linux-kernel - Re: [PATCH v4 15/16] ARM: add uprobes support

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Date:	Fri, 20 Dec 2013 19:47:18 +0000
From:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
To:	Rabin Vincent <rabin@....in>
Cc:	David Long <dave.long@...aro.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	Russell King <linux@....linux.org.uk>,
	Oleg Nesterov <oleg@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	davem@...emloft.net, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 15/16] ARM: add uprobes support

On Fri, 2013-12-20 at 20:00 +0100, Rabin Vincent wrote:
> 2013/12/20 Jon Medhurst (Tixy) <tixy@...aro.org>
> > On Sun, 2013-12-15 at 23:08 -0500, David Long wrote:
> > > +static int uprobes_substitute_pc(unsigned long *pinsn, u32 oregs)
> > > +{
> > > +     probes_opcode_t insn = __mem_to_opcode_arm(*pinsn);
> > > +     probes_opcode_t temp;
> > > +     probes_opcode_t mask;
> > > +     int freereg;
> > > +     u32 free = 0xffff;
> > > +     u32 regs;
> > > +
> > > +     for (regs = oregs; regs; regs >>= 4, insn >>= 4) {
> > > +             if ((regs & 0xf) == REG_TYPE_NONE)
> > > +                     continue;
> > > +
> > > +             free &= ~(1 << (insn & 0xf));
> > > +     }
> > > +
> > > +     /* No PC, no problem */
> > > +     if (free & (1 << 15))
> > > +             return 15;
> > > +
> > > +     if (!free)
> > > +             return -1;
> > > +
> > > +     /*
> > > +      * fls instead of ffs ensures that for "ldrd r0, r1, [pc]" we would
> > > +      * pick LR instead of R1.
> >
> > Do we know why this is desirable, i.e. preferring the higher numbered
> > registers? If there isn't a preference, then no need for comment really.
> >
> > Also, the comment as is is wrong, should be "...pick LR instead of R2"
> > because R1 wouldn't be chosen as the instruction already uses it.
> 
> The second destination register of LDRD (R1 in the example above) is
> not encoded in the instruction

Ah, that's the fact I'd missed, so the code and comment in this patch is
correct.

Thanks

-- 
Tixy


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