Re: [PATCH v2] xen/arm: do not read MVFR2 when is not defined

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.01.2021 20:22, Stefano Stabellini wrote:
> MVFR2 is not available on ARMv7. It is available on ARMv8 aarch32 and
> aarch64. If Xen reads MVFR2 on ARMv7 it could crash.
> 
> Avoid the issue by doing the following:
> 
> - define MVFR2_MAYBE_UNDEFINED on arm32
> - if MVFR2_MAYBE_UNDEFINED, do not attempt to read MVFR2 in Xen
> - keep the 3rd register_t in struct cpuinfo_arm.mvfr on arm32 so that a
>   guest read to the register returns '0' instead of crashing the guest.
> 
> '0' is an appropriate value to return to the guest because it is defined
> as "no support for miscellaneous features".
> 
> Aarch64 Xen is not affected by this patch.

But it looks to also be affected by ...

> Fixes: 9cfdb489af81 ("xen/arm: Add ID registers and complete cpuinfo")

... this, faulting (according to osstest logs) early during boot on

000000000025D314        mrs     x1, id_pfr2_el1

afaict.

Jan