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 <a...@linux.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
