[Qemu-devel] [PATCH v3 3/3] utils: Add prefetch for Thunderx platform

[vijay.kilari@gmail.com]

From: Vijaya Kumar K <Vijaya.Kumar@cavium.com>

Thunderx pass2 chip requires explicit prefetch
instruction to give prefetch hint.

To speed up live migration on Thunderx platform,
prefetch instruction is added in zero buffer check
function.The below results show live migration time improvement
with prefetch instruction. VM with 4 VCPUs, 8GB RAM is migrated.

Without prefetch total migration time is ~13 seconds
adding prefetch total migration time is 9.5 seconds

Code for decoding cache size is taken from Richard's
patch

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@cavium.com>
---
 util/bufferiszero.c | 37 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 36 insertions(+), 1 deletion(-)

diff --git a/util/bufferiszero.c b/util/bufferiszero.c
index 421d945..f50b8df 100644
--- a/util/bufferiszero.c
+++ b/util/bufferiszero.c
@@ -25,6 +25,10 @@
 #include "qemu-common.h"
 #include "qemu/cutils.h"
 #include "qemu/bswap.h"
+#include <math.h>
+
+static uint32_t cache_line_factor = 1;
+static uint32_t prefetch_line_dist = 1;
 
 static bool
 buffer_zero_int(const void *buf, size_t len)
@@ -49,7 +53,8 @@ buffer_zero_int(const void *buf, size_t len)
         const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8);
 
         for (; p + 8 <= e; p += 8) {
-            __builtin_prefetch(p + 8, 0, 0);
+            __builtin_prefetch(p +
+               (8 * cache_line_factor * prefetch_line_dist), 0, 0);
             if (t) {
                 return false;
             }
@@ -293,6 +298,30 @@ bool test_buffer_is_zero_next_accel(void)
 }
 #endif
 
+#if defined(__aarch64__)
+#include "qemu/aarch64-cpuid.h"
+
+static void __attribute__((constructor)) aarch64_init_cache_size(void)
+{
+    uint64_t t;
+
+    /* Use the DZP block size as a proxy for the cacheline size,
+       since the later is not available to userspace.  This seems
+       to work in practice for existing implementations.  */
+    asm("mrs %0, dczid_el0" : "=r"(t));
+    if (pow(2, (t & 0xf)) * 4 >= 128) {
+        cache_line_factor = 2;
+    } else {
+        cache_line_factor = 1;
+    }
+
+    get_aarch64_cpu_id();
+    if (is_thunderx_pass2_cpu()) {
+        prefetch_line_dist = 3;
+    }
+}
+#endif
+
 /*
  * Checks if a buffer is all zeroes
  */
@@ -305,6 +334,12 @@ bool buffer_is_zero(const void *buf, size_t len)
     /* Fetch the beginning of the buffer while we select the accelerator.  */
     __builtin_prefetch(buf, 0, 0);
 
+#if defined(__aarch64__)
+    if (is_thunderx_pass2_cpu()) {
+        __builtin_prefetch(buf + 16, 0, 0);
+        __builtin_prefetch(buf + 32, 0, 0);
+    }
+#endif
     /* Use an optimized zero check if possible.  Note that this also
        includes a check for an unrolled loop over 64-bit integers.  */
     return select_accel_fn(buf, len);
-- 
1.9.1