[Qemu-devel] [PATCH v2 04/13] tcg: keep TCGContext's read-mostly fields in a separate cache line

["Emilio G. Cota" <cota@braap.org>]

Upcoming changes will require reading from TCGContext from a
parallel fast path. Prepare for this by keeping the struct's
read-mostly fields in a separate cache line, thereby preventing
false cache line sharing.

Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 tcg/tcg.h | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/tcg/tcg.h b/tcg/tcg.h
index 5fdbfe3..b26f0ef 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -642,6 +642,20 @@ QEMU_BUILD_BUG_ON(OPPARAM_BUF_SIZE > (1 << 14));
 QEMU_BUILD_BUG_ON(sizeof(TCGOp) > 8);
 
 struct TCGContext {
+    /* Read-mostly fields go here to prevent false sharing */
+    struct {
+        GHashTable *helpers;
+
+        void *code_gen_prologue;
+        void *code_gen_buffer;
+        size_t code_gen_buffer_size;
+
+        /* Threshold to flush the translated code buffer.  */
+        void *code_gen_highwater;
+
+        int code_gen_max_blocks;
+    } QEMU_ALIGNED(64);
+
     uint8_t *pool_cur, *pool_end;
     TCGPool *pool_first, *pool_current, *pool_first_large;
     int nb_labels;
@@ -663,8 +677,6 @@ struct TCGContext {
 
     tcg_insn_unit *code_ptr;
 
-    GHashTable *helpers;
-
 #ifdef CONFIG_PROFILER
     /* profiling info */
     int64_t tb_count1;
@@ -697,15 +709,8 @@ struct TCGContext {
        here, because there's too much arithmetic throughout that relies
        on addition and subtraction working on bytes.  Rely on the GCC
        extension that allows arithmetic on void*.  */
-    int code_gen_max_blocks;
-    void *code_gen_prologue;
-    void *code_gen_buffer;
-    size_t code_gen_buffer_size;
     void *code_gen_ptr;
 
-    /* Threshold to flush the translated code buffer.  */
-    void *code_gen_highwater;
-
     TBContext tb_ctx;
 
     /* Track which vCPU triggers events */
-- 
2.7.4