Re: [PATCH] i386 uaccess to fixmap pages

[Jamie Lokier <jamie@shareable.org>]

H. Peter Anvin wrote:
> 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.

It doesn't need a PTE.  The vsyscall code could be _copied_ to the end
of the page at 0xbffff000, with the stack immediately preceding it.

In fact that's 1 fewer TLB entries than the current arrangement.  The
fork-time copy should be negligable as it is a very small code
fragment.

I suspect this would cause difficulties with the latest ELF and unwind
tables, but apart from that, why _not_ place the vsyscall trampoline
at the end of the stack?

-- Jamie