Re: 64bit port

H. Peter Anvin (hpa@transmeta.com)
Sat, 01 May 1999 03:47:53 -0700


David Miller wrote:
>
> Date: Sat, 01 May 1999 03:32:48 -0700
> From: "H. Peter Anvin" <hpa@transmeta.com>
>
> I'd be really curious to hear the details. Are you making it
> illegal to map a page both writable and executable?
>
> This is the crux of it, see my other mail. When you have seperate,
> software controlled, instruction and data TLBs you can pull all of
> this off cleanly I believe.
>

I actually think that you don't need either of those constraints. What
you *do* need, however, are separate r, w and x permissions on each
page. Also the instruction set needs to be amenable to these kinds of
scans; most RISCs are. You only need to keep each page in one of two
states: writable or executable. When you write to an executable page,
you get a trap, and flip it to the other state; when you execute, you
again get a trap, scan the page, and put it back in the executable
state.

-hpa

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