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SubjectRe: arm64: kernel v4.6-rc1 hangs on QEMU
On Wed, Mar 30, 2016 at 12:32:42AM +0200, Arnd Bergmann wrote:
> On Wednesday 30 March 2016 01:22:17 Yury Norov wrote:
> > >
> > > Undefined instruction in cpuinfo_store_boot_cpu() could be related
> > > to the SYS_ID_AA64MMFR2_EL1 access that was recently added.
> > >
> > > What does the architecture say about reading unknown cpuid registers?
> > >
> > > Arnd
> >
> > ThunderX has some unimplemented system registers. AFAIR, attempt to access it
> > causes data abort.
>
> Ok, if that is the case, maybe the read_cpuid() macro can be changed
> so it contains a fixup for the trap? That should handle both data abort
> and undefinstr.
>
> Arnd

Sounds alluring, but not clear what we'd return that way. I mean, how
we'd distinguish between correct value and error code (0, -1 or whatever).
But I think, we can do like this:

val = read_cpuid_safe(reg, impossible_val);
if (val == impossible_val)
goto err;

I think it will work for many cases.

Yury.

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