The System Call Interface

Normally, native UnixWare 7 processes enter the kernel through a mechanism known to Intel CPUs as a ``call gate''. Linux binaries, however, use a different mechanism known as an ``int80'' software interrupt. This difference provides the means by which UnixWare 7 distinguishes between UNIX and Linux processes.

Native UnixWare 7 processes do not issue int80 interrupts. If the LKP is not installed, the UnixWare 7 kernel terminates any process that makes an int80 call.

If the LKP is installed, the UnixWare 7 kernel traps int80 interrupts, understanding them to be Linux process requests for kernel functions. When a process executes an int80 instruction, the UnixWare 7 kernel marks it as a Linux process and moves it into a Linux working environment. In effect, the kernel switches personality from UnixWare 7 to Linux for that process.

For each int80 software interrupt it receives, the LKP layer of the UnixWare 7 kernel identifies the type of call, and retrieves its arguments. Depending on the system call type, the LKP layer then does one of the following:

• It calls an equivalent underlying UnixWare 7 kernel function, sometimes mapping or converting the arguments. This is possible where the Linux and UnixWare 7 versions of a system call are identical or very similar (for example: read, write, exit).

• It handles the complete call itself. Ptrace is an example of a Linux system call that has no close UnixWare 7 equivalent.

• It refuses the call altogether. The LKP layer does this if it cannot support or permit the system call. The Linux reboot system call is an example.

When the call is complete, the UnixWare 7 kernel sends the results back to the caller process.

Limitations

The LKP supports most Linux system calls, but cannot support system calls that get or set Linux kernel attributes. The following Linux system calls are not currently supported.

acct	adjtimex	bdflush	capget	capset
create_module	delete_module	get_kernel_syms	init_module	mount
nfsservctl	old_umount	old_vm86	pivot_root	prctl
query_module	quotactl	reboot	sendfile	setdomainname
setfsuid	setfsuid16	setfsgid	setfsgid16	sethostname
sigqueueinfo	swapoff	swapon	sysfs	sysinfo
syslog	umount	uselib	ustat	vhangup
vm86