead and