Re: [PATCH v6 3/6] KVM: arm64: Enable writable for ID_AA64DFR0_EL1 and ID_DFR0_EL1

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Jul 18 2023, Jing Zhang <jingzhangos@google.com> wrote:

> All valid fields in ID_AA64DFR0_EL1 and ID_DFR0_EL1 are writable
> from usrespace with this change.

Typo: s/usrespace/userspace/

>
> Signed-off-by: Jing Zhang <jingzhangos@google.com>
> ---
>  arch/arm64/kvm/sys_regs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 053d8057ff1e..f33aec83f1b4 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2008,7 +2008,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	  .set_user = set_id_dfr0_el1,
>  	  .visibility = aa32_id_visibility,
>  	  .reset = read_sanitised_id_dfr0_el1,
> -	  .val = ID_DFR0_EL1_PerfMon_MASK, },
> +	  .val = GENMASK(63, 0), },
>  	ID_HIDDEN(ID_AFR0_EL1),
>  	AA32_ID_SANITISED(ID_MMFR0_EL1),
>  	AA32_ID_SANITISED(ID_MMFR1_EL1),
> @@ -2057,7 +2057,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  	  .get_user = get_id_reg,
>  	  .set_user = set_id_aa64dfr0_el1,
>  	  .reset = read_sanitised_id_aa64dfr0_el1,
> -	  .val = ID_AA64DFR0_EL1_PMUVer_MASK, },
> +	  .val = GENMASK(63, 0), },
>  	ID_SANITISED(ID_AA64DFR1_EL1),
>  	ID_UNALLOCATED(5,2),
>  	ID_UNALLOCATED(5,3),

How does userspace find out whether a given id reg is actually writable,
other than trying to write to it?