[Qemu-devel] [PATCH v2 3/5] target-arm: A64: fix use 12 bit page tables for AArch64

[Alex Bennée]

The AArch64 architecture only support 4k+ pages so using a smaller value
for QEMU's internal page table handling only makes us less efficient. I
ran some simple benchmarks and measured a 25-30% speed improvement for
CPU bound tasks like booting the kernel or compressing a section of a
file-system.

Signed-off-by: Alex Bennée <address@hidden>

---

v2:
  - fix AArch64 references
  - add benchmark notes to commit msg

diff --git a/target-arm/cpu.h b/target-arm/cpu.h
index c83f249..83df513 100644
--- a/target-arm/cpu.h
+++ b/target-arm/cpu.h
@@ -1051,11 +1051,18 @@ bool write_cpustate_to_list(ARMCPU *cpu);
 #if defined(CONFIG_USER_ONLY)
 #define TARGET_PAGE_BITS 12
 #else
-/* The ARM MMU allows 1k pages.  */
-/* ??? Linux doesn't actually use these, and they're deprecated in recent
-   architecture revisions.  Maybe a configure option to disable them.  */
+#if defined(TARGET_AARCH64)
+/* You can't configure 1k pages on AArch64 hardware */
+#define TARGET_PAGE_BITS 12
+#else
+/* The ARM MMU allows 1k pages - although they are not used by Linux
+ * FIXME?: they're deprecated in recent architecture revisions and
+ * this does create a performance hit. Maybe a configure option to
+ * disable them?
+ */
 #define TARGET_PAGE_BITS 10
 #endif
+#endif
 
 #if defined(TARGET_AARCH64)
 #  define TARGET_PHYS_ADDR_SPACE_BITS 48
-- 
2.0.3