The Design and Implementation of the Anykernel and Rump Kernels

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5. Some calling conventions (e.g. ARM EABI) require 64bit parameters to be
passed in even numbered registers. For example, consider the lseek call.
The first parameter is an integer and is passed to the system call in register
0. The second parameter is 64bit, and according to the ABI it needs to
be passed in registers 2+3 instead of registers 1+2. To ensure the alignment
constraint matches in the kernel, the system call description table
syscalls.master contains padding parameters. For example, lseek is defined
as lseek(int fd, int pad, off_t offset, int whence). Since “pad”
is not a part of the application API, we do not want to include it in the
rump kernel system call signature. However, we must include padding in
the struct sys_lseek_args parameter which is passed to the kernel. We
solved the issue by first renaming all pad parameters to the uppercase “PAD”
to decrease the possibility of conflict with an actual parameter called “pad”.
Then, we modified makesyscalls.sh to ignore all parameters named “PAD” for
the application interface side.
A possibility outside of the scope of this work is to examine if the libc system call
stubs and prototypes can now be autogenerated from syscalls.master instead of
requiring separate code in the NetBSD libc Makefiles and system headers.
3.6.2 vnode Interface
The vnode interface is a kernel internal interface. The vnode interface routines take a
vnode object along with other parameters, and call the respective method of the file
system associated with the vnode. For example, the interface for reading is the following:
int VOP_READ(struct vnode *, struct uio *, int, kauth_cred_t);
if the first parameter is a pointer to a FFS vnode, the call will be passed to the FFS
driver.