linux-kernel - Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

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Date:	Sat, 16 Jan 2010 19:12:28 -0500
From:	Bryan Donlan <bdonlan@...il.com>
To:	Jim Keniston <jkenisto@...ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	utrace-devel <utrace-devel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Mark Wielaard <mjw@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

On Fri, Jan 15, 2010 at 7:58 PM, Jim Keniston <jkenisto@...ibm.com> wrote:

> 4. Emulation removes the need for the XOL area, but requires pretty much
> total knowledge of the instruction set.  It's also a performance win for
> architectures that can't do #3.  I see kvm implemented on 4
> architectures (ia64, powerpc, s390, x86).  Coincidentally, those are the
> architectures to which uprobes (old uprobes, with ubp and xol bundled
> in) has already been ported (though Intel hasn't been maintaining their
> ia64 port).  So it sort of comes down to how objectionable the XOL vma
> (or page) really is.

On x86 at least, wouldn't one option to be to run the instruction to
be emulated in CPL ('ring') 2, from a XOL page above the user-kernel
split, not accessible to userspace at CPL 3? Linux hasn't
traditionally used anything other than CPL 0 and CPL 3 (plus CPL 1 on
Xen), but it would seem to avoid many of the problems here - it's
invisible to normal userspace code and so doesn't pollute userspace
memory maps with kernel-private stuff, but since it's running at a
higher CPL than the kernel, we can still protect kernel memory and
protect against privileged instructions.
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