✎ soc:2010:cooldavid:journal:week12

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==== Port wscale branch against current gPXE master ====
Recent TCP updates is a lot different from previous base of wscale branch.

=== Hit a GCC optimization trap ===

== For the following code ==
<code>
#include <stdio.h>

struct foo {
        char huge_gap[0x4005d0];
        int v;
};

void store_and_print(int *val, struct foo *f)
{
        *val = f->v;
        if ( !f ) {
                printf("Value is wrong.\n");
        } else {
                printf("Value is correct.\n");
        }
}

int main()
{
        int unidentified_value;

        printf("%p\n", main);
        store_and_print(&unidentified_value, NULL);

        return 0;
}
</code>

The value ''0x4005d0'' is from ''printf("%p\n", main);'' in order to get an
mapped memory address to prevent ''Segment fault'' error while running.

== The output if compiled with ''-O2'' ==
<code>
$ ./null_deref_optimized_away
0x4005d0
Value is correct.
$ 
</code>
And its assembly code:
<code>
void store_and_print(int *val, struct foo *f)
{
        *val = f->v;
  4005b0:       8b 86 d0 05 40 00       mov    0x4005d0(%rsi),%eax
  4005b6:       89 07                   mov    %eax,(%rdi)
        if ( !f ) {
                printf("Value is wrong.\n");
        } else {
                printf("Value is correct.\n");
  4005b8:       bf ec 06 40 00          mov    $0x4006ec,%edi
  4005bd:       e9 c6 fe ff ff          jmpq   400488 <puts@plt>
  4005c2:       66 66 66 66 66 2e 0f    nopw   %cs:0x0(%rax,%rax,1)
  4005c9:       1f 84 00 00 00 00 00

00000000004005d0 <main>:
        }
}
</code>

== The output if compiled with ''-O2 -fno-delete-null-pointer-checks'' ==
<code>
$ ./null_deref_optimized_away
0x4005d0
Value is wrong.
$ 
</code>
And its assembly code:
<code>
00000000004005b0 <store_and_print>:
        int v;
};

void store_and_print(int *val, struct foo *f)
{
        *val = f->v;
  4005b0:       8b 86 d0 05 40 00       mov    0x4005d0(%rsi),%eax
        if ( !f ) {
  4005b6:       48 85 f6                test   %rsi,%rsi
        int v;
};

void store_and_print(int *val, struct foo *f)
{
        *val = f->v;
  4005b9:       89 07                   mov    %eax,(%rdi)
        if ( !f ) {
  4005bb:       74 13                   je     4005d0 <store_and_print+0x20>
                printf("Value is wrong.\n");
        } else {
                printf("Value is correct.\n");
  4005bd:       bf 0c 07 40 00          mov    $0x40070c,%edi
  4005c2:       e9 c1 fe ff ff          jmpq   400488 <puts@plt>
  4005c7:       66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
  4005ce:       00 00

void store_and_print(int *val, struct foo *f)
{
        *val = f->v;
        if ( !f ) {
                printf("Value is wrong.\n");
  4005d0:       bf fc 06 40 00          mov    $0x4006fc,%edi
  4005d5:       e9 ae fe ff ff          jmpq   400488 <puts@plt>
  4005da:       66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)

00000000004005e0 <main>:
                printf("Value is correct.\n");
        }
}
</code>

==== Adding some facilities for testing convenience ====
  * Add a command to modify TCP window size at runtime.
  * print "\b%c" instead of "." in monojob, keep the console clean.
  * Add a ''pause'' command.

[[http://git.etherboot.org/?p=people/cooldavid/gpxe.git;a=shortlog;h=refs/heads/wscale2_test|wscale2 testing branch]]

[[http://git.etherboot.org/?p=people/cooldavid/gpxe.git;a=shortlog;h=refs/heads/tcpfix2_test|tcpfix2 testing branch]]

==== Window size difference ====
  * Test target image on BKO: http://130.239.17.17/bko/live/fedora/f13-alpha/i686/initrd0.img
  * Image size: 43,355,267 bytes
  * Linux wget speed: 1MByte/s (My Internet link speed is 10Mbit/s, it's about the full spped.)
  * Tested it with emulated rtl8139 of KVM, and Native on rtl8139 hardware.

^ Window Size ^   tcpfix2 branch     ^^    wscale2 branch    ^^    sackws2 branch    ^^
^             ^ KVM TAP/NAT ^  Native ^ KVM TAP/NAT ^   Native   ^ KVM TAP/NAT ^  Native ^
|           8K|       17KB/s|   16KB/s|       16KB/s|      16KB/s|         KB/s|     KB/s|
|          16K|       43KB/s|   43KB/s|         KB/s|      41KB/s|         KB/s|     KB/s|
|          32K|       94KB/s|   93KB/s|         KB/s|      91KB/s|         KB/s|     KB/s|
|          64K|      198KB/s|  192KB/s|         KB/s|     187KB/s|         KB/s|     KB/s|
|         128K|             |         |         KB/s|     374KB/s|         KB/s|     KB/s|
|         256K|             |         |         KB/s|     742KB/s|         KB/s|     KB/s|
|         512K|             |         |         KB/s|  [1]344KB/s|         KB/s|     KB/s|
|        1024K|             |         |         KB/s|  [1]403KB/s|         KB/s|     KB/s|

[1]: While TCP rx window is large enough to hit the bandwidth limit(1MB/s),
packets start to drop. According to captured data, the retransmittion is very slow.
The sending host wait a long time for transmit next missing packet required by client.
This issue should be able to fixed with Selective ACK support.
Trace: • info • ubuntubko • ideas • dnsmasq • memsearch • qemupxeknifehttp • tg3 • 2009 • freedos
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