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

[Joey Gouly]

On Tue, Aug 13, 2024 at 01:47:48PM +0100, Marc Zyngier wrote:
> On Tue, 13 Aug 2024 11:57:10 +0100,
> Joey Gouly <joey.gouly at arm.com> wrote:
> > 
> > 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).
> 
> Yeah, that's an interesting point. I'm afraid I have lost track of the
> many helpers that have been added over time.
> 
> Something like this?

LGTM!

> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index da6d017f24a1..2d1e45178422 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1539,6 +1539,10 @@ static u64 __kvm_read_sanitised_id_reg(const struct kvm_vcpu *vcpu,
>  
>  		val &= ~ARM64_FEATURE_MASK(ID_AA64PFR1_EL1_SME);
>  		break;
> +	case SYS_ID_AA64PFR2_EL1:
> +		/* We only expose FPMR */
> +		val &= ID_AA64PFR2_EL1_FPMR;
> +		break;
>  	case SYS_ID_AA64ISAR1_EL1:
>  		if (!vcpu_has_ptrauth(vcpu))
>  			val &= ~(ARM64_FEATURE_MASK(ID_AA64ISAR1_EL1_APA) |
> @@ -1722,15 +1726,6 @@ 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;
> -}
> -
>  #define ID_REG_LIMIT_FIELD_ENUM(val, reg, field, limit)			       \
>  ({									       \
>  	u64 __f_val = FIELD_GET(reg##_##field##_MASK, val);		       \
> @@ -2390,12 +2385,7 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>  		   ID_AA64PFR0_EL1_AdvSIMD |
>  		   ID_AA64PFR0_EL1_FP), },
>  	ID_SANITISED(ID_AA64PFR1_EL1),
> -	{ 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, },
> +	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,
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.