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

[Julien Grall <julien@xxxxxxx>]

Hi Stefano,

On 08/01/2021 19:22, 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.
>
> Fixes: 9cfdb489af81 ("xen/arm: Add ID registers and complete cpuinfo")
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

With one question below:

> ---
>   xen/arch/arm/cpufeature.c           | 2 ++
>   xen/include/asm-arm/arm32/sysregs.h | 3 +++
>   2 files changed, 5 insertions(+)
>
> diff --git a/xen/arch/arm/cpufeature.c b/xen/arch/arm/cpufeature.c
> index 1f6a85aafe..698bfa0201 100644
> --- a/xen/arch/arm/cpufeature.c
> +++ b/xen/arch/arm/cpufeature.c
> @@ -150,7 +150,9 @@ void identify_cpu(struct cpuinfo_arm *c)
>   
>           c->mvfr.bits[0] = READ_SYSREG(MVFR0_EL1);
>           c->mvfr.bits[1] = READ_SYSREG(MVFR1_EL1);
> +#ifndef MVFR2_MAYBE_UNDEFINED
>           c->mvfr.bits[2] = READ_SYSREG(MVFR2_EL1);
> +#endif
Is there any guarantee that c->mvfr.bits[2] will be zeroed by default?

Cheers,

--
Julien Grall