Re: instruction storage exception handling

[Jacques de Laval <jacques.delaval@protonmail.com>]

On Thursday, October 28th, 2021 at 5:01 AM, Nicholas Piggin <npiggin@gmail.com> wrote:
> Excerpts from Jacques de Laval's message of October 27, 2021 10:03 pm:
>
> > On Wednesday, October 27th, 2021 at 9:52 AM, Christophe Leroy christophe.leroy@csgroup.eu wrote:
> >
> > > Le 27/10/2021 à 09:47, Nicholas Piggin a écrit :
> > >
> > > > You're right. In that case it shouldn't change anything unless there
> > > >
> > > > was a BO fault. I'm not sure what the problem is then. Guessing based
> > > >
> > > > on the NIP and instructions, it looks like it's probably got the correct
> > > >
> > > > user address that it's storing into vmf on the stack, so it has got past
> > > >
> > > > the access checks so my theory would be wrong anyway.
> > > >
> > > > Must be something simple but I can't see it yet.
> > >
> > > Anyway, I think it is still worth doing the check with setting 0 in
> > >
> > > _ESR(r11), maybe the Reference Manual is wrong.
> > >
> > > So Jacques, please do the test anyway if you can.
> > >
> > > Thanks
> > >
> > > Christophe
> >
> > I tested with the last patch from Nicholas, and with that I can not
> >
> > reproduce the issue, so it seems like that solves it for my case and setup
> >
> > at least.
> >
> > Big thanks Christophe and Nicholas for looking in to this!
>
> Thanks for reporting and testing. We can certainly send this patch
>
> upstream to fix the regression, but I'm still not exactly sure what is
>
> going on. If it is an errata or part of specification we missed that
>
> could explain it but it would be good to understand and comment it.
>
> If you have time to test again with only the following patch applied,
>
> it might give a better clue. This patch should keep running but it
>
> would print some dmesg output.
>
> Thanks,
>
> Nick
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>
> index a8d0ce85d39a..cf56f23ff90a 100644
>
> --- a/arch/powerpc/mm/fault.c
>
> +++ b/arch/powerpc/mm/fault.c
>
> @@ -548,6 +548,12 @@ static __always_inline void __do_page_fault(struct pt_regs *regs)
>
> DEFINE_INTERRUPT_HANDLER(do_page_fault)
>
> {
>
> -   if (TRAP(regs) == INTERRUPT_INST_STORAGE) {
>
> -         if (regs->dsisr != 0) {
>
>
> -         	printk("ISI pc:%lx msr:%lx dsisr:%lx ESR:%lx\\n", regs->nip, regs->msr, regs->dsisr, mfspr(SPRN_ESR));
>
>
> -         	regs->dsisr = 0; // fix?
>
>
> -         }
>
>
> -   }
>
>     __do_page_fault(regs);
>
>     }
>

As expected it keeps running. The output, and number of prints is naturally
a bit different from time to time, but dsisr/ESR is always 0x800000.

Here's a representative output from one run:

	ISI pc:b789e6c0 msr:2d002 dsisr:800000 ESR:800000
	ISI pc:b7884220 msr:2d002 dsisr:800000 ESR:800000
	ISI pc:b78c18a4 msr:2d002 dsisr:800000 ESR:800000
	ISI pc:55a238 msr:2f902 dsisr:800000 ESR:800000
	ISI pc:412380 msr:2f902 dsisr:800000 ESR:800000
	ISI pc:3aabe0 msr:2f902 dsisr:800000 ESR:800000
	ISI pc:47a0e0 msr:2f902 dsisr:800000 ESR:800000
	ISI pc:443290 msr:2f902 dsisr:800000 ESR:800000
	ISI pc:43b350 msr:2d002 dsisr:800000 ESR:800000

Thanks,
Jacques