[9fans] rfork(), getss() etc etc

[nigel@9fs.org]

Hmm.

clone() not splitting the stack is a feature of Linux, and probably not
worth wasting brain power over. The fact that rfork() in FreeBSD copied
clone() semantics, but the Plan 9 manual page is hard to
believe. I did get some indication recently that the FreeBSD manual page
would be changed to own up to the fact that it was rfork() by name
but clone() by nature! FreeBSD does not split either because of the way
the VM works (reading between the lines, allegedly).

Now, what is the problem with this? Firslty, the only way to tell whether
you are parent or child after the split is to check the return result from
the system call. The inevitable conclusion is that assembly code is
required to establish a new stack. This is a retrograde step, and I am
staggered to find open source systems promoting the use of assembly
code by including system call variants which cannot be sensibly used
without.

Secondly, the stack now established is not managed or protected by
the kernel. Good grief, this is what we all criticise Win9x and MacOS
for.

Thirdly, you can only identify which process you are by calling getpid(),
rather than referencing a data structure in your own stack. This is
expensive, as it involves a system call, plus some form of mapping
(?hash table?) from pid to per process data structure.

This is might be why gettss() was used. It produces a number with
a smaller range that getpid(), allowing a simple index to per process
data.

The Vita Nuova FreeBSD port of the Inferno emu uses rfork() and getpid().