Date: Sat, 10 May 2003 16:31:56 +0100
From: Jamie Lokier <jamie@shareable.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] i386 uaccess to fixmap pages
Message-ID: <20030510153156.GA29271@mail.jlokier.co.uk>
References: <20030509124042.GB25569@mail.jlokier.co.uk> <Pine.LNX.4.44.0305090856500.9705-100000@home.transmeta.com> <b9hrhg$5v7$1@cesium.transmeta.com>
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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
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