[Qemu-arm] [RFC PATCH] include/exec/cpu-defs.h: try and make SoftMMU page size match target

[Alex Bennée]

While the SoftMMU is not emulating the target MMU of a system there is
a relationship between its page size and that of the target. If the
target MMU is full featured the functions called to re-fill the
entries in the SoftMMU entries start moving up the perf profiles. If
we can we should try and prevent too much thrashing around by having
the page sizes the same.

Ideally we should use TARGET_PAGE_BITS_MIN but that potentially
involves a fair bit of #include re-jigging so I went for 10 bits (1k
pages) which I think is the smallest of all our emulated systems.

Some quick numbers show a reasonable performance win on an x86_64
host:

 ./aarch64-softmmu/qemu-system-aarch64 -machine type=virt \
   -display none -m 16384 -cpu cortex-a57 -serial mon:stdio \
   -drive file=../jessie-arm64.qcow2,id=myblock,index=0,if=none \
   -device virtio-blk-device,drive=myblock \
   -append "console=ttyAMA0 root=/dev/vda1 
systemd.unit=benchmark-build.service" \
   -kernel ../aarch64-current-linux-kernel-only.img -machine gic-version=3 -smp 
4

8 bit TLB:
  run 1: ret=0 (PASS), time=425.202797 (1/1)
  run 2: ret=0 (PASS), time=410.421742 (2/2)
  run 3: ret=0 (PASS), time=417.666752 (3/3)
  run 4: ret=0 (PASS), time=411.158793 (4/4)
  run 5: ret=0 (PASS), time=417.133068 (5/5)
  Results summary:
  0: 5 times (100.00%), avg time 416.317 (35.70 varience/5.98 deviation)

10 bit TLB
  run 1: ret=0 (PASS), time=359.310380 (1/1)
  run 2: ret=0 (PASS), time=387.826981 (2/2)
  run 3: ret=0 (PASS), time=381.097123 (3/3)
  run 4: ret=0 (PASS), time=393.826197 (4/4)
  run 5: ret=0 (PASS), time=384.340781 (5/5)
  Results summary:
  0: 5 times (100.00%), avg time 381.280 (173.08 varience/13.16 deviation)

CC: Pranith Kumar <address@hidden>
Signed-off-by: Alex Bennée <address@hidden>
---
 include/exec/cpu-defs.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index bc8e7f848d..a0f9249752 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -89,7 +89,7 @@ typedef uint64_t target_ulong;
  * of tlb_table inside env (which is non-trivial but not huge).
  */
 #define CPU_TLB_BITS                                             \
-    MIN(8,                                                       \
+    MIN(10,                                                      \
         TCG_TARGET_TLB_DISPLACEMENT_BITS - CPU_TLB_ENTRY_BITS -  \
         (NB_MMU_MODES <= 1 ? 0 :                                 \
          NB_MMU_MODES <= 2 ? 1 :                                 \
-- 
2.13.0