[Qemu-devel] [v2 1/2] cutils: add avx2 instruction optimization

[Liang Li]

buffer_find_nonzero_offset() is a hot function during live migration.
Now it use SSE2 intructions for optimization. For platform supports
AVX2 instructions, use the AVX2 instructions for optimization can help
to improve the performance about 30% comparing to SSE2.
Zero page check can be faster with this optimization, the test result
shows that for an 8GB RAM idle guest, this patch can help to shorten
the total live migration time about 6%.

This patch use the ifunc mechanism to select the proper function when
running, for platform supports AVX2, excute the AVX2 instructions,
else, excute the original code.

Signed-off-by: Liang Li <address@hidden>
---
 include/qemu-common.h | 28 +++++++++++++++------
 util/Makefile.objs    |  2 ++
 util/avx2.c           | 69 +++++++++++++++++++++++++++++++++++++++++++++++++++
 util/cutils.c         | 53 +++++++++++++++++++++++++++++++++++++--
 4 files changed, 143 insertions(+), 9 deletions(-)
 create mode 100644 util/avx2.c

diff --git a/include/qemu-common.h b/include/qemu-common.h
index 2f74540..9fa7501 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -484,15 +484,29 @@ void qemu_hexdump(const char *buf, FILE *fp, const char 
*prefix, size_t size);
 #endif
 
 #define BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR 8
-static inline bool
-can_use_buffer_find_nonzero_offset(const void *buf, size_t len)
-{
-    return (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
-                   * sizeof(VECTYPE)) == 0
-            && ((uintptr_t) buf) % sizeof(VECTYPE) == 0);
-}
+bool can_use_buffer_find_nonzero_offset(const void *buf, size_t len);
+
 size_t buffer_find_nonzero_offset(const void *buf, size_t len);
 
+extern bool
+can_use_buffer_find_nonzero_offset_avx2(const void *buf, size_t len);
+
+extern size_t buffer_find_nonzero_offset_avx2(const void *buf, size_t len);
+
+extern bool
+can_use_buffer_find_nonzero_offset_inner(const void *buf, size_t len);
+
+extern size_t buffer_find_nonzero_offset_inner(const void *buf, size_t len);
+
+__asm__(".type can_use_buffer_find_nonzero_offset, \%gnu_indirect_function");
+__asm__(".type buffer_find_nonzero_offset, \%gnu_indirect_function");
+
+
+void *can_use_buffer_find_nonzero_offset_ifunc(void) \
+                     __asm__("can_use_buffer_find_nonzero_offset");
+
+void *buffer_find_nonzero_offset_ifunc(void) \
+                     __asm__("buffer_find_nonzero_offset");
 /*
  * helper to parse debug environment variables
  */
diff --git a/util/Makefile.objs b/util/Makefile.objs
index d7cc399..6aacad7 100644
--- a/util/Makefile.objs
+++ b/util/Makefile.objs
@@ -1,4 +1,5 @@
 util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o
+util-obj-y += avx2.o
 util-obj-$(CONFIG_POSIX) += compatfd.o
 util-obj-$(CONFIG_POSIX) += event_notifier-posix.o
 util-obj-$(CONFIG_POSIX) += mmap-alloc.o
@@ -29,3 +30,4 @@ util-obj-y += qemu-coroutine.o qemu-coroutine-lock.o 
qemu-coroutine-io.o
 util-obj-y += qemu-coroutine-sleep.o
 util-obj-y += coroutine-$(CONFIG_COROUTINE_BACKEND).o
 util-obj-y += buffer.o
+avx2.o-cflags      := $(AVX2_CFLAGS)
diff --git a/util/avx2.c b/util/avx2.c
new file mode 100644
index 0000000..0e6915a
--- /dev/null
+++ b/util/avx2.c
@@ -0,0 +1,69 @@
+#include "qemu-common.h"
+
+#ifdef __AVX2__
+#include <immintrin.h>
+#define AVX2_VECTYPE        __m256i
+#define AVX2_SPLAT(p)       _mm256_set1_epi8(*(p))
+#define AVX2_ALL_EQ(v1, v2) \
+    (_mm256_movemask_epi8(_mm256_cmpeq_epi8(v1, v2)) == 0xFFFFFFFF)
+#define AVX2_VEC_OR(v1, v2) (_mm256_or_si256(v1, v2))
+
+inline bool
+can_use_buffer_find_nonzero_offset_avx2(const void *buf, size_t len)
+{
+    return (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
+                   * sizeof(AVX2_VECTYPE)) == 0
+            && ((uintptr_t) buf) % sizeof(AVX2_VECTYPE) == 0);
+}
+
+size_t buffer_find_nonzero_offset_avx2(const void *buf, size_t len)
+{
+    const AVX2_VECTYPE *p = buf;
+    const AVX2_VECTYPE zero = (AVX2_VECTYPE){0};
+    size_t i;
+
+    assert(can_use_buffer_find_nonzero_offset_avx2(buf, len));
+
+    if (!len) {
+        return 0;
+    }
+
+    for (i = 0; i < BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; i++) {
+        if (!AVX2_ALL_EQ(p[i], zero)) {
+            return i * sizeof(AVX2_VECTYPE);
+        }
+    }
+
+    for (i = BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR;
+         i < len / sizeof(AVX2_VECTYPE);
+         i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) {
+        AVX2_VECTYPE tmp0 = AVX2_VEC_OR(p[i + 0], p[i + 1]);
+        AVX2_VECTYPE tmp1 = AVX2_VEC_OR(p[i + 2], p[i + 3]);
+        AVX2_VECTYPE tmp2 = AVX2_VEC_OR(p[i + 4], p[i + 5]);
+        AVX2_VECTYPE tmp3 = AVX2_VEC_OR(p[i + 6], p[i + 7]);
+        AVX2_VECTYPE tmp01 = AVX2_VEC_OR(tmp0, tmp1);
+        AVX2_VECTYPE tmp23 = AVX2_VEC_OR(tmp2, tmp3);
+        if (!AVX2_ALL_EQ(AVX2_VEC_OR(tmp01, tmp23), zero)) {
+            break;
+        }
+    }
+
+    return i * sizeof(AVX2_VECTYPE);
+}
+
+#else
+/* use the original functions if avx2 is not enabled when buiding*/
+
+inline bool
+can_use_buffer_find_nonzero_offset_avx2(const void *buf, size_t len)
+{
+    return can_use_buffer_find_nonzero_offset_inner(buf, len);
+}
+
+inline size_t buffer_find_nonzero_offset_avx2(const void *buf, size_t len)
+{
+    return buffer_find_nonzero_offset_inner(buf, len);
+}
+
+#endif
+
diff --git a/util/cutils.c b/util/cutils.c
index cfeb848..cd478ce 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -26,6 +26,7 @@
 #include <math.h>
 #include <limits.h>
 #include <errno.h>
+#include <cpuid.h>
 
