Re: [PATCH] i386 uaccess to fixmap pages

[H. Peter Anvin]

Followup to:  <[e-mail]>
By author:    Linus Torvalds <[e-mail]>
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
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