[PATCH v3 8/8] KVM: arm64: Expose ID_AA64PFR2_EL1 to userspace and guests

[Joey Gouly]

Hello!

On Tue, Aug 13, 2024 at 11:44:00AM +0100, Marc Zyngier wrote:
> Everything is now in place for a guest to "enjoy" FP8 support.
> Expose ID_AA64PFR2_EL1 to both userspace and guests, with the
> explicit restriction of only being able to clear FPMR.
> 
> All other features (MTE* at the time of writing) are hidden
> and not writable.
> 
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 51627add0a72..da6d017f24a1 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1722,6 +1722,15 @@ static u64 read_sanitised_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
>  	return val;
>  }
>  
> +static u64 read_sanitised_id_aa64pfr2_el1(struct kvm_vcpu *vcpu,
> +					  const struct sys_reg_desc *rd)
> +{
> +	u64 val = read_sanitised_ftr_reg(SYS_ID_AA64PFR2_EL1);
> +
> +	/* We only expose FPMR */
> +	return val & ID_AA64PFR2_EL1_FPMR;
> +}

Wondering why you're adding this function instead of extending __kvm_read_sanitised_id_reg()?

> +
>  #define ID_REG_LIMIT_FIELD_ENUM(val, reg, field, limit)			       \
>  ({									       \
>  	u64 __f_val = FIELD_GET(reg##_##field##_MASK, val);		       \
> @@ -2381,7 +2390,12 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  		   ID_AA64PFR0_EL1_AdvSIMD |
>  		   ID_AA64PFR0_EL1_FP), },
>  	ID_SANITISED(ID_AA64PFR1_EL1),
> -	ID_UNALLOCATED(4,2),
> +	{ SYS_DESC(SYS_ID_AA64PFR2_EL1),
> +	  .access	= access_id_reg,
> +	  .get_user	= get_id_reg,
> +	  .set_user	= set_id_reg,
> +	  .reset	= read_sanitised_id_aa64pfr2_el1,
> +	  .val		= ID_AA64PFR2_EL1_FPMR, },

Then I think this would just be ID_WRITABLE(ID_AA64PFR2_EL1, ID_AA64PFR2_EL1_FPMR).

>  	ID_UNALLOCATED(4,3),
>  	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
>  	ID_HIDDEN(ID_AA64SMFR0_EL1),

Thanks,
Joey