Re: [PATCH] i386 uaccess to fixmap pages

From: H. Peter Anvin (hpa@zytor.com)
Date: Fri May 09 2003 - 22:26:40 EST


Followup to: <Pine.LNX.4.44.0305090856500.9705-100000@home.transmeta.com>
By author: Linus Torvalds <torvalds@transmeta.com>
In newsgroup: linux.dev.kernel
>
> It actually does have some cost in that form, namely the fact that the
> kernel 1:1 mapping needs to be 4MB-aligned in order to take advantage of
> large-pte support. So we'd have to move the kernel to something like
> 0xc0400000 (and preferably higher, to make sure there is a nice hole in
> between - say 0xc1000000), which in turn has a cost of verifying that
> nothing assumes the current lay-out (we've had the 1/2/3GB TASK_SIZE
> patches floating around, but they've never had "odd sizes").
>
> There's another cost, which is that right now we share the pgd with the
> kernel fixmaps, and this would mean that we'd have a new one. That's just
> a single page, though.
>
> But it might "just work", and it would be interesting to see what the
> patch would look like. Hint hint.
>

Another option would be to put it in the "user" part of the address
space at 0xbffff000 (and move the %esp base value.) That would have
the nice side benefit that stuff like UML or whatever who wanted to
map something over the vsyscall could do so. Downside: each process
needs a PTE for this.

        -hpa

-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
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