Re: Ideal Architecture for Linux

["H. Peter Anvin" <hpa@zytor.com>]

James Crosby wrote:
> A stack based architecture is best, it makes nested interrupts, and general
> interruptibility very easy.

If you mean stack-based register file, then that's *not* desirable; 
Linux depends on gcc which prefers a flat register file.  16 registers 
minimum.

> Two hardware stack pointers, one for kernel stack and one for user mode
> stack of each task, but maybe a third, interrupt stack.

Don't necessarily have to be hardware registers, although it speed 
things up.  Interrupt stacks are good.

> At least two corresponding processor modes, a user mode with access only
> to 'safe' instructions, and a privileged one that can do anything. But would
> additional modes for interrupt handlers and system call handlers be
> beneficial?

Not in Linux.

> I would also consider it a significant security problem if when a task enters
> the kernel, kernel data is stored on the task's user-mode stack, since the
> task could fish around beyond the end of its stack and sniff private data. (This
> is, if I understand correctly, the reason ARM system mode is unused.)
> 
> But on the other hand, if each task didn't need a separate kernel stack, then
> there could be 4k or 8k less memory use per thread.

No, it would just be in a different place.  Linux will *NOT* accommodate 
this model.

> Is it necessarily the case that the more similar to x86, the better Linux 'fits'
> the architecture?

No, the x86 is lacking quite a few key features.

Linux would prefer to have address space numbers, instructions for 
user-space memory access in kernel mode, and a larger page size (4K is 
really too small for modern multi-gigabyte machines.)

The design of the I/O infrastructure is another matter, too.

	-hpa