Re: [Xen-devel] [PATCH v2] xen/arm: Correctly support WARN_ON

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Ian,

On 10/09/14 02:35, Ian Campbell wrote:
> On Tue, 2014-09-09 at 11:31 -0700, Julien Grall wrote:
> > Hi Ian,
> >
> > On 09/09/14 02:17, Ian Campbell wrote:
> > > On Mon, 2014-09-08 at 12:19 -0700, Julien Grall wrote:
> > > > Hi Ian,
> > > >
> > > > On 08/09/14 06:01, Ian Campbell wrote:
> > > > > > +    /* PC is always 4-byte align, as Xen is using ARM instruction set */
> > > > > "aligned"
> > > > Ok.
> > > >
> > > > > Is it worth a check here? I presume the nested fault if PC were
> > > > > misaligned would be pretty exciting, print+goto die would seem
> > > > > appropriate.
> > > > I don't think so. The "undefined instruction" is only used when the
> > > > processor is unable to decode the instruction. If it fails to load the
> > > > instruction (because a wrong memory address, pc misaligned), Xen would
> > > > have received a "prefetch abort".
> > > I was thinking of something either wrongly or rightly transitioning to
> > > thumb mode. It'd have to be something horrible like a broken firmware
> > > returning from a PSCI call in the wrong mode or something like that
> > > though.
> > The thumb mode is stored in SPSR which is a banked registers. Your use
> > case can only happen if the secure mode decides to modify the registers
> > of the hypervisor. It would be very stupid from the firmware and other
> > bad things could happen.
> Yes, as I said it would need to be horribly broken firmware. I wouldn't
> rule out the ability of firmware to do horribly broken things though.
Ok. I will add a check and directly go to die.

Regards,


--
Julien Grall

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