 #include "qemu/sockets.h"
 #include "qemu/iov.h"
@@ -161,6 +162,54 @@ int qemu_fdatasync(int fd)
 #endif
 }
 
+/* old compiler maynot define bit_AVX2 */
+#ifndef bit_AVX2
+#define bit_AVX2 (1 << 5)
+#endif
+
+static inline bool avx2_support(void)
+{
+    int a, b, c, d;
+
+    if (__get_cpuid_max(0, NULL) < 7) {
+        printf("max cpuid < 7\n");
+        return false;
+    }
+
+    __cpuid_count(7, 0, a, b, c, d);
+    printf("b = %x\n", b);
+    return b & bit_AVX2;
+}
+
+void *buffer_find_nonzero_offset_ifunc(void)
+{
+    printf("deciding %s\n", __func__);
+
+    typeof(buffer_find_nonzero_offset) *func = (avx2_support()) ?
+        buffer_find_nonzero_offset_avx2 : buffer_find_nonzero_offset_inner;
+
+    return func;
+}
+
+void *can_use_buffer_find_nonzero_offset_ifunc(void)
+{
+    printf("deciding %s\n", __func__);
+
+    typeof(can_use_buffer_find_nonzero_offset) *func = (avx2_support()) ?
+        can_use_buffer_find_nonzero_offset_avx2 :
+        can_use_buffer_find_nonzero_offset_inner;
+
+    return func;
+}
+
+inline bool
+can_use_buffer_find_nonzero_offset_inner(const void *buf, size_t len)
+{
+    return (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
+                   * sizeof(VECTYPE)) == 0
+            && ((uintptr_t) buf) % sizeof(VECTYPE) == 0);
+}
+
 /*
  * Searches for an area with non-zero content in a buffer
  *
@@ -181,13 +230,13 @@ int qemu_fdatasync(int fd)
  * If the buffer is all zero the return value is equal to len.
  */
 
-size_t buffer_find_nonzero_offset(const void *buf, size_t len)
+size_t buffer_find_nonzero_offset_inner(const void *buf, size_t len)
 {
     const VECTYPE *p = buf;
     const VECTYPE zero = (VECTYPE){0};
     size_t i;
 
-    assert(can_use_buffer_find_nonzero_offset(buf, len));
+    assert(can_use_buffer_find_nonzero_offset_inner(buf, len));
 
     if (!len) {
         return 0;
-- 
1.9.1