Questions with the in_cksumdata() function in sys/amd64/amd64/in_cksum.c

[btw at mail.ustc.edu.cn]

Hi All,

I'm reading the in_cksumdata() function in sys/amd64/amd64/in_cksum.c, and
I have some questions with the following comment and code:

static u_int64_t
in_cksumdata(const void *buf, int len)
{
        ......

        /*
         * access prefilling to start load of next cache line.
         * then add current cache line
         * save result of prefilling for loop iteration.
         */
        prefilled = lw[0];
        while ((len -= 32) >= 4) {
                u_int64_t prefilling = lw[8];
                sum += prefilled + lw[1] + lw[2] + lw[3]
                        + lw[4] + lw[5] + lw[6] + lw[7];
                lw += 8;
                prefilled = prefilling;
        }

        ......
}

It said that it adds the current cache line, and it adds 32 bytes actually,
while on amd64 platform, the size of each cache line is 64 bytes. So I think
the correct code should be something like this:

static u_int64_t
in_cksumdata(const void *buf, int len)
{
        ......

        /*
         * access prefilling to start load of next cache line.
         * then add current cache line
         * save result of prefilling for loop iteration.
         */
        prefilled = lw[0];
        while ((len -= 64) >= 4) {
                u_int64_t prefilling = lw[16];
                sum += prefilled + lw[1] + lw[2] + lw[3]
                        + lw[4] + lw[5] + lw[6] + lw[7]
                        + lw[8] + lw[9] + lw[10] + lw[11]
                        + lw[12] + lw[13] + lw[14] + lw[15];
                lw += 16;
                prefilled = prefilling;
        }

        ......
}

The full patch is:

diff --git a/in_cksum.c b/in_cksum.c
index 2ae3a0c..4f141f8 100644
--- a/in_cksum.c
+++ b/in_cksum.c
@@ -140,19 +140,23 @@ in_cksumdata(const void *buf, int len)
         * save result of prefilling for loop iteration.
         */
        prefilled = lw[0];
-       while ((len -= 32) >= 4) {
-               u_int64_t prefilling = lw[8];
+       while ((len -= 64) >= 4) {
+               u_int64_t prefilling = lw[16];
                sum += prefilled + lw[1] + lw[2] + lw[3]
-                       + lw[4] + lw[5] + lw[6] + lw[7];
-               lw += 8;
+                       + lw[4] + lw[5] + lw[6] + lw[7]
+                       + lw[8] + lw[9] + lw[10] + lw[11]
+                       + lw[12] + lw[13] + lw[14] + lw[15];
+               lw += 16;
                prefilled = prefilling;
        }
        if (len >= 0) {
                sum += prefilled + lw[1] + lw[2] + lw[3]
-                       + lw[4] + lw[5] + lw[6] + lw[7];
-               lw += 8;
+                       + lw[4] + lw[5] + lw[6] + lw[7]
+                       + lw[8] + lw[9] + lw[10] + lw[11]
+                       + lw[12] + lw[13] + lw[14] + lw[15];
+               lw += 16;
        } else {
-               len += 32;
+               len += 64;
        }
        while ((len -= 16) >= 0) {
                sum += (u_int64_t) lw[0] + lw[1] + lw[2] + lw[3];

Sorry about the confusion if I did something wrong.

- twb