linux-kernel - Re: [PATCH V3 3/4] arch/powerpc: Implement Optprobes

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Message-Id: <20170203193949.GD4090@naverao1-tp.localdomain>
Date:   Sat, 4 Feb 2017 01:09:49 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Anju T Sudhakar <anju@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        ananth@...ibm.com, mahesh@...ux.vnet.ibm.com, paulus@...ba.org,
        mhiramat@...nel.org, srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH V3 3/4] arch/powerpc: Implement Optprobes

Hi Michael,
Thanks for the review! I'll defer to Anju on most of the aspects, but...

On 2017/02/01 09:53PM, Michael Ellerman wrote:
> Anju T Sudhakar <anju@...ux.vnet.ibm.com> writes:
> 
> > +static void optimized_callback(struct optimized_kprobe *op,
> > +			       struct pt_regs *regs)
> > +{
> > +	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
> > +	unsigned long flags;
> > +
> > +	/* This is possible if op is under delayed unoptimizing */
> > +	if (kprobe_disabled(&op->kp))
> > +		return;
> > +
> > +	local_irq_save(flags);
> 
> What is that protecting against? Because on powerpc it doesn't actually
> disable interrupts, it just masks some of them, the perf interrupt for
> example can still run.

That's an excellent catch, as always! :)

This is meant to prevent us from missing kprobe hits while processing 
interrupts that arrive when this optprobe is being handled. And you are 
totally right -- we would miss kprobe hits during PMI handling with the 
current approach. We need a hard_irq_disable() there.

> > +	/*
> > +	 * Optprobe template:
> > +	 * This template gets copied into one of the slots in optinsn_slot
> > +	 * and gets fixed up with real optprobe structures et al.
> > +	 */
> > +	.global optprobe_template_entry
> > +optprobe_template_entry:
> > +	/* Create an in-memory pt_regs */
> > +	stdu	r1,-INT_FRAME_SIZE(r1)
> > +	SAVE_GPR(0,r1)
> > +	/* Save the previous SP into stack */
> > +	addi	r0,r1,INT_FRAME_SIZE
> > +	std	r0,GPR1(r1)
> > +	SAVE_10GPRS(2,r1)
> > +	SAVE_10GPRS(12,r1)
> > +	SAVE_10GPRS(22,r1)
> > +	/* Save SPRS */
> > +	mfmsr	r5
> > +	std	r5,_MSR(r1)
> > +	li	r5,0x700
> > +	std	r5,_TRAP(r1)
> > +	li	r5,0
> > +	std	r5,ORIG_GPR3(r1)
> > +	std	r5,RESULT(r1)
> > +	mfctr	r5
> > +	std	r5,_CTR(r1)
> > +	mflr	r5
> > +	std	r5,_LINK(r1)
> > +	mfspr	r5,SPRN_XER
> > +	std	r5,_XER(r1)
> > +	mfcr	r5
> > +	std	r5,_CCR(r1)
> > +	lbz     r5,PACASOFTIRQEN(r13)
> > +	std     r5,SOFTE(r1)
> > +	mfdar	r5
> > +	std	r5,_DAR(r1)
> > +	mfdsisr	r5
> > +	std	r5,_DSISR(r1)
> 
> So this is what made me originally reply to this patch. This
> save/restore sequence.
> 
> I'm not clear on why this is what we need to save/restore.
> 
> Aren't we essentially just interposing a function call? If so do we need
> to save/restore all of these? eg. MSR/DAR/DSISR. Non-volatile GPRs? And
> why are we pretending there was a 0x700 trap?
> 
> Is it because we're going to end up emulating the instruction and so we
> need everything in pt_regs ?

Yes, that and also for the kprobe pre_handler() which takes pt_regs.


Regards,
- Naveen