Re: Ideal Architecture for Linux
From: H. Peter Anvin
Date: Tue Feb 19 2008 - 12:38:19 EST
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
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