linux-kernel - Re: [PATCH v2 2.6.38-rc8-tip 6/20] 6: x86: analyze instruction and determine fixups.

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Date:	Sat, 19 Mar 2011 00:19:22 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Roland McGrath <roland@...k.frob.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	SystemTap <systemtap@...rces.redhat.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 2.6.38-rc8-tip 6/20] 6: x86: analyze instruction and
 determine fixups.

* Roland McGrath <roland@...k.frob.com> [2011-03-18 11:36:29]:

> > handle_riprel_insn() returns 0 if the instruction is not rip-relative
> > returns 1 if its rip-relative but can use XOL slots.
> > returns -1 if its rip-relative but cannot use XOL.
> > 
> > We dont see any instructions that are rip-relative and cannot use XOL.
> > so the check and return are redundant and I will remove that in the next
> > patch.
> 
> How is that?  You can only adjust a rip-relative instruction correctly if
> the instruction copy is within 2GB of the original target address, which
> cannot be presumed to always be the case in user address space layout
> (unlike the kernel).
> 

So we rewrite the copy of instruction (stored in XOL) such that it
accesses its memory operand indirectly thro a scratch register.
The contents of the scratch register are stored before singlestep and
restored later.

Can you please tell us if this doesnt work?

-- 
Thanks and Regards
Srikar

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