__attribute__ is a keyword introduced by GCC. It is regarded as an extension of a language. It helps the compiler optimize calls, check code more carefully for correctness, control memory placement and code generation options.

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X11 is designed as client-server mode. The communication between the X client and server complies with TCP protocol. Recently I have a Windows X server VcXsrv installed on my Windows 10 and I debug an OpenGL demo glxgears on the WSL2 with tcpdump.

Environment

WSL2 is equipped with its own networking interface like a virtual machine.

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$ ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 172.28.233.193 netmask 255.255.240.0 broadcast 172.28.239.255
inet6 fe80::215:5dff:fe27:9562 prefixlen 64 scopeid 0x20<link>
ether 00:15:5d:27:95:62 txqueuelen 1000 (Ethernet)
RX packets 221579 bytes 15160108 (15.1 MB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 306691 bytes 15714800233 (15.7 GB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 392 bytes 43340 (43.3 KB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 392 bytes 43340 (43.3 KB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

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$ /mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -C "ipconfig"

Windows IP Configuration


Ethernet adapter vEthernet (WSL):

Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::412b:ad3:5548:67ba%38
IPv4 Address. . . . . . . . . . . : 172.28.224.1
Subnet Mask . . . . . . . . . . . : 255.255.240.0
Default Gateway . . . . . . . . . :

Wireless LAN adapter ±¾µØÁ¬½Ó* 1:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :

Wireless LAN adapter ±¾µØÁ¬½Ó* 10:

Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :

Wireless LAN adapter WLAN:

Connection-specific DNS Suffix . : DHCP HOST
Link-local IPv6 Address . . . . . : fe80::954b:e66f:4065:20af%20
IPv4 Address. . . . . . . . . . . : 192.168.2.100
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.2.1

After starting up the X server VcXsrv, you need to export the environmnet variable DISPLAY on the WSL. In case of the vEthernet configuration changed after rebooting you’d better do it like this:

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export DISPLAY=$(grep 'nameserver' /etc/resolv.conf | awk '{print $2}'):0

NOTE: The Firewall between the host and WSL2 must be disabled or your X client can not connect VcXsrv.

Debugging

I trace the demo glxgears using gdb and tcpdump at the same time.

  • gdb
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gdb -q -tui glxgears
  • tcpdump
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sudo tcpdump -vvX not icmp and not arp and not udp and portrange 37900-37999 -w x110224.pcap
  • vv: verboser than -v
  • X: show the packet’s content
  • not icmp: filter out icmp packets
  • not arp: filter out arp packets
  • not udp: filter out udp packets
  • portrange 37900-37999: listening on the ports from 37900 to 37999
  • w x110224.pcap: save the packet captures into the file
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tcpdump -X -r x110224.pcap
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reading from file /home/luc/github/x110224.pcap, link-type EN10MB (Ethernet)
09:11:56.659816 IP 172.28.233.193.37950 > 172.28.224.1.x11: Flags [P.], seq 2796598646:2796598658, ack 1053772981, win 259, length 12
0x0000: 4500 0034 1d84 4000 4006 fb43 ac1c e9c1 E..4..@.@..C....
0x0010: ac1c e001 943e 1770 a6b0 b576 3ecf 4cb5 .....>.p...v>.L.
0x0020: 5018 0103 2223 0000 6200 0300 0400 0000 P..."#..b.......
0x0030: 4452 4932 DRI2
09:11:56.660218 IP 172.28.224.1.x11 > 172.28.233.193.37950: Flags [P.], seq 1:33, ack 12, win 8211, length 32
0x0000: 4500 0048 d9d7 4000 8006 fedb ac1c e001 E..H..@.........
0x0010: ac1c e9c1 1770 943e 3ecf 4cb5 a6b0 b582 .....p.>>.L.....
0x0020: 5018 2013 cf36 0000 0100 0a00 0000 0000 P....6..........
0x0030: 0000 0000 0000 0000 0000 0000 0000 0000 ................
0x0040: 0000 0000 0000 0000 ........
09:11:56.660232 IP 172.28.233.193.37950 > 172.28.224.1.x11: Flags [.], ack 33, win 259, length 0
0x0000: 4500 0028 1d85 4000 4006 fb4e ac1c e9c1 E..(..@.@..N....
0x0010: ac1c e001 943e 1770 a6b0 b582 3ecf 4cd5 .....>.p....>.L.
0x0020: 5010 0103 2217 0000 P..."...

what codes sends and receives these packets? The first two twenty-byted segments are IP header (20 bytes without option) and TCP header (20 bytes without option) separately in these packets.

The source code snippet:

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Bool
XQueryExtension(
register Display *dpy,
_Xconst char *name,
int *major_opcode, /* RETURN */
int *first_event, /* RETURN */
int *first_error) /* RETURN */
{
xQueryExtensionReply rep;
register xQueryExtensionReq *req;

LockDisplay(dpy);
GetReq(QueryExtension, req);
req->nbytes = name ? strlen(name) : 0;
req->length += (req->nbytes+(unsigned)3)>>2;
_XSend(dpy, name, (long)req->nbytes);
(void) _XReply (dpy, (xReply *)&rep, 0, xTrue);
*major_opcode = rep.major_opcode;
*first_event = rep.first_event;
*first_error = rep.first_error;
UnlockDisplay(dpy);
SyncHandle();
return (rep.present);
}

The gdb log:

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Starting program: /mnt/c/Users/lulu/Documents/github/demos/src/xdemos/glxgears 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

Breakpoint 1, XextAddDisplay (extinfo=0x7ffff7e77380 <_dri2Info_data>, dpy=0x55555555d2a0,
ext_name=0x7ffff7e76780 <dri2ExtensionName> "DRI2", hooks=0x7ffff7e767a0 <dri2ExtensionHooks>, nevents=0, data=0x0)
at extutil.c:103
XInitExtension (dpy=dpy@entry=0x55555555d2a0, name=name@entry=0x7ffff7e76780 <dri2ExtensionName> "DRI2")
at InitExt.c:44
XQueryExtension (dpy=dpy@entry=0x55555555d2a0, name=name@entry=0x7ffff7e76780 <dri2ExtensionName> "DRI2",
major_opcode=major_opcode@entry=0x7fffffffd984, first_event=first_event@entry=0x7fffffffd988,
first_error=first_error@entry=0x7fffffffd98c) at QuExt.c:39
$2 = {
reqType = 0x62,
pad = 0x0,
length = 0x3,
nbytes = 0x4,
pad1 = 0x0,
pad2 = 0x0
}
$3 = {
type = 0x1,
pad1 = 0x0,
sequenceNumber = 0xa,
length = 0x0,
present = 0x0,
major_opcode = 0x0,
first_event = 0x0,
first_error = 0x0,
pad3 = 0x0,
pad4 = 0x0,
pad5 = 0x0,
pad6 = 0x0,
pad7 = 0x0
}

$2 is Request packet content to VcXsrv, $3 is Reply packet content from VcXsrv. Even that we can notice the three-way handshake of TCP from the zero-lengthed packet in x110224.pcap.

Overview

Mesa 包含了各种 GPU/CPU 的 OpenGL, OpenCL, Vulkan 实现(Usermode Driver), 也包括 GLX, EGL, GBM 等协议的实现。

  • Vulkan 驱动
codename directories platforms
anv src/intel/vulkan, src/intel/vulkan_hasvk Alder Lake-P
asahi src/asahi/vulkan Apple M1, M2
lpv src/gallium/drivers/llvmpipe CPU
panvk src/panfrost/vulkan RK3399
v3dv src/broadcom/vulkan Raspberry Pi

Build

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meson build -Dprefix=/usr -Dplatforms=x11 -Dgallium-drivers=swrast,panfrost,radeonsi -Dvulkan-drivers=swrast -Dglx=dri -Dllvm=enabled -Dcpp_rtti=false -Dxmlconfig=enabled -Dglvnd=true
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ninja -C build && sudo ninja -C build install
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