Re: I/O permission control in OSKit-Mach

[Roland McGrath]

If copying it all every time is good enough for Linux, it's good enough for us.

You can change the bitmap offset in the tss with little cost, so you can
always use a smaller bitmap stored at the tail end of the tss segment.  For
each task, keep track of the highest port number whose bit is set.  At task
switch, copy only that number of bits into the tail end of the tss segment,
and reset the tss io bitmap slot to start that many bits back (actually you
have to round this all to bytes).

Linux could be said to do the limiting case of this, since it always either
sets the offset at the end of the segment and copies no bits, or always
copies exactly 1024 bits.  That's what you'll get here for a bitmap-less
task and a task using port 0x3ff